checker.ts

/* @internal */
namespace ts {
    const ambientModuleSymbolRegex = /^".+"$/;

    let nextSymbolId = 1;
    let nextNodeId = 1;
    let nextMergeId = 1;
    let nextFlowId = 1;

    export function getNodeId(node: Node): number {
        if (!node.id) {
            node.id = nextNodeId;
            nextNodeId++;
        }
        return node.id;
    }

    export function getSymbolId(symbol: Symbol): number {
        if (!symbol.id) {
            symbol.id = nextSymbolId;
            nextSymbolId++;
        }

        return symbol.id;
    }

    export function isInstantiatedModule(node: ModuleDeclaration, preserveConstEnums: boolean) {
        const moduleState = getModuleInstanceState(node);
        return moduleState === ModuleInstanceState.Instantiated ||
            (preserveConstEnums && moduleState === ModuleInstanceState.ConstEnumOnly);
    }

    export function createTypeChecker(host: TypeCheckerHost, produceDiagnostics: boolean): TypeChecker {
        const getPackagesSet: () => Map<true> = memoize(() => {
            const set = createMap<true>();
            host.getSourceFiles().forEach(sf => {
                if (!sf.resolvedModules) return;

                forEachEntry(sf.resolvedModules, r => {
                    if (r && r.packageId) set.set(r.packageId.name, true);
                });
            });
            return set;
        });

        // Cancellation that controls whether or not we can cancel in the middle of type checking.
        // In general cancelling is *not* safe for the type checker.  We might be in the middle of
        // computing something, and we will leave our internals in an inconsistent state.  Callers
        // who set the cancellation token should catch if a cancellation exception occurs, and
        // should throw away and create a new TypeChecker.
        //
        // Currently we only support setting the cancellation token when getting diagnostics.  This
        // is because diagnostics can be quite expensive, and we want to allow hosts to bail out if
        // they no longer need the information (for example, if the user started editing again).
        let cancellationToken: CancellationToken | undefined;
        let requestedExternalEmitHelpers: ExternalEmitHelpers;
        let externalHelpersModule: Symbol;

        // tslint:disable variable-name
        const Symbol = objectAllocator.getSymbolConstructor();
        const Type = objectAllocator.getTypeConstructor();
        const Signature = objectAllocator.getSignatureConstructor();
        // tslint:enable variable-name

        let typeCount = 0;
        let symbolCount = 0;
        let enumCount = 0;
        let instantiationDepth = 0;
        let constraintDepth = 0;

        const emptySymbols = createSymbolTable();
        const identityMapper: (type: Type) => Type = identity;

        const compilerOptions = host.getCompilerOptions();
        const languageVersion = getEmitScriptTarget(compilerOptions);
        const moduleKind = getEmitModuleKind(compilerOptions);
        const allowSyntheticDefaultImports = getAllowSyntheticDefaultImports(compilerOptions);
        const strictNullChecks = getStrictOptionValue(compilerOptions, "strictNullChecks");
        const strictFunctionTypes = getStrictOptionValue(compilerOptions, "strictFunctionTypes");
        const strictPropertyInitialization = getStrictOptionValue(compilerOptions, "strictPropertyInitialization");
        const noImplicitAny = getStrictOptionValue(compilerOptions, "noImplicitAny");
        const noImplicitThis = getStrictOptionValue(compilerOptions, "noImplicitThis");
        const keyofStringsOnly = !!compilerOptions.keyofStringsOnly;

        const emitResolver = createResolver();
        const nodeBuilder = createNodeBuilder();

        const undefinedSymbol = createSymbol(SymbolFlags.Property, "undefined" as __String);
        undefinedSymbol.declarations = [];
        const argumentsSymbol = createSymbol(SymbolFlags.Property, "arguments" as __String);
        const requireSymbol = createSymbol(SymbolFlags.Property, "require" as __String);

        /** This will be set during calls to `getResolvedSignature` where services determines an apparent number of arguments greater than what is actually provided. */
        let apparentArgumentCount: number | undefined;

        // for public members that accept a Node or one of its subtypes, we must guard against
        // synthetic nodes created during transformations by calling `getParseTreeNode`.
        // for most of these, we perform the guard only on `checker` to avoid any possible
        // extra cost of calling `getParseTreeNode` when calling these functions from inside the
        // checker.
        const checker: TypeChecker = {
            getNodeCount: () => sum(host.getSourceFiles(), "nodeCount"),
            getIdentifierCount: () => sum(host.getSourceFiles(), "identifierCount"),
            getSymbolCount: () => sum(host.getSourceFiles(), "symbolCount") + symbolCount,
            getTypeCount: () => typeCount,
            isUndefinedSymbol: symbol => symbol === undefinedSymbol,
            isArgumentsSymbol: symbol => symbol === argumentsSymbol,
            isUnknownSymbol: symbol => symbol === unknownSymbol,
            getMergedSymbol,
            getDiagnostics,
            getGlobalDiagnostics,
            getTypeOfSymbolAtLocation: (symbol, location) => {
                location = getParseTreeNode(location);
                return location ? getTypeOfSymbolAtLocation(symbol, location) : errorType;
            },
            getSymbolsOfParameterPropertyDeclaration: (parameterIn, parameterName) => {
                const parameter = getParseTreeNode(parameterIn, isParameter);
                if (parameter === undefined) return Debug.fail("Cannot get symbols of a synthetic parameter that cannot be resolved to a parse-tree node.");
                return getSymbolsOfParameterPropertyDeclaration(parameter, escapeLeadingUnderscores(parameterName));
            },
            getDeclaredTypeOfSymbol,
            getPropertiesOfType,
            getPropertyOfType: (type, name) => getPropertyOfType(type, escapeLeadingUnderscores(name)),
            getTypeOfPropertyOfType: (type, name) => getTypeOfPropertyOfType(type, escapeLeadingUnderscores(name)),
            getIndexInfoOfType,
            getSignaturesOfType,
            getIndexTypeOfType,
            getBaseTypes,
            getBaseTypeOfLiteralType,
            getWidenedType,
            getTypeFromTypeNode: nodeIn => {
                const node = getParseTreeNode(nodeIn, isTypeNode);
                return node ? getTypeFromTypeNode(node) : errorType;
            },
            getParameterType: getTypeAtPosition,
            getPromisedTypeOfPromise,
            getReturnTypeOfSignature,
            getNullableType,
            getNonNullableType,
            typeToTypeNode: nodeBuilder.typeToTypeNode,
            indexInfoToIndexSignatureDeclaration: nodeBuilder.indexInfoToIndexSignatureDeclaration,
            signatureToSignatureDeclaration: nodeBuilder.signatureToSignatureDeclaration,
            symbolToEntityName: nodeBuilder.symbolToEntityName as (symbol: Symbol, meaning: SymbolFlags, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => EntityName, // TODO: GH#18217
            symbolToExpression: nodeBuilder.symbolToExpression as (symbol: Symbol, meaning: SymbolFlags, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => Expression, // TODO: GH#18217
            symbolToTypeParameterDeclarations: nodeBuilder.symbolToTypeParameterDeclarations,
            symbolToParameterDeclaration: nodeBuilder.symbolToParameterDeclaration as (symbol: Symbol, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => ParameterDeclaration, // TODO: GH#18217
            typeParameterToDeclaration: nodeBuilder.typeParameterToDeclaration as (parameter: TypeParameter, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => TypeParameterDeclaration, // TODO: GH#18217
            getSymbolsInScope: (location, meaning) => {
                location = getParseTreeNode(location);
                return location ? getSymbolsInScope(location, meaning) : [];
            },
            getSymbolAtLocation: node => {
                node = getParseTreeNode(node);
                return node ? getSymbolAtLocation(node) : undefined;
            },
            getShorthandAssignmentValueSymbol: node => {
                node = getParseTreeNode(node);
                return node ? getShorthandAssignmentValueSymbol(node) : undefined;
            },
            getExportSpecifierLocalTargetSymbol: nodeIn => {
                const node = getParseTreeNode(nodeIn, isExportSpecifier);
                return node ? getExportSpecifierLocalTargetSymbol(node) : undefined;
            },
            getExportSymbolOfSymbol(symbol) {
                return getMergedSymbol(symbol.exportSymbol || symbol);
            },
            getTypeAtLocation: node => {
                node = getParseTreeNode(node);
                return node ? getTypeOfNode(node) : errorType;
            },
            getPropertySymbolOfDestructuringAssignment: locationIn => {
                const location = getParseTreeNode(locationIn, isIdentifier);
                return location ? getPropertySymbolOfDestructuringAssignment(location) : undefined;
            },
            signatureToString: (signature, enclosingDeclaration, flags, kind) => {
                return signatureToString(signature, getParseTreeNode(enclosingDeclaration), flags, kind);
            },
            typeToString: (type, enclosingDeclaration, flags) => {
                return typeToString(type, getParseTreeNode(enclosingDeclaration), flags);
            },
            symbolToString: (symbol, enclosingDeclaration, meaning, flags) => {
                return symbolToString(symbol, getParseTreeNode(enclosingDeclaration), meaning, flags);
            },
            typePredicateToString: (predicate, enclosingDeclaration, flags) => {
                return typePredicateToString(predicate, getParseTreeNode(enclosingDeclaration), flags);
            },
            writeSignature: (signature, enclosingDeclaration, flags, kind, writer) => {
                return signatureToString(signature, getParseTreeNode(enclosingDeclaration), flags, kind, writer);
            },
            writeType: (type, enclosingDeclaration, flags, writer) => {
                return typeToString(type, getParseTreeNode(enclosingDeclaration), flags, writer);
            },
            writeSymbol: (symbol, enclosingDeclaration, meaning, flags, writer) => {
                return symbolToString(symbol, getParseTreeNode(enclosingDeclaration), meaning, flags, writer);
            },
            writeTypePredicate: (predicate, enclosingDeclaration, flags, writer) => {
                return typePredicateToString(predicate, getParseTreeNode(enclosingDeclaration), flags, writer);
            },
            getAugmentedPropertiesOfType,
            getRootSymbols,
            getContextualType: nodeIn => {
                const node = getParseTreeNode(nodeIn, isExpression);
                return node ? getContextualType(node) : undefined;
            },
            getContextualTypeForObjectLiteralElement: nodeIn => {
                const node = getParseTreeNode(nodeIn, isObjectLiteralElementLike);
                return node ? getContextualTypeForObjectLiteralElement(node) : undefined;
            },
            getContextualTypeForArgumentAtIndex: (nodeIn, argIndex) => {
                const node = getParseTreeNode(nodeIn, isCallLikeExpression);
                return node && getContextualTypeForArgumentAtIndex(node, argIndex);
            },
            getContextualTypeForJsxAttribute: (nodeIn) => {
                const node = getParseTreeNode(nodeIn, isJsxAttributeLike);
                return node && getContextualTypeForJsxAttribute(node);
            },
            isContextSensitive,
            getFullyQualifiedName,
            getResolvedSignature: (node, candidatesOutArray, agumentCount) =>
                getResolvedSignatureWorker(node, candidatesOutArray, agumentCount, /*isForSignatureHelp*/ false),
            getResolvedSignatureForSignatureHelp: (node, candidatesOutArray, agumentCount) =>
                getResolvedSignatureWorker(node, candidatesOutArray, agumentCount, /*isForSignatureHelp*/ true),
            getConstantValue: nodeIn => {
                const node = getParseTreeNode(nodeIn, canHaveConstantValue);
                return node ? getConstantValue(node) : undefined;
            },
            isValidPropertyAccess: (nodeIn, propertyName) => {
                const node = getParseTreeNode(nodeIn, isPropertyAccessOrQualifiedNameOrImportTypeNode);
                return !!node && isValidPropertyAccess(node, escapeLeadingUnderscores(propertyName));
            },
            isValidPropertyAccessForCompletions: (nodeIn, type, property) => {
                const node = getParseTreeNode(nodeIn, isPropertyAccessExpression);
                return !!node && isValidPropertyAccessForCompletions(node, type, property);
            },
            getSignatureFromDeclaration: declarationIn => {
                const declaration = getParseTreeNode(declarationIn, isFunctionLike);
                return declaration ? getSignatureFromDeclaration(declaration) : undefined;
            },
            isImplementationOfOverload: node => {
                const parsed = getParseTreeNode(node, isFunctionLike);
                return parsed ? isImplementationOfOverload(parsed) : undefined;
            },
            getImmediateAliasedSymbol: symbol => {
                Debug.assert((symbol.flags & SymbolFlags.Alias) !== 0, "Should only get Alias here.");
                const links = getSymbolLinks(symbol);
                if (!links.immediateTarget) {
                    const node = getDeclarationOfAliasSymbol(symbol);
                    if (!node) return Debug.fail();
                    links.immediateTarget = getTargetOfAliasDeclaration(node, /*dontRecursivelyResolve*/ true);
                }

                return links.immediateTarget;
            },
            getAliasedSymbol: resolveAlias,
            getEmitResolver,
            getExportsOfModule: getExportsOfModuleAsArray,
            getExportsAndPropertiesOfModule,
            getSymbolWalker: createGetSymbolWalker(
                getRestTypeOfSignature,
                getTypePredicateOfSignature,
                getReturnTypeOfSignature,
                getBaseTypes,
                resolveStructuredTypeMembers,
                getTypeOfSymbol,
                getResolvedSymbol,
                getIndexTypeOfStructuredType,
                getConstraintOfTypeParameter,
                getFirstIdentifier,
            ),
            getAmbientModules,
            getAllAttributesTypeFromJsxOpeningLikeElement: nodeIn => {
                const node = getParseTreeNode(nodeIn, isJsxOpeningLikeElement);
                return node ? getAllAttributesTypeFromJsxOpeningLikeElement(node) : undefined;
            },
            getJsxIntrinsicTagNamesAt,
            isOptionalParameter: nodeIn => {
                const node = getParseTreeNode(nodeIn, isParameter);
                return node ? isOptionalParameter(node) : false;
            },
            tryGetMemberInModuleExports: (name, symbol) => tryGetMemberInModuleExports(escapeLeadingUnderscores(name), symbol),
            tryGetMemberInModuleExportsAndProperties: (name, symbol) => tryGetMemberInModuleExportsAndProperties(escapeLeadingUnderscores(name), symbol),
            tryFindAmbientModuleWithoutAugmentations: moduleName => {
                // we deliberately exclude augmentations
                // since we are only interested in declarations of the module itself
                return tryFindAmbientModule(moduleName, /*withAugmentations*/ false);
            },
            getApparentType,
            getUnionType,
            createAnonymousType,
            createSignature,
            createSymbol,
            createIndexInfo,
            getAnyType: () => anyType,
            getStringType: () => stringType,
            getNumberType: () => numberType,
            createPromiseType,
            createArrayType,
            getBooleanType: () => booleanType,
            getFalseType: () => falseType,
            getTrueType: () => trueType,
            getVoidType: () => voidType,
            getUndefinedType: () => undefinedType,
            getNullType: () => nullType,
            getESSymbolType: () => esSymbolType,
            getNeverType: () => neverType,
            isSymbolAccessible,
            isArrayLikeType,
            isTypeInvalidDueToUnionDiscriminant,
            getAllPossiblePropertiesOfTypes,
            getSuggestionForNonexistentProperty: (node, type) => getSuggestionForNonexistentProperty(node, type),
            getSuggestionForNonexistentSymbol: (location, name, meaning) => getSuggestionForNonexistentSymbol(location, escapeLeadingUnderscores(name), meaning),
            getSuggestionForNonexistentExport: (node, target) => getSuggestionForNonexistentExport(node, target),
            getBaseConstraintOfType,
            getDefaultFromTypeParameter: type => type && type.flags & TypeFlags.TypeParameter ? getDefaultFromTypeParameter(type as TypeParameter) : undefined,
            resolveName(name, location, meaning, excludeGlobals) {
                return resolveName(location, escapeLeadingUnderscores(name), meaning, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ false, excludeGlobals);
            },
            getJsxNamespace: n => unescapeLeadingUnderscores(getJsxNamespace(n)),
            getAccessibleSymbolChain,
            getTypePredicateOfSignature: getTypePredicateOfSignature as (signature: Signature) => TypePredicate, // TODO: GH#18217
            resolveExternalModuleSymbol,
            tryGetThisTypeAt: node => {
                node = getParseTreeNode(node);
                return node && tryGetThisTypeAt(node);
            },
            getTypeArgumentConstraint: nodeIn => {
                const node = getParseTreeNode(nodeIn, isTypeNode);
                return node && getTypeArgumentConstraint(node);
            },
            getSuggestionDiagnostics: (file, ct) => {
                if (skipTypeChecking(file, compilerOptions)) {
                    return emptyArray;
                }

                let diagnostics: DiagnosticWithLocation[] | undefined;
                try {
                    // Record the cancellation token so it can be checked later on during checkSourceElement.
                    // Do this in a finally block so we can ensure that it gets reset back to nothing after
                    // this call is done.
                    cancellationToken = ct;

                    // Ensure file is type checked
                    checkSourceFile(file);
                    Debug.assert(!!(getNodeLinks(file).flags & NodeCheckFlags.TypeChecked));

                    diagnostics = addRange(diagnostics, suggestionDiagnostics.get(file.fileName));
                    if (!file.isDeclarationFile && (!unusedIsError(UnusedKind.Local) || !unusedIsError(UnusedKind.Parameter))) {
                        addUnusedDiagnostics();
                    }
                    return diagnostics || emptyArray;
                }
                finally {
                    cancellationToken = undefined;
                }

                function addUnusedDiagnostics() {
                    checkUnusedIdentifiers(getPotentiallyUnusedIdentifiers(file), (containingNode, kind, diag) => {
                        if (!containsParseError(containingNode) && !unusedIsError(kind)) {
                            (diagnostics || (diagnostics = [])).push({ ...diag, category: DiagnosticCategory.Suggestion });
                        }
                    });
                }
            },

            runWithCancellationToken: (token, callback) => {
                try {
                    cancellationToken = token;
                    return callback(checker);
                }
                finally {
                    cancellationToken = undefined;
                }
            },

            getLocalTypeParametersOfClassOrInterfaceOrTypeAlias,
        };

        function getResolvedSignatureWorker(nodeIn: CallLikeExpression, candidatesOutArray: Signature[] | undefined, argumentCount: number | undefined, isForSignatureHelp: boolean): Signature | undefined {
            const node = getParseTreeNode(nodeIn, isCallLikeExpression);
            apparentArgumentCount = argumentCount;
            const res = node ? getResolvedSignature(node, candidatesOutArray, isForSignatureHelp) : undefined;
            apparentArgumentCount = undefined;
            return res;
        }

        const tupleTypes = createMap<GenericType>();
        const unionTypes = createMap<UnionType>();
        const intersectionTypes = createMap<IntersectionType>();
        const literalTypes = createMap<LiteralType>();
        const indexedAccessTypes = createMap<IndexedAccessType>();
        const evolvingArrayTypes: EvolvingArrayType[] = [];
        const undefinedProperties = createMap<Symbol>() as UnderscoreEscapedMap<Symbol>;

        const unknownSymbol = createSymbol(SymbolFlags.Property, "unknown" as __String);
        const resolvingSymbol = createSymbol(0, InternalSymbolName.Resolving);

        const anyType = createIntrinsicType(TypeFlags.Any, "any");
        const autoType = createIntrinsicType(TypeFlags.Any, "any");
        const wildcardType = createIntrinsicType(TypeFlags.Any, "any");
        const errorType = createIntrinsicType(TypeFlags.Any, "error");
        const unknownType = createIntrinsicType(TypeFlags.Unknown, "unknown");
        const undefinedType = createIntrinsicType(TypeFlags.Undefined, "undefined");
        const undefinedWideningType = strictNullChecks ? undefinedType : createIntrinsicType(TypeFlags.Undefined | TypeFlags.ContainsWideningType, "undefined");
        const nullType = createIntrinsicType(TypeFlags.Null, "null");
        const nullWideningType = strictNullChecks ? nullType : createIntrinsicType(TypeFlags.Null | TypeFlags.ContainsWideningType, "null");
        const stringType = createIntrinsicType(TypeFlags.String, "string");
        const numberType = createIntrinsicType(TypeFlags.Number, "number");
        const falseType = createIntrinsicType(TypeFlags.BooleanLiteral, "false");
        const trueType = createIntrinsicType(TypeFlags.BooleanLiteral, "true");
        const booleanType = createBooleanType([falseType, trueType]);
        const esSymbolType = createIntrinsicType(TypeFlags.ESSymbol, "symbol");
        const voidType = createIntrinsicType(TypeFlags.Void, "void");
        const neverType = createIntrinsicType(TypeFlags.Never, "never");
        const silentNeverType = createIntrinsicType(TypeFlags.Never, "never");
        const implicitNeverType = createIntrinsicType(TypeFlags.Never, "never");
        const nonPrimitiveType = createIntrinsicType(TypeFlags.NonPrimitive, "object");
        const stringNumberSymbolType = getUnionType([stringType, numberType, esSymbolType]);
        const keyofConstraintType = keyofStringsOnly ? stringType : stringNumberSymbolType;

        const emptyObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);

        const emptyTypeLiteralSymbol = createSymbol(SymbolFlags.TypeLiteral, InternalSymbolName.Type);
        emptyTypeLiteralSymbol.members = createSymbolTable();
        const emptyTypeLiteralType = createAnonymousType(emptyTypeLiteralSymbol, emptySymbols, emptyArray, emptyArray, undefined, undefined);

        const emptyGenericType = <GenericType><ObjectType>createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        emptyGenericType.instantiations = createMap<TypeReference>();

        const anyFunctionType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        // The anyFunctionType contains the anyFunctionType by definition. The flag is further propagated
        // in getPropagatingFlagsOfTypes, and it is checked in inferFromTypes.
        anyFunctionType.flags |= TypeFlags.ContainsAnyFunctionType;

        const noConstraintType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        const circularConstraintType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        const resolvingDefaultType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);

        const markerSuperType = <TypeParameter>createType(TypeFlags.TypeParameter);
        const markerSubType = <TypeParameter>createType(TypeFlags.TypeParameter);
        markerSubType.constraint = markerSuperType;
        const markerOtherType = <TypeParameter>createType(TypeFlags.TypeParameter);

        const noTypePredicate = createIdentifierTypePredicate("<<unresolved>>", 0, anyType);

        const anySignature = createSignature(undefined, undefined, undefined, emptyArray, anyType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
        const unknownSignature = createSignature(undefined, undefined, undefined, emptyArray, errorType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
        const resolvingSignature = createSignature(undefined, undefined, undefined, emptyArray, anyType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
        const silentNeverSignature = createSignature(undefined, undefined, undefined, emptyArray, silentNeverType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
        const resolvingSignaturesArray = [resolvingSignature];

        const enumNumberIndexInfo = createIndexInfo(stringType, /*isReadonly*/ true);

        const globals = createSymbolTable();
        let amalgamatedDuplicates: Map<{ firstFile: SourceFile, secondFile: SourceFile, firstFileInstances: Map<{ instances: Node[], blockScoped: boolean }>, secondFileInstances: Map<{ instances: Node[], blockScoped: boolean }> }> | undefined;
        const reverseMappedCache = createMap<Type | undefined>();
        let ambientModulesCache: Symbol[] | undefined;
        /**
         * List of every ambient module with a "*" wildcard.
         * Unlike other ambient modules, these can't be stored in `globals` because symbol tables only deal with exact matches.
         * This is only used if there is no exact match.
         */
        let patternAmbientModules: PatternAmbientModule[];

        let globalObjectType: ObjectType;
        let globalFunctionType: ObjectType;
        let globalArrayType: GenericType;
        let globalReadonlyArrayType: GenericType;
        let globalStringType: ObjectType;
        let globalNumberType: ObjectType;
        let globalBooleanType: ObjectType;
        let globalRegExpType: ObjectType;
        let globalThisType: GenericType;
        let anyArrayType: Type;
        let autoArrayType: Type;
        let anyReadonlyArrayType: Type;
        let deferredGlobalNonNullableTypeAlias: Symbol;

        // The library files are only loaded when the feature is used.
        // This allows users to just specify library files they want to used through --lib
        // and they will not get an error from not having unrelated library files
        let deferredGlobalESSymbolConstructorSymbol: Symbol | undefined;
        let deferredGlobalESSymbolType: ObjectType;
        let deferredGlobalTypedPropertyDescriptorType: GenericType;
        let deferredGlobalPromiseType: GenericType;
        let deferredGlobalPromiseConstructorSymbol: Symbol | undefined;
        let deferredGlobalPromiseConstructorLikeType: ObjectType;
        let deferredGlobalIterableType: GenericType;
        let deferredGlobalIteratorType: GenericType;
        let deferredGlobalIterableIteratorType: GenericType;
        let deferredGlobalAsyncIterableType: GenericType;
        let deferredGlobalAsyncIteratorType: GenericType;
        let deferredGlobalAsyncIterableIteratorType: GenericType;
        let deferredGlobalTemplateStringsArrayType: ObjectType;
        let deferredGlobalImportMetaType: ObjectType;
        let deferredGlobalExtractSymbol: Symbol;

        let deferredNodes: Map<Node> | undefined;
        const allPotentiallyUnusedIdentifiers = createMap<PotentiallyUnusedIdentifier[]>(); // key is file name

        let flowLoopStart = 0;
        let flowLoopCount = 0;
        let sharedFlowCount = 0;
        let flowAnalysisDisabled = false;

        const emptyStringType = getLiteralType("");
        const zeroType = getLiteralType(0);

        const resolutionTargets: TypeSystemEntity[] = [];
        const resolutionResults: boolean[] = [];
        const resolutionPropertyNames: TypeSystemPropertyName[] = [];

        let suggestionCount = 0;
        const maximumSuggestionCount = 10;
        const mergedSymbols: Symbol[] = [];
        const symbolLinks: SymbolLinks[] = [];
        const nodeLinks: NodeLinks[] = [];
        const flowLoopCaches: Map<Type>[] = [];
        const flowLoopNodes: FlowNode[] = [];
        const flowLoopKeys: string[] = [];
        const flowLoopTypes: Type[][] = [];
        const sharedFlowNodes: FlowNode[] = [];
        const sharedFlowTypes: FlowType[] = [];
        const potentialThisCollisions: Node[] = [];
        const potentialNewTargetCollisions: Node[] = [];
        const awaitedTypeStack: number[] = [];

        const diagnostics = createDiagnosticCollection();
        // Suggestion diagnostics must have a file. Keyed by source file name.
        const suggestionDiagnostics = createMultiMap<DiagnosticWithLocation>();

        const enum TypeFacts {
            None = 0,
            TypeofEQString = 1 << 0,      // typeof x === "string"
            TypeofEQNumber = 1 << 1,      // typeof x === "number"
            TypeofEQBoolean = 1 << 2,     // typeof x === "boolean"
            TypeofEQSymbol = 1 << 3,      // typeof x === "symbol"
            TypeofEQObject = 1 << 4,      // typeof x === "object"
            TypeofEQFunction = 1 << 5,    // typeof x === "function"
            TypeofEQHostObject = 1 << 6,  // typeof x === "xxx"
            TypeofNEString = 1 << 7,      // typeof x !== "string"
            TypeofNENumber = 1 << 8,      // typeof x !== "number"
            TypeofNEBoolean = 1 << 9,     // typeof x !== "boolean"
            TypeofNESymbol = 1 << 10,     // typeof x !== "symbol"
            TypeofNEObject = 1 << 11,     // typeof x !== "object"
            TypeofNEFunction = 1 << 12,   // typeof x !== "function"
            TypeofNEHostObject = 1 << 13, // typeof x !== "xxx"
            EQUndefined = 1 << 14,        // x === undefined
            EQNull = 1 << 15,             // x === null
            EQUndefinedOrNull = 1 << 16,  // x === undefined / x === null
            NEUndefined = 1 << 17,        // x !== undefined
            NENull = 1 << 18,             // x !== null
            NEUndefinedOrNull = 1 << 19,  // x != undefined / x != null
            Truthy = 1 << 20,             // x
            Falsy = 1 << 21,              // !x
            All = (1 << 22) - 1,
            // The following members encode facts about particular kinds of types for use in the getTypeFacts function.
            // The presence of a particular fact means that the given test is true for some (and possibly all) values
            // of that kind of type.
            BaseStringStrictFacts = TypeofEQString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull,
            BaseStringFacts = BaseStringStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            StringStrictFacts = BaseStringStrictFacts | Truthy | Falsy,
            StringFacts = BaseStringFacts | Truthy,
            EmptyStringStrictFacts = BaseStringStrictFacts | Falsy,
            EmptyStringFacts = BaseStringFacts,
            NonEmptyStringStrictFacts = BaseStringStrictFacts | Truthy,
            NonEmptyStringFacts = BaseStringFacts | Truthy,
            BaseNumberStrictFacts = TypeofEQNumber | TypeofNEString | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull,
            BaseNumberFacts = BaseNumberStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            NumberStrictFacts = BaseNumberStrictFacts | Truthy | Falsy,
            NumberFacts = BaseNumberFacts | Truthy,
            ZeroStrictFacts = BaseNumberStrictFacts | Falsy,
            ZeroFacts = BaseNumberFacts,
            NonZeroStrictFacts = BaseNumberStrictFacts | Truthy,
            NonZeroFacts = BaseNumberFacts | Truthy,
            BaseBooleanStrictFacts = TypeofEQBoolean | TypeofNEString | TypeofNENumber | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull,
            BaseBooleanFacts = BaseBooleanStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            BooleanStrictFacts = BaseBooleanStrictFacts | Truthy | Falsy,
            BooleanFacts = BaseBooleanFacts | Truthy,
            FalseStrictFacts = BaseBooleanStrictFacts | Falsy,
            FalseFacts = BaseBooleanFacts,
            TrueStrictFacts = BaseBooleanStrictFacts | Truthy,
            TrueFacts = BaseBooleanFacts | Truthy,
            SymbolStrictFacts = TypeofEQSymbol | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
            SymbolFacts = SymbolStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            ObjectStrictFacts = TypeofEQObject | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
            ObjectFacts = ObjectStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            FunctionStrictFacts = TypeofEQFunction | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
            FunctionFacts = FunctionStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            UndefinedFacts = TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | EQUndefined | EQUndefinedOrNull | NENull | Falsy,
            NullFacts = TypeofEQObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | TypeofNEHostObject | EQNull | EQUndefinedOrNull | NEUndefined | Falsy,
        }

        const typeofEQFacts = createMapFromTemplate({
            string: TypeFacts.TypeofEQString,
            number: TypeFacts.TypeofEQNumber,
            boolean: TypeFacts.TypeofEQBoolean,
            symbol: TypeFacts.TypeofEQSymbol,
            undefined: TypeFacts.EQUndefined,
            object: TypeFacts.TypeofEQObject,
            function: TypeFacts.TypeofEQFunction
        });
        const typeofNEFacts = createMapFromTemplate({
            string: TypeFacts.TypeofNEString,
            number: TypeFacts.TypeofNENumber,
            boolean: TypeFacts.TypeofNEBoolean,
            symbol: TypeFacts.TypeofNESymbol,
            undefined: TypeFacts.NEUndefined,
            object: TypeFacts.TypeofNEObject,
            function: TypeFacts.TypeofNEFunction
        });
        const typeofTypesByName = createMapFromTemplate<Type>({
            string: stringType,
            number: numberType,
            boolean: booleanType,
            symbol: esSymbolType,
            undefined: undefinedType
        });
        const typeofType = createTypeofType();

        let _jsxNamespace: __String;
        let _jsxFactoryEntity: EntityName | undefined;

        const subtypeRelation = createMap<RelationComparisonResult>();
        const assignableRelation = createMap<RelationComparisonResult>();
        const definitelyAssignableRelation = createMap<RelationComparisonResult>();
        const comparableRelation = createMap<RelationComparisonResult>();
        const identityRelation = createMap<RelationComparisonResult>();
        const enumRelation = createMap<RelationComparisonResult>();

        type TypeSystemEntity = Symbol | Type | Signature;

        const enum TypeSystemPropertyName {
            Type,
            ResolvedBaseConstructorType,
            DeclaredType,
            ResolvedReturnType,
            ImmediateBaseConstraint,
        }

        const enum CheckMode {
            Normal = 0,                // Normal type checking
            SkipContextSensitive = 1,  // Skip context sensitive function expressions
            Inferential = 2,           // Inferential typing
            Contextual = 3,            // Normal type checking informed by a contextual type, therefore not cacheable
        }

        const enum CallbackCheck {
            None,
            Bivariant,
            Strict,
        }

        const enum MappedTypeModifiers {
            IncludeReadonly = 1 << 0,
            ExcludeReadonly = 1 << 1,
            IncludeOptional = 1 << 2,
            ExcludeOptional = 1 << 3,
        }

        const enum ExpandingFlags {
            None = 0,
            Source = 1,
            Target = 1 << 1,
            Both = Source | Target,
        }

        const enum MembersOrExportsResolutionKind {
            resolvedExports = "resolvedExports",
            resolvedMembers = "resolvedMembers"
        }

        const enum UnusedKind {
            Local,
            Parameter,
        }
        /** @param containingNode Node to check for parse error */
        type AddUnusedDiagnostic = (containingNode: Node, type: UnusedKind, diagnostic: DiagnosticWithLocation) => void;

        const builtinGlobals = createSymbolTable();
        builtinGlobals.set(undefinedSymbol.escapedName, undefinedSymbol);

        const isNotOverloadAndNotAccessor = and(isNotOverload, isNotAccessor);

        initializeTypeChecker();

        return checker;

        function getJsxNamespace(location: Node | undefined): __String {
            if (location) {
                const file = getSourceFileOfNode(location);
                if (file) {
                    if (file.localJsxNamespace) {
                        return file.localJsxNamespace;
                    }
                    const jsxPragma = file.pragmas.get("jsx");
                    if (jsxPragma) {
                        const chosenpragma: any = isArray(jsxPragma) ? jsxPragma[0] : jsxPragma; // TODO: GH#18217
                        file.localJsxFactory = parseIsolatedEntityName(chosenpragma.arguments.factory, languageVersion);
                        if (file.localJsxFactory) {
                            return file.localJsxNamespace = getFirstIdentifier(file.localJsxFactory).escapedText;
                        }
                    }
                }
            }
            if (!_jsxNamespace) {
                _jsxNamespace = "React" as __String;
                if (compilerOptions.jsxFactory) {
                    _jsxFactoryEntity = parseIsolatedEntityName(compilerOptions.jsxFactory, languageVersion);
                    if (_jsxFactoryEntity) {
                        _jsxNamespace = getFirstIdentifier(_jsxFactoryEntity).escapedText;
                    }
                }
                else if (compilerOptions.reactNamespace) {
                    _jsxNamespace = escapeLeadingUnderscores(compilerOptions.reactNamespace);
                }
            }
            return _jsxNamespace;
        }

        function getEmitResolver(sourceFile: SourceFile, cancellationToken: CancellationToken) {
            // Ensure we have all the type information in place for this file so that all the
            // emitter questions of this resolver will return the right information.
            getDiagnostics(sourceFile, cancellationToken);
            return emitResolver;
        }

        function lookupOrIssueError(location: Node | undefined, message: DiagnosticMessage, arg0?: string | number, arg1?: string | number, arg2?: string | number, arg3?: string | number): Diagnostic {
            const diagnostic = location
                ? createDiagnosticForNode(location, message, arg0, arg1, arg2, arg3)
                : createCompilerDiagnostic(message, arg0, arg1, arg2, arg3);
            const existing = diagnostics.lookup(diagnostic);
            if (existing) {
                return existing;
            }
            else {
                diagnostics.add(diagnostic);
                return diagnostic;
            }
        }

        function addRelatedInfo(diagnostic: Diagnostic, ...relatedInformation: [DiagnosticRelatedInformation, ...DiagnosticRelatedInformation[]]) {
            if (!diagnostic.relatedInformation) {
                diagnostic.relatedInformation = [];
            }
            diagnostic.relatedInformation.push(...relatedInformation);
            return diagnostic;
        }

        function error(location: Node | undefined, message: DiagnosticMessage, arg0?: string | number, arg1?: string | number, arg2?: string | number, arg3?: string | number): Diagnostic {
            const diagnostic = location
                ? createDiagnosticForNode(location, message, arg0, arg1, arg2, arg3)
                : createCompilerDiagnostic(message, arg0, arg1, arg2, arg3);
            diagnostics.add(diagnostic);
            return diagnostic;
        }

        function addErrorOrSuggestion(isError: boolean, diagnostic: DiagnosticWithLocation) {
            if (isError) {
                diagnostics.add(diagnostic);
            }
            else {
                suggestionDiagnostics.add(diagnostic.file.fileName, { ...diagnostic, category: DiagnosticCategory.Suggestion });
            }
        }
        function errorOrSuggestion(isError: boolean, location: Node, message: DiagnosticMessage | DiagnosticMessageChain, arg0?: string | number, arg1?: string | number, arg2?: string | number, arg3?: string | number): void {
            addErrorOrSuggestion(isError, "message" in message ? createDiagnosticForNode(location, message, arg0, arg1, arg2, arg3) : createDiagnosticForNodeFromMessageChain(location, message));
        }

        function createSymbol(flags: SymbolFlags, name: __String, checkFlags?: CheckFlags) {
            symbolCount++;
            const symbol = <TransientSymbol>(new Symbol(flags | SymbolFlags.Transient, name));
            symbol.checkFlags = checkFlags || 0;
            return symbol;
        }

        function isTransientSymbol(symbol: Symbol): symbol is TransientSymbol {
            return (symbol.flags & SymbolFlags.Transient) !== 0;
        }

        function getExcludedSymbolFlags(flags: SymbolFlags): SymbolFlags {
            let result: SymbolFlags = 0;
            if (flags & SymbolFlags.BlockScopedVariable) result |= SymbolFlags.BlockScopedVariableExcludes;
            if (flags & SymbolFlags.FunctionScopedVariable) result |= SymbolFlags.FunctionScopedVariableExcludes;
            if (flags & SymbolFlags.Property) result |= SymbolFlags.PropertyExcludes;
            if (flags & SymbolFlags.EnumMember) result |= SymbolFlags.EnumMemberExcludes;
            if (flags & SymbolFlags.Function) result |= SymbolFlags.FunctionExcludes;
            if (flags & SymbolFlags.Class) result |= SymbolFlags.ClassExcludes;
            if (flags & SymbolFlags.Interface) result |= SymbolFlags.InterfaceExcludes;
            if (flags & SymbolFlags.RegularEnum) result |= SymbolFlags.RegularEnumExcludes;
            if (flags & SymbolFlags.ConstEnum) result |= SymbolFlags.ConstEnumExcludes;
            if (flags & SymbolFlags.ValueModule) result |= SymbolFlags.ValueModuleExcludes;
            if (flags & SymbolFlags.Method) result |= SymbolFlags.MethodExcludes;
            if (flags & SymbolFlags.GetAccessor) result |= SymbolFlags.GetAccessorExcludes;
            if (flags & SymbolFlags.SetAccessor) result |= SymbolFlags.SetAccessorExcludes;
            if (flags & SymbolFlags.TypeParameter) result |= SymbolFlags.TypeParameterExcludes;
            if (flags & SymbolFlags.TypeAlias) result |= SymbolFlags.TypeAliasExcludes;
            if (flags & SymbolFlags.Alias) result |= SymbolFlags.AliasExcludes;
            return result;
        }

        function recordMergedSymbol(target: Symbol, source: Symbol) {
            if (!source.mergeId) {
                source.mergeId = nextMergeId;
                nextMergeId++;
            }
            mergedSymbols[source.mergeId] = target;
        }

        function cloneSymbol(symbol: Symbol): Symbol {
            const result = createSymbol(symbol.flags, symbol.escapedName);
            result.declarations = symbol.declarations ? symbol.declarations.slice() : [];
            result.parent = symbol.parent;
            if (symbol.valueDeclaration) result.valueDeclaration = symbol.valueDeclaration;
            if (symbol.constEnumOnlyModule) result.constEnumOnlyModule = true;
            if (symbol.members) result.members = cloneMap(symbol.members);
            if (symbol.exports) result.exports = cloneMap(symbol.exports);
            recordMergedSymbol(result, symbol);
            return result;
        }

        /**
         * Note: if target is transient, then it is mutable, and mergeSymbol with both mutate and return it.
         * If target is not transient, mergeSymbol will produce a transient clone, mutate that and return it.
         */
        function mergeSymbol(target: Symbol, source: Symbol): Symbol {
            if (!(target.flags & getExcludedSymbolFlags(source.flags)) ||
                (source.flags | target.flags) & SymbolFlags.Assignment) {
                Debug.assert(source !== target);
                if (!(target.flags & SymbolFlags.Transient)) {
                    target = cloneSymbol(target);
                }
                // Javascript static-property-assignment declarations always merge, even though they are also values
                if (source.flags & SymbolFlags.ValueModule && target.flags & SymbolFlags.ValueModule && target.constEnumOnlyModule && !source.constEnumOnlyModule) {
                    // reset flag when merging instantiated module into value module that has only const enums
                    target.constEnumOnlyModule = false;
                }
                target.flags |= source.flags;
                if (source.valueDeclaration &&
                    (!target.valueDeclaration ||
                     isAssignmentDeclaration(target.valueDeclaration) ||
                     isEffectiveModuleDeclaration(target.valueDeclaration) && !isEffectiveModuleDeclaration(source.valueDeclaration))) {
                    // other kinds of value declarations take precedence over modules and assignment declarations
                    target.valueDeclaration = source.valueDeclaration;
                }
                addRange(target.declarations, source.declarations);
                if (source.members) {
                    if (!target.members) target.members = createSymbolTable();
                    mergeSymbolTable(target.members, source.members);
                }
                if (source.exports) {
                    if (!target.exports) target.exports = createSymbolTable();
                    mergeSymbolTable(target.exports, source.exports);
                }
                recordMergedSymbol(target, source);
            }
            else if (target.flags & SymbolFlags.NamespaceModule) {
                error(getNameOfDeclaration(source.declarations[0]), Diagnostics.Cannot_augment_module_0_with_value_exports_because_it_resolves_to_a_non_module_entity, symbolToString(target));
            }
            else {
                const isEitherEnum = !!(target.flags & SymbolFlags.Enum || source.flags & SymbolFlags.Enum);
                const isEitherBlockScoped = !!(target.flags & SymbolFlags.BlockScopedVariable || source.flags & SymbolFlags.BlockScopedVariable);
                const message = isEitherEnum
                    ? Diagnostics.Enum_declarations_can_only_merge_with_namespace_or_other_enum_declarations
                    : isEitherBlockScoped
                        ? Diagnostics.Cannot_redeclare_block_scoped_variable_0
                        : Diagnostics.Duplicate_identifier_0;
                const sourceSymbolFile = source.declarations && getSourceFileOfNode(source.declarations[0]);
                const targetSymbolFile = target.declarations && getSourceFileOfNode(target.declarations[0]);

                // Collect top-level duplicate identifier errors into one mapping, so we can then merge their diagnostics if there are a bunch
                if (sourceSymbolFile && targetSymbolFile && amalgamatedDuplicates && !isEitherEnum && sourceSymbolFile !== targetSymbolFile) {
                    const firstFile = comparePaths(sourceSymbolFile.path, targetSymbolFile.path) === Comparison.LessThan ? sourceSymbolFile : targetSymbolFile;
                    const secondFile = firstFile === sourceSymbolFile ? targetSymbolFile : sourceSymbolFile;
                    const cacheKey = `${firstFile.path}|${secondFile.path}`;
                    const existing = amalgamatedDuplicates.get(cacheKey) || { firstFile, secondFile, firstFileInstances: createMap(), secondFileInstances: createMap() };
                    const symbolName = symbolToString(source);
                    const firstInstanceList = existing.firstFileInstances.get(symbolName) || { instances: [], blockScoped: isEitherBlockScoped };
                    const secondInstanceList = existing.secondFileInstances.get(symbolName) || { instances: [], blockScoped: isEitherBlockScoped };

                    forEach(source.declarations, node => {
                        const errorNode = (getExpandoInitializer(node, /*isPrototypeAssignment*/ false) ? getNameOfExpando(node) : getNameOfDeclaration(node)) || node;
                        const targetList = sourceSymbolFile === firstFile ? firstInstanceList : secondInstanceList;
                        targetList.instances.push(errorNode);
                    });
                    forEach(target.declarations, node => {
                        const errorNode = (getExpandoInitializer(node, /*isPrototypeAssignment*/ false) ? getNameOfExpando(node) : getNameOfDeclaration(node)) || node;
                        const targetList = targetSymbolFile === firstFile ? firstInstanceList : secondInstanceList;
                        targetList.instances.push(errorNode);
                    });

                    existing.firstFileInstances.set(symbolName, firstInstanceList);
                    existing.secondFileInstances.set(symbolName, secondInstanceList);
                    amalgamatedDuplicates.set(cacheKey, existing);
                    return target;
                }
                const symbolName = symbolToString(source);
                addDuplicateDeclarationErrorsForSymbols(source, message, symbolName, target);
                addDuplicateDeclarationErrorsForSymbols(target, message, symbolName, source);
            }
            return target;
        }

        function addDuplicateDeclarationErrorsForSymbols(target: Symbol, message: DiagnosticMessage, symbolName: string, source: Symbol) {
            forEach(target.declarations, node => {
                const errorNode = (getExpandoInitializer(node, /*isPrototypeAssignment*/ false) ? getNameOfExpando(node) : getNameOfDeclaration(node)) || node;
                addDuplicateDeclarationError(errorNode, message, symbolName, source.declarations && source.declarations[0]);
            });
        }

        function addDuplicateDeclarationError(errorNode: Node, message: DiagnosticMessage, symbolName: string, relatedNode: Node | undefined) {
            const err = lookupOrIssueError(errorNode, message, symbolName);
            if (relatedNode && length(err.relatedInformation) < 5) {
                addRelatedInfo(err, !length(err.relatedInformation) ? createDiagnosticForNode(relatedNode, Diagnostics._0_was_also_declared_here, symbolName) : createDiagnosticForNode(relatedNode, Diagnostics.and_here));
            }
        }

        function combineSymbolTables(first: SymbolTable | undefined, second: SymbolTable | undefined): SymbolTable | undefined {
            if (!hasEntries(first)) return second;
            if (!hasEntries(second)) return first;
            const combined = createSymbolTable();
            mergeSymbolTable(combined, first);
            mergeSymbolTable(combined, second);
            return combined;
        }

        function mergeSymbolTable(target: SymbolTable, source: SymbolTable) {
            source.forEach((sourceSymbol, id) => {
                target.set(id, target.has(id) ? mergeSymbol(target.get(id)!, sourceSymbol) : sourceSymbol);
            });
        }

        function mergeModuleAugmentation(moduleName: StringLiteral | Identifier): void {
            const moduleAugmentation = <ModuleDeclaration>moduleName.parent;
            if (moduleAugmentation.symbol.declarations[0] !== moduleAugmentation) {
                // this is a combined symbol for multiple augmentations within the same file.
                // its symbol already has accumulated information for all declarations
                // so we need to add it just once - do the work only for first declaration
                Debug.assert(moduleAugmentation.symbol.declarations.length > 1);
                return;
            }

            if (isGlobalScopeAugmentation(moduleAugmentation)) {
                mergeSymbolTable(globals, moduleAugmentation.symbol.exports!);
            }
            else {
                // find a module that about to be augmented
                // do not validate names of augmentations that are defined in ambient context
                const moduleNotFoundError = !(moduleName.parent.parent.flags & NodeFlags.Ambient)
                    ? Diagnostics.Invalid_module_name_in_augmentation_module_0_cannot_be_found
                    : undefined;
                let mainModule = resolveExternalModuleNameWorker(moduleName, moduleName, moduleNotFoundError, /*isForAugmentation*/ true);
                if (!mainModule) {
                    return;
                }
                // obtain item referenced by 'export='
                mainModule = resolveExternalModuleSymbol(mainModule);
                if (mainModule.flags & SymbolFlags.Namespace) {
                    mainModule = mergeSymbol(mainModule, moduleAugmentation.symbol);
                }
                else {
                    // moduleName will be a StringLiteral since this is not `declare global`.
                    error(moduleName, Diagnostics.Cannot_augment_module_0_because_it_resolves_to_a_non_module_entity, (moduleName as StringLiteral).text);
                }
            }
        }

        function addToSymbolTable(target: SymbolTable, source: SymbolTable, message: DiagnosticMessage) {
            source.forEach((sourceSymbol, id) => {
                const targetSymbol = target.get(id);
                if (targetSymbol) {
                    // Error on redeclarations
                    forEach(targetSymbol.declarations, addDeclarationDiagnostic(unescapeLeadingUnderscores(id), message));
                }
                else {
                    target.set(id, sourceSymbol);
                }
            });

            function addDeclarationDiagnostic(id: string, message: DiagnosticMessage) {
                return (declaration: Declaration) => diagnostics.add(createDiagnosticForNode(declaration, message, id));
            }
        }

        function getSymbolLinks(symbol: Symbol): SymbolLinks {
            if (symbol.flags & SymbolFlags.Transient) return <TransientSymbol>symbol;
            const id = getSymbolId(symbol);
            return symbolLinks[id] || (symbolLinks[id] = {});
        }

        function getNodeLinks(node: Node): NodeLinks {
            const nodeId = getNodeId(node);
            return nodeLinks[nodeId] || (nodeLinks[nodeId] = { flags: 0 } as NodeLinks);
        }

        function isGlobalSourceFile(node: Node) {
            return node.kind === SyntaxKind.SourceFile && !isExternalOrCommonJsModule(<SourceFile>node);
        }

        function getSymbol(symbols: SymbolTable, name: __String, meaning: SymbolFlags): Symbol | undefined {
            if (meaning) {
                const symbol = symbols.get(name);
                if (symbol) {
                    Debug.assert((getCheckFlags(symbol) & CheckFlags.Instantiated) === 0, "Should never get an instantiated symbol here.");
                    if (symbol.flags & meaning) {
                        return symbol;
                    }
                    if (symbol.flags & SymbolFlags.Alias) {
                        const target = resolveAlias(symbol);
                        // Unknown symbol means an error occurred in alias resolution, treat it as positive answer to avoid cascading errors
                        if (target === unknownSymbol || target.flags & meaning) {
                            return symbol;
                        }
                    }
                }
            }
            // return undefined if we can't find a symbol.
        }

        /**
         * Get symbols that represent parameter-property-declaration as parameter and as property declaration
         * @param parameter a parameterDeclaration node
         * @param parameterName a name of the parameter to get the symbols for.
         * @return a tuple of two symbols
         */
        function getSymbolsOfParameterPropertyDeclaration(parameter: ParameterDeclaration, parameterName: __String): [Symbol, Symbol] {
            const constructorDeclaration = parameter.parent;
            const classDeclaration = parameter.parent.parent;

            const parameterSymbol = getSymbol(constructorDeclaration.locals!, parameterName, SymbolFlags.Value);
            const propertySymbol = getSymbol(getMembersOfSymbol(classDeclaration.symbol), parameterName, SymbolFlags.Value);

            if (parameterSymbol && propertySymbol) {
                return [parameterSymbol, propertySymbol];
            }

            return Debug.fail("There should exist two symbols, one as property declaration and one as parameter declaration");
        }

        function isBlockScopedNameDeclaredBeforeUse(declaration: Declaration, usage: Node): boolean {
            const declarationFile = getSourceFileOfNode(declaration);
            const useFile = getSourceFileOfNode(usage);
            if (declarationFile !== useFile) {
                if ((moduleKind && (declarationFile.externalModuleIndicator || useFile.externalModuleIndicator)) ||
                    (!compilerOptions.outFile && !compilerOptions.out) ||
                    isInTypeQuery(usage) ||
                    declaration.flags & NodeFlags.Ambient) {
                    // nodes are in different files and order cannot be determined
                    return true;
                }
                // declaration is after usage
                // can be legal if usage is deferred (i.e. inside function or in initializer of instance property)
                if (isUsedInFunctionOrInstanceProperty(usage, declaration)) {
                    return true;
                }
                const sourceFiles = host.getSourceFiles();
                return sourceFiles.indexOf(declarationFile) <= sourceFiles.indexOf(useFile);
            }

            if (declaration.pos <= usage.pos) {
                // declaration is before usage
                if (declaration.kind === SyntaxKind.BindingElement) {
                    // still might be illegal if declaration and usage are both binding elements (eg var [a = b, b = b] = [1, 2])
                    const errorBindingElement = getAncestor(usage, SyntaxKind.BindingElement) as BindingElement;
                    if (errorBindingElement) {
                        return findAncestor(errorBindingElement, isBindingElement) !== findAncestor(declaration, isBindingElement) ||
                            declaration.pos < errorBindingElement.pos;
                    }
                    // or it might be illegal if usage happens before parent variable is declared (eg var [a] = a)
                    return isBlockScopedNameDeclaredBeforeUse(getAncestor(declaration, SyntaxKind.VariableDeclaration) as Declaration, usage);
                }
                else if (declaration.kind === SyntaxKind.VariableDeclaration) {
                    // still might be illegal if usage is in the initializer of the variable declaration (eg var a = a)
                    return !isImmediatelyUsedInInitializerOfBlockScopedVariable(declaration as VariableDeclaration, usage);
                }
                else if (isClassDeclaration(declaration)) {
                    // still might be illegal if the usage is within a computed property name in the class (eg class A { static p = "a"; [A.p]() {} })
                    return !findAncestor(usage, n => isComputedPropertyName(n) && n.parent.parent === declaration);
                }
                return true;
            }


            // declaration is after usage, but it can still be legal if usage is deferred:
            // 1. inside an export specifier
            // 2. inside a function
            // 3. inside an instance property initializer, a reference to a non-instance property
            // 4. inside a static property initializer, a reference to a static method in the same class
            // 5. inside a TS export= declaration (since we will move the export statement during emit to avoid TDZ)
            // or if usage is in a type context:
            // 1. inside a type query (typeof in type position)
            // 2. inside a jsdoc comment
            if (usage.parent.kind === SyntaxKind.ExportSpecifier || (usage.parent.kind === SyntaxKind.ExportAssignment && (usage.parent as ExportAssignment).isExportEquals)) {
                // export specifiers do not use the variable, they only make it available for use
                return true;
            }
            // When resolving symbols for exports, the `usage` location passed in can be the export site directly
            if (usage.kind === SyntaxKind.ExportAssignment && (usage as ExportAssignment).isExportEquals) {
                return true;
            }

            const container = getEnclosingBlockScopeContainer(declaration);
            return !!(usage.flags & NodeFlags.JSDoc) || isInTypeQuery(usage) || isUsedInFunctionOrInstanceProperty(usage, declaration, container);

            function isImmediatelyUsedInInitializerOfBlockScopedVariable(declaration: VariableDeclaration, usage: Node): boolean {
                const container = getEnclosingBlockScopeContainer(declaration);

                switch (declaration.parent.parent.kind) {
                    case SyntaxKind.VariableStatement:
                    case SyntaxKind.ForStatement:
                    case SyntaxKind.ForOfStatement:
                        // variable statement/for/for-of statement case,
                        // use site should not be inside variable declaration (initializer of declaration or binding element)
                        if (isSameScopeDescendentOf(usage, declaration, container)) {
                            return true;
                        }
                        break;
                }

                // ForIn/ForOf case - use site should not be used in expression part
                const grandparent = declaration.parent.parent;
                return isForInOrOfStatement(grandparent) && isSameScopeDescendentOf(usage, grandparent.expression, container);
            }

            function isUsedInFunctionOrInstanceProperty(usage: Node, declaration: Node, container?: Node): boolean {
                return !!findAncestor(usage, current => {
                    if (current === container) {
                        return "quit";
                    }
                    if (isFunctionLike(current)) {
                        return true;
                    }

                    const initializerOfProperty = current.parent &&
                        current.parent.kind === SyntaxKind.PropertyDeclaration &&
                        (<PropertyDeclaration>current.parent).initializer === current;

                    if (initializerOfProperty) {
                        if (hasModifier(current.parent, ModifierFlags.Static)) {
                            if (declaration.kind === SyntaxKind.MethodDeclaration) {
                                return true;
                            }
                        }
                        else {
                            const isDeclarationInstanceProperty = declaration.kind === SyntaxKind.PropertyDeclaration && !hasModifier(declaration, ModifierFlags.Static);
                            if (!isDeclarationInstanceProperty || getContainingClass(usage) !== getContainingClass(declaration)) {
                                return true;
                            }
                        }
                    }
                    return false;
                });
            }
        }

        /**
         * Resolve a given name for a given meaning at a given location. An error is reported if the name was not found and
         * the nameNotFoundMessage argument is not undefined. Returns the resolved symbol, or undefined if no symbol with
         * the given name can be found.
         *
         * @param isUse If true, this will count towards --noUnusedLocals / --noUnusedParameters.
         */
        function resolveName(
            location: Node | undefined,
            name: __String,
            meaning: SymbolFlags,
            nameNotFoundMessage: DiagnosticMessage | undefined,
            nameArg: __String | Identifier | undefined,
            isUse: boolean,
            excludeGlobals = false,
            suggestedNameNotFoundMessage?: DiagnosticMessage): Symbol | undefined {
            return resolveNameHelper(location, name, meaning, nameNotFoundMessage, nameArg, isUse, excludeGlobals, getSymbol, suggestedNameNotFoundMessage);
        }

        function resolveNameHelper(
            location: Node | undefined,
            name: __String,
            meaning: SymbolFlags,
            nameNotFoundMessage: DiagnosticMessage | undefined,
            nameArg: __String | Identifier | undefined,
            isUse: boolean,
            excludeGlobals: boolean,
            lookup: typeof getSymbol,
            suggestedNameNotFoundMessage?: DiagnosticMessage): Symbol | undefined {
            const originalLocation = location; // needed for did-you-mean error reporting, which gathers candidates starting from the original location
            let result: Symbol | undefined;
            let lastLocation: Node | undefined;
            let lastSelfReferenceLocation: Node | undefined;
            let propertyWithInvalidInitializer: Node | undefined;
            const errorLocation = location;
            let grandparent: Node;
            let isInExternalModule = false;

            loop: while (location) {
                // Locals of a source file are not in scope (because they get merged into the global symbol table)
                if (location.locals && !isGlobalSourceFile(location)) {
                    if (result = lookup(location.locals, name, meaning)) {
                        let useResult = true;
                        if (isFunctionLike(location) && lastLocation && lastLocation !== (<FunctionLikeDeclaration>location).body) {
                            // symbol lookup restrictions for function-like declarations
                            // - Type parameters of a function are in scope in the entire function declaration, including the parameter
                            //   list and return type. However, local types are only in scope in the function body.
                            // - parameters are only in the scope of function body
                            // This restriction does not apply to JSDoc comment types because they are parented
                            // at a higher level than type parameters would normally be
                            if (meaning & result.flags & SymbolFlags.Type && lastLocation.kind !== SyntaxKind.JSDocComment) {
                                useResult = result.flags & SymbolFlags.TypeParameter
                                    // type parameters are visible in parameter list, return type and type parameter list
                                    ? lastLocation === (<FunctionLikeDeclaration>location).type ||
                                    lastLocation.kind === SyntaxKind.Parameter ||
                                    lastLocation.kind === SyntaxKind.TypeParameter
                                    // local types not visible outside the function body
                                    : false;
                            }
                            if (meaning & SymbolFlags.Value && result.flags & SymbolFlags.FunctionScopedVariable) {
                                // parameters are visible only inside function body, parameter list and return type
                                // technically for parameter list case here we might mix parameters and variables declared in function,
                                // however it is detected separately when checking initializers of parameters
                                // to make sure that they reference no variables declared after them.
                                useResult =
                                    lastLocation.kind === SyntaxKind.Parameter ||
                                    (
                                        lastLocation === (<FunctionLikeDeclaration>location).type &&
                                        !!findAncestor(result.valueDeclaration, isParameter)
                                    );
                            }
                        }
                        else if (location.kind === SyntaxKind.ConditionalType) {
                            // A type parameter declared using 'infer T' in a conditional type is visible only in
                            // the true branch of the conditional type.
                            useResult = lastLocation === (<ConditionalTypeNode>location).trueType;
                        }

                        if (useResult) {
                            break loop;
                        }
                        else {
                            result = undefined;
                        }
                    }
                }
                switch (location.kind) {
                    case SyntaxKind.SourceFile:
                        if (!isExternalOrCommonJsModule(<SourceFile>location)) break;
                        isInExternalModule = true;
                        // falls through
                    case SyntaxKind.ModuleDeclaration:
                        const moduleExports = getSymbolOfNode(location as SourceFile | ModuleDeclaration).exports!;
                        if (location.kind === SyntaxKind.SourceFile || isAmbientModule(location)) {

                            // It's an external module. First see if the module has an export default and if the local
                            // name of that export default matches.
                            if (result = moduleExports.get(InternalSymbolName.Default)) {
                                const localSymbol = getLocalSymbolForExportDefault(result);
                                if (localSymbol && (result.flags & meaning) && localSymbol.escapedName === name) {
                                    break loop;
                                }
                                result = undefined;
                            }

                            // Because of module/namespace merging, a module's exports are in scope,
                            // yet we never want to treat an export specifier as putting a member in scope.
                            // Therefore, if the name we find is purely an export specifier, it is not actually considered in scope.
                            // Two things to note about this:
                            //     1. We have to check this without calling getSymbol. The problem with calling getSymbol
                            //        on an export specifier is that it might find the export specifier itself, and try to
                            //        resolve it as an alias. This will cause the checker to consider the export specifier
                            //        a circular alias reference when it might not be.
                            //     2. We check === SymbolFlags.Alias in order to check that the symbol is *purely*
                            //        an alias. If we used &, we'd be throwing out symbols that have non alias aspects,
                            //        which is not the desired behavior.
                            const moduleExport = moduleExports.get(name);
                            if (moduleExport &&
                                moduleExport.flags === SymbolFlags.Alias &&
                                getDeclarationOfKind(moduleExport, SyntaxKind.ExportSpecifier)) {
                                break;
                            }
                        }

                        // ES6 exports are also visible locally (except for 'default'), but commonjs exports are not (except typedefs)
                        if (name !== InternalSymbolName.Default && (result = lookup(moduleExports, name, meaning & SymbolFlags.ModuleMember))) {
                            if (isSourceFile(location) && location.commonJsModuleIndicator && !result.declarations.some(isJSDocTypeAlias)) {
                                result = undefined;
                            }
                            else {
                                break loop;
                            }
                        }
                        break;
                    case SyntaxKind.EnumDeclaration:
                        if (result = lookup(getSymbolOfNode(location)!.exports!, name, meaning & SymbolFlags.EnumMember)) {
                            break loop;
                        }
                        break;
                    case SyntaxKind.PropertyDeclaration:
                    case SyntaxKind.PropertySignature:
                        // TypeScript 1.0 spec (April 2014): 8.4.1
                        // Initializer expressions for instance member variables are evaluated in the scope
                        // of the class constructor body but are not permitted to reference parameters or
                        // local variables of the constructor. This effectively means that entities from outer scopes
                        // by the same name as a constructor parameter or local variable are inaccessible
                        // in initializer expressions for instance member variables.
                        if (isClassLike(location.parent) && !hasModifier(location, ModifierFlags.Static)) {
                            const ctor = findConstructorDeclaration(location.parent);
                            if (ctor && ctor.locals) {
                                if (lookup(ctor.locals, name, meaning & SymbolFlags.Value)) {
                                    // Remember the property node, it will be used later to report appropriate error
                                    propertyWithInvalidInitializer = location;
                                }
                            }
                        }
                        break;
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.ClassExpression:
                    case SyntaxKind.InterfaceDeclaration:
                        if (result = lookup(getMembersOfSymbol(getSymbolOfNode(location as ClassLikeDeclaration | InterfaceDeclaration)), name, meaning & SymbolFlags.Type)) {
                            if (!isTypeParameterSymbolDeclaredInContainer(result, location)) {
                                // ignore type parameters not declared in this container
                                result = undefined;
                                break;
                            }
                            if (lastLocation && hasModifier(lastLocation, ModifierFlags.Static)) {
                                // TypeScript 1.0 spec (April 2014): 3.4.1
                                // The scope of a type parameter extends over the entire declaration with which the type
                                // parameter list is associated, with the exception of static member declarations in classes.
                                error(errorLocation, Diagnostics.Static_members_cannot_reference_class_type_parameters);
                                return undefined;
                            }
                            break loop;
                        }
                        if (location.kind === SyntaxKind.ClassExpression && meaning & SymbolFlags.Class) {
                            const className = (<ClassExpression>location).name;
                            if (className && name === className.escapedText) {
                                result = location.symbol;
                                break loop;
                            }
                        }
                        break;
                    case SyntaxKind.ExpressionWithTypeArguments:
                        // The type parameters of a class are not in scope in the base class expression.
                        if (lastLocation === (<ExpressionWithTypeArguments>location).expression && (<HeritageClause>location.parent).token === SyntaxKind.ExtendsKeyword) {
                            const container = location.parent.parent;
                            if (isClassLike(container) && (result = lookup(getSymbolOfNode(container).members!, name, meaning & SymbolFlags.Type))) {
                                if (nameNotFoundMessage) {
                                    error(errorLocation, Diagnostics.Base_class_expressions_cannot_reference_class_type_parameters);
                                }
                                return undefined;
                            }
                        }
                        break;
                    // It is not legal to reference a class's own type parameters from a computed property name that
                    // belongs to the class. For example:
                    //
                    //   function foo<T>() { return '' }
                    //   class C<T> { // <-- Class's own type parameter T
                    //       [foo<T>()]() { } // <-- Reference to T from class's own computed property
                    //   }
                    //
                    case SyntaxKind.ComputedPropertyName:
                        grandparent = location.parent.parent;
                        if (isClassLike(grandparent) || grandparent.kind === SyntaxKind.InterfaceDeclaration) {
                            // A reference to this grandparent's type parameters would be an error
                            if (result = lookup(getSymbolOfNode(grandparent as ClassLikeDeclaration | InterfaceDeclaration).members!, name, meaning & SymbolFlags.Type)) {
                                error(errorLocation, Diagnostics.A_computed_property_name_cannot_reference_a_type_parameter_from_its_containing_type);
                                return undefined;
                            }
                        }
                        break;
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                    case SyntaxKind.Constructor:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.ArrowFunction:
                        if (meaning & SymbolFlags.Variable && name === "arguments") {
                            result = argumentsSymbol;
                            break loop;
                        }
                        break;
                    case SyntaxKind.FunctionExpression:
                        if (meaning & SymbolFlags.Variable && name === "arguments") {
                            result = argumentsSymbol;
                            break loop;
                        }

                        if (meaning & SymbolFlags.Function) {
                            const functionName = (<FunctionExpression>location).name;
                            if (functionName && name === functionName.escapedText) {
                                result = location.symbol;
                                break loop;
                            }
                        }
                        break;
                    case SyntaxKind.Decorator:
                        // Decorators are resolved at the class declaration. Resolving at the parameter
                        // or member would result in looking up locals in the method.
                        //
                        //   function y() {}
                        //   class C {
                        //       method(@y x, y) {} // <-- decorator y should be resolved at the class declaration, not the parameter.
                        //   }
                        //
                        if (location.parent && location.parent.kind === SyntaxKind.Parameter) {
                            location = location.parent;
                        }
                        //
                        //   function y() {}
                        //   class C {
                        //       @y method(x, y) {} // <-- decorator y should be resolved at the class declaration, not the method.
                        //   }
                        //
                        if (location.parent && isClassElement(location.parent)) {
                            location = location.parent;
                        }
                        break;
                    case SyntaxKind.JSDocTypedefTag:
                    case SyntaxKind.JSDocCallbackTag:
                        // js type aliases do not resolve names from their host, so skip past it
                        location = getJSDocHost(location);
                        break;
                }
                if (isSelfReferenceLocation(location)) {
                    lastSelfReferenceLocation = location;
                }
                lastLocation = location;
                location = location.parent;
            }

            // We just climbed up parents looking for the name, meaning that we started in a descendant node of `lastLocation`.
            // If `result === lastSelfReferenceLocation.symbol`, that means that we are somewhere inside `lastSelfReferenceLocation` looking up a name, and resolving to `lastLocation` itself.
            // That means that this is a self-reference of `lastLocation`, and shouldn't count this when considering whether `lastLocation` is used.
            if (isUse && result && (!lastSelfReferenceLocation || result !== lastSelfReferenceLocation.symbol)) {
                result.isReferenced! |= meaning;
            }

            if (!result) {
                if (lastLocation) {
                    Debug.assert(lastLocation.kind === SyntaxKind.SourceFile);
                    if ((lastLocation as SourceFile).commonJsModuleIndicator && name === "exports") {
                        return lastLocation.symbol;
                    }
                }

                if (!excludeGlobals) {
                    result = lookup(globals, name, meaning);
                }
            }
            if (!result) {
                if (originalLocation && isInJSFile(originalLocation) && originalLocation.parent) {
                    if (isRequireCall(originalLocation.parent, /*checkArgumentIsStringLiteralLike*/ false)) {
                        return requireSymbol;
                    }
                }
            }
            if (!result) {
                if (nameNotFoundMessage) {
                    if (!errorLocation ||
                        !checkAndReportErrorForMissingPrefix(errorLocation, name, nameArg!) && // TODO: GH#18217
                        !checkAndReportErrorForExtendingInterface(errorLocation) &&
                        !checkAndReportErrorForUsingTypeAsNamespace(errorLocation, name, meaning) &&
                        !checkAndReportErrorForUsingTypeAsValue(errorLocation, name, meaning) &&
                        !checkAndReportErrorForUsingNamespaceModuleAsValue(errorLocation, name, meaning)) {
                        let suggestion: Symbol | undefined;
                        if (suggestedNameNotFoundMessage && suggestionCount < maximumSuggestionCount) {
                            suggestion = getSuggestedSymbolForNonexistentSymbol(originalLocation, name, meaning);
                            if (suggestion) {
                                const suggestionName = symbolToString(suggestion);
                                const diagnostic = error(errorLocation, suggestedNameNotFoundMessage, diagnosticName(nameArg!), suggestionName);
                                if (suggestion.valueDeclaration) {
                                    addRelatedInfo(
                                        diagnostic,
                                        createDiagnosticForNode(suggestion.valueDeclaration, Diagnostics._0_is_declared_here, suggestionName)
                                    );
                                }
                            }
                        }
                        if (!suggestion) {
                            error(errorLocation, nameNotFoundMessage, diagnosticName(nameArg!));
                        }
                        suggestionCount++;
                    }
                }
                return undefined;
            }

            // Perform extra checks only if error reporting was requested
            if (nameNotFoundMessage) {
                if (propertyWithInvalidInitializer) {
                    // We have a match, but the reference occurred within a property initializer and the identifier also binds
                    // to a local variable in the constructor where the code will be emitted.
                    const propertyName = (<PropertyDeclaration>propertyWithInvalidInitializer).name;
                    error(errorLocation, Diagnostics.Initializer_of_instance_member_variable_0_cannot_reference_identifier_1_declared_in_the_constructor,
                        declarationNameToString(propertyName), diagnosticName(nameArg!));
                    return undefined;
                }

                // Only check for block-scoped variable if we have an error location and are looking for the
                // name with variable meaning
                //      For example,
                //          declare module foo {
                //              interface bar {}
                //          }
                //      const foo/*1*/: foo/*2*/.bar;
                // The foo at /*1*/ and /*2*/ will share same symbol with two meanings:
                // block-scoped variable and namespace module. However, only when we
                // try to resolve name in /*1*/ which is used in variable position,
                // we want to check for block-scoped
                if (errorLocation &&
                    (meaning & SymbolFlags.BlockScopedVariable ||
                     ((meaning & SymbolFlags.Class || meaning & SymbolFlags.Enum) && (meaning & SymbolFlags.Value) === SymbolFlags.Value))) {
                    const exportOrLocalSymbol = getExportSymbolOfValueSymbolIfExported(result);
                    if (exportOrLocalSymbol.flags & SymbolFlags.BlockScopedVariable || exportOrLocalSymbol.flags & SymbolFlags.Class || exportOrLocalSymbol.flags & SymbolFlags.Enum) {
                        checkResolvedBlockScopedVariable(exportOrLocalSymbol, errorLocation);
                    }
                }

                // If we're in an external module, we can't reference value symbols created from UMD export declarations
                if (result && isInExternalModule && (meaning & SymbolFlags.Value) === SymbolFlags.Value && !(originalLocation!.flags & NodeFlags.JSDoc)) {
                    const decls = result.declarations;
                    if (decls && decls.length === 1 && decls[0].kind === SyntaxKind.NamespaceExportDeclaration) {
                        error(errorLocation!, Diagnostics._0_refers_to_a_UMD_global_but_the_current_file_is_a_module_Consider_adding_an_import_instead, unescapeLeadingUnderscores(name)); // TODO: GH#18217
                    }
                }
            }
            return result;
        }

        function isSelfReferenceLocation(node: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.InterfaceDeclaration:
                case SyntaxKind.EnumDeclaration:
                case SyntaxKind.TypeAliasDeclaration:
                case SyntaxKind.ModuleDeclaration: // For `namespace N { N; }`
                    return true;
                default:
                    return false;
            }
        }

        function diagnosticName(nameArg: __String | Identifier) {
            return isString(nameArg) ? unescapeLeadingUnderscores(nameArg as __String) : declarationNameToString(nameArg as Identifier);
        }

        function isTypeParameterSymbolDeclaredInContainer(symbol: Symbol, container: Node) {
            for (const decl of symbol.declarations) {
                if (decl.kind === SyntaxKind.TypeParameter) {
                    const parent = isJSDocTemplateTag(decl.parent) ? getJSDocHost(decl.parent) : decl.parent;
                    if (parent === container) {
                        return !(isJSDocTemplateTag(decl.parent) && find((decl.parent.parent as JSDoc).tags!, isJSDocTypeAlias)); // TODO: GH#18217
                    }
                }
            }

            return false;
        }

        function checkAndReportErrorForMissingPrefix(errorLocation: Node, name: __String, nameArg: __String | Identifier): boolean {
            if (!isIdentifier(errorLocation) || errorLocation.escapedText !== name || isTypeReferenceIdentifier(errorLocation) || isInTypeQuery(errorLocation)) {
                return false;
            }

            const container = getThisContainer(errorLocation, /*includeArrowFunctions*/ false);
            let location = container;
            while (location) {
                if (isClassLike(location.parent)) {
                    const classSymbol = getSymbolOfNode(location.parent);
                    if (!classSymbol) {
                        break;
                    }

                    // Check to see if a static member exists.
                    const constructorType = getTypeOfSymbol(classSymbol);
                    if (getPropertyOfType(constructorType, name)) {
                        error(errorLocation, Diagnostics.Cannot_find_name_0_Did_you_mean_the_static_member_1_0, diagnosticName(nameArg), symbolToString(classSymbol));
                        return true;
                    }

                    // No static member is present.
                    // Check if we're in an instance method and look for a relevant instance member.
                    if (location === container && !hasModifier(location, ModifierFlags.Static)) {
                        const instanceType = (<InterfaceType>getDeclaredTypeOfSymbol(classSymbol)).thisType!; // TODO: GH#18217
                        if (getPropertyOfType(instanceType, name)) {
                            error(errorLocation, Diagnostics.Cannot_find_name_0_Did_you_mean_the_instance_member_this_0, diagnosticName(nameArg));
                            return true;
                        }
                    }
                }

                location = location.parent;
            }
            return false;
        }


        function checkAndReportErrorForExtendingInterface(errorLocation: Node): boolean {
            const expression = getEntityNameForExtendingInterface(errorLocation);
            if (expression && resolveEntityName(expression, SymbolFlags.Interface, /*ignoreErrors*/ true)) {
                error(errorLocation, Diagnostics.Cannot_extend_an_interface_0_Did_you_mean_implements, getTextOfNode(expression));
                return true;
            }
            return false;
        }
        /**
         * Climbs up parents to an ExpressionWithTypeArguments, and returns its expression,
         * but returns undefined if that expression is not an EntityNameExpression.
         */
        function getEntityNameForExtendingInterface(node: Node): EntityNameExpression | undefined {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                case SyntaxKind.PropertyAccessExpression:
                    return node.parent ? getEntityNameForExtendingInterface(node.parent) : undefined;
                case SyntaxKind.ExpressionWithTypeArguments:
                    if (isEntityNameExpression((<ExpressionWithTypeArguments>node).expression)) {
                        return <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression;
                    }
                    // falls through
                default:
                    return undefined;
            }
        }

        function checkAndReportErrorForUsingTypeAsNamespace(errorLocation: Node, name: __String, meaning: SymbolFlags): boolean {
            const namespaceMeaning = SymbolFlags.Namespace | (isInJSFile(errorLocation) ? SymbolFlags.Value : 0);
            if (meaning === namespaceMeaning) {
                const symbol = resolveSymbol(resolveName(errorLocation, name, SymbolFlags.Type & ~namespaceMeaning, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined, /*isUse*/ false));
                const parent = errorLocation.parent;
                if (symbol) {
                    if (isQualifiedName(parent)) {
                        Debug.assert(parent.left === errorLocation, "Should only be resolving left side of qualified name as a namespace");
                        const propName = parent.right.escapedText;
                        const propType = getPropertyOfType(getDeclaredTypeOfSymbol(symbol), propName);
                        if (propType) {
                            error(
                                parent,
                                Diagnostics.Cannot_access_0_1_because_0_is_a_type_but_not_a_namespace_Did_you_mean_to_retrieve_the_type_of_the_property_1_in_0_with_0_1,
                                unescapeLeadingUnderscores(name),
                                unescapeLeadingUnderscores(propName),
                            );
                            return true;
                        }
                    }
                    error(errorLocation, Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_namespace_here, unescapeLeadingUnderscores(name));
                    return true;
                }
            }

            return false;
        }

        function checkAndReportErrorForUsingTypeAsValue(errorLocation: Node, name: __String, meaning: SymbolFlags): boolean {
            if (meaning & (SymbolFlags.Value & ~SymbolFlags.NamespaceModule)) {
                if (name === "any" || name === "string" || name === "number" || name === "boolean" || name === "never") {
                    error(errorLocation, Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here, unescapeLeadingUnderscores(name));
                    return true;
                }
                const symbol = resolveSymbol(resolveName(errorLocation, name, SymbolFlags.Type & ~SymbolFlags.Value, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined, /*isUse*/ false));
                if (symbol && !(symbol.flags & SymbolFlags.NamespaceModule)) {
                    const message = (name === "Promise" || name === "Symbol")
                        ? Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here_Do_you_need_to_change_your_target_library_Try_changing_the_lib_compiler_option_to_es2015_or_later
                        : Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here;
                    error(errorLocation, message, unescapeLeadingUnderscores(name));
                    return true;
                }
            }
            return false;
        }

        function checkAndReportErrorForUsingNamespaceModuleAsValue(errorLocation: Node, name: __String, meaning: SymbolFlags): boolean {
            if (meaning & (SymbolFlags.Value & ~SymbolFlags.NamespaceModule & ~SymbolFlags.Type)) {
                const symbol = resolveSymbol(resolveName(errorLocation, name, SymbolFlags.NamespaceModule & ~SymbolFlags.Value, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined, /*isUse*/ false));
                if (symbol) {
                    error(errorLocation, Diagnostics.Cannot_use_namespace_0_as_a_value, unescapeLeadingUnderscores(name));
                    return true;
                }
            }
            else if (meaning & (SymbolFlags.Type & ~SymbolFlags.NamespaceModule & ~SymbolFlags.Value)) {
                const symbol = resolveSymbol(resolveName(errorLocation, name, (SymbolFlags.ValueModule | SymbolFlags.NamespaceModule) & ~SymbolFlags.Type, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined, /*isUse*/ false));
                if (symbol) {
                    error(errorLocation, Diagnostics.Cannot_use_namespace_0_as_a_type, unescapeLeadingUnderscores(name));
                    return true;
                }
            }
            return false;
        }

        function checkResolvedBlockScopedVariable(result: Symbol, errorLocation: Node): void {
            Debug.assert(!!(result.flags & SymbolFlags.BlockScopedVariable || result.flags & SymbolFlags.Class || result.flags & SymbolFlags.Enum));
            // Block-scoped variables cannot be used before their definition
            const declaration = forEach(result.declarations, d => isBlockOrCatchScoped(d) || isClassLike(d) || (d.kind === SyntaxKind.EnumDeclaration) ? d : undefined);

            if (declaration === undefined) return Debug.fail("Declaration to checkResolvedBlockScopedVariable is undefined");

            if (!(declaration.flags & NodeFlags.Ambient) && !isBlockScopedNameDeclaredBeforeUse(declaration, errorLocation)) {
                let diagnosticMessage;
                const declarationName = declarationNameToString(getNameOfDeclaration(declaration));
                if (result.flags & SymbolFlags.BlockScopedVariable) {
                    diagnosticMessage = error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, declarationName);
                }
                else if (result.flags & SymbolFlags.Class) {
                    diagnosticMessage = error(errorLocation, Diagnostics.Class_0_used_before_its_declaration, declarationName);
                }
                else if (result.flags & SymbolFlags.RegularEnum) {
                    diagnosticMessage = error(errorLocation, Diagnostics.Enum_0_used_before_its_declaration, declarationName);
                }
                else {
                    Debug.assert(!!(result.flags & SymbolFlags.ConstEnum));
                    if (compilerOptions.preserveConstEnums) {
                        diagnosticMessage = error(errorLocation, Diagnostics.Class_0_used_before_its_declaration, declarationName);
                    }
                }

                if (diagnosticMessage) {
                    addRelatedInfo(diagnosticMessage,
                        createDiagnosticForNode(declaration, Diagnostics._0_is_declared_here, declarationName)
                    );
                }
            }
        }

        /* Starting from 'initial' node walk up the parent chain until 'stopAt' node is reached.
         * If at any point current node is equal to 'parent' node - return true.
         * Return false if 'stopAt' node is reached or isFunctionLike(current) === true.
         */
        function isSameScopeDescendentOf(initial: Node, parent: Node | undefined, stopAt: Node): boolean {
            return !!parent && !!findAncestor(initial, n => n === stopAt || isFunctionLike(n) ? "quit" : n === parent);
        }

        function getAnyImportSyntax(node: Node): AnyImportSyntax | undefined {
            switch (node.kind) {
                case SyntaxKind.ImportEqualsDeclaration:
                    return node as ImportEqualsDeclaration;
                case SyntaxKind.ImportClause:
                    return (node as ImportClause).parent;
                case SyntaxKind.NamespaceImport:
                    return (node as NamespaceImport).parent.parent;
                case SyntaxKind.ImportSpecifier:
                    return (node as ImportSpecifier).parent.parent.parent;
                default:
                    return undefined;
            }
        }

        function getDeclarationOfAliasSymbol(symbol: Symbol): Declaration | undefined {
            return find<Declaration>(symbol.declarations, isAliasSymbolDeclaration);
        }

        function getTargetOfImportEqualsDeclaration(node: ImportEqualsDeclaration, dontResolveAlias: boolean): Symbol | undefined {
            if (node.moduleReference.kind === SyntaxKind.ExternalModuleReference) {
                return resolveExternalModuleSymbol(resolveExternalModuleName(node, getExternalModuleImportEqualsDeclarationExpression(node)));
            }
            return getSymbolOfPartOfRightHandSideOfImportEquals(node.moduleReference, dontResolveAlias);
        }

        function resolveExportByName(moduleSymbol: Symbol, name: __String, dontResolveAlias: boolean) {
            const exportValue = moduleSymbol.exports!.get(InternalSymbolName.ExportEquals);
            return exportValue
                ? getPropertyOfType(getTypeOfSymbol(exportValue), name)
                : resolveSymbol(moduleSymbol.exports!.get(name), dontResolveAlias);
        }

        function isSyntacticDefault(node: Node) {
            return ((isExportAssignment(node) && !node.isExportEquals) || hasModifier(node, ModifierFlags.Default) || isExportSpecifier(node));
        }

        function canHaveSyntheticDefault(file: SourceFile | undefined, moduleSymbol: Symbol, dontResolveAlias: boolean) {
            if (!allowSyntheticDefaultImports) {
                return false;
            }
            // Declaration files (and ambient modules)
            if (!file || file.isDeclarationFile) {
                // Definitely cannot have a synthetic default if they have a syntactic default member specified
                const defaultExportSymbol = resolveExportByName(moduleSymbol, InternalSymbolName.Default, /*dontResolveAlias*/ true); // Dont resolve alias because we want the immediately exported symbol's declaration
                if (defaultExportSymbol && some(defaultExportSymbol.declarations, isSyntacticDefault)) {
                    return false;
                }
                // It _might_ still be incorrect to assume there is no __esModule marker on the import at runtime, even if there is no `default` member
                // So we check a bit more,
                if (resolveExportByName(moduleSymbol, escapeLeadingUnderscores("__esModule"), dontResolveAlias)) {
                    // If there is an `__esModule` specified in the declaration (meaning someone explicitly added it or wrote it in their code),
                    // it definitely is a module and does not have a synthetic default
                    return false;
                }
                // There are _many_ declaration files not written with esmodules in mind that still get compiled into a format with __esModule set
                // Meaning there may be no default at runtime - however to be on the permissive side, we allow access to a synthetic default member
                // as there is no marker to indicate if the accompanying JS has `__esModule` or not, or is even native esm
                return true;
            }
            // TypeScript files never have a synthetic default (as they are always emitted with an __esModule marker) _unless_ they contain an export= statement
            if (!isSourceFileJS(file)) {
                return hasExportAssignmentSymbol(moduleSymbol);
            }
            // JS files have a synthetic default if they do not contain ES2015+ module syntax (export = is not valid in js) _and_ do not have an __esModule marker
            return !file.externalModuleIndicator && !resolveExportByName(moduleSymbol, escapeLeadingUnderscores("__esModule"), dontResolveAlias);
        }

        function getTargetOfImportClause(node: ImportClause, dontResolveAlias: boolean): Symbol | undefined {
            const moduleSymbol = resolveExternalModuleName(node, node.parent.moduleSpecifier);

            if (moduleSymbol) {
                let exportDefaultSymbol: Symbol | undefined;
                if (isShorthandAmbientModuleSymbol(moduleSymbol)) {
                    exportDefaultSymbol = moduleSymbol;
                }
                else {
                    exportDefaultSymbol = resolveExportByName(moduleSymbol, InternalSymbolName.Default, dontResolveAlias);
                }

                const file = find(moduleSymbol.declarations, isSourceFile);
                const hasSyntheticDefault = canHaveSyntheticDefault(file, moduleSymbol, dontResolveAlias);
                if (!exportDefaultSymbol && !hasSyntheticDefault) {
                    error(node.name, Diagnostics.Module_0_has_no_default_export, symbolToString(moduleSymbol));
                }
                else if (hasSyntheticDefault) {
                    // per emit behavior, a synthetic default overrides a "real" .default member if `__esModule` is not present
                    return resolveExternalModuleSymbol(moduleSymbol, dontResolveAlias) || resolveSymbol(moduleSymbol, dontResolveAlias);
                }
                return exportDefaultSymbol;
            }
        }

        function getTargetOfNamespaceImport(node: NamespaceImport, dontResolveAlias: boolean): Symbol | undefined {
            const moduleSpecifier = node.parent.parent.moduleSpecifier;
            return resolveESModuleSymbol(resolveExternalModuleName(node, moduleSpecifier), moduleSpecifier, dontResolveAlias);
        }

        // This function creates a synthetic symbol that combines the value side of one symbol with the
        // type/namespace side of another symbol. Consider this example:
        //
        //   declare module graphics {
        //       interface Point {
        //           x: number;
        //           y: number;
        //       }
        //   }
        //   declare var graphics: {
        //       Point: new (x: number, y: number) => graphics.Point;
        //   }
        //   declare module "graphics" {
        //       export = graphics;
        //   }
        //
        // An 'import { Point } from "graphics"' needs to create a symbol that combines the value side 'Point'
        // property with the type/namespace side interface 'Point'.
        function combineValueAndTypeSymbols(valueSymbol: Symbol, typeSymbol: Symbol): Symbol {
            if (valueSymbol === unknownSymbol && typeSymbol === unknownSymbol) {
                return unknownSymbol;
            }
            if (valueSymbol.flags & (SymbolFlags.Type | SymbolFlags.Namespace)) {
                return valueSymbol;
            }
            const result = createSymbol(valueSymbol.flags | typeSymbol.flags, valueSymbol.escapedName);
            result.declarations = deduplicate(concatenate(valueSymbol.declarations, typeSymbol.declarations), equateValues);
            result.parent = valueSymbol.parent || typeSymbol.parent;
            if (valueSymbol.valueDeclaration) result.valueDeclaration = valueSymbol.valueDeclaration;
            if (typeSymbol.members) result.members = typeSymbol.members;
            if (valueSymbol.exports) result.exports = valueSymbol.exports;
            return result;
        }

        function getExportOfModule(symbol: Symbol, name: __String, dontResolveAlias: boolean): Symbol | undefined {
            if (symbol.flags & SymbolFlags.Module) {
                return resolveSymbol(getExportsOfSymbol(symbol).get(name)!, dontResolveAlias);
            }
        }

        function getPropertyOfVariable(symbol: Symbol, name: __String): Symbol | undefined {
            if (symbol.flags & SymbolFlags.Variable) {
                const typeAnnotation = (<VariableDeclaration>symbol.valueDeclaration).type;
                if (typeAnnotation) {
                    return resolveSymbol(getPropertyOfType(getTypeFromTypeNode(typeAnnotation), name));
                }
            }
        }

        function getExternalModuleMember(node: ImportDeclaration | ExportDeclaration, specifier: ImportOrExportSpecifier, dontResolveAlias = false): Symbol | undefined {
            const moduleSymbol = resolveExternalModuleName(node, node.moduleSpecifier!)!; // TODO: GH#18217
            const targetSymbol = resolveESModuleSymbol(moduleSymbol, node.moduleSpecifier!, dontResolveAlias);
            if (targetSymbol) {
                const name = specifier.propertyName || specifier.name;
                if (name.escapedText) {
                    if (isShorthandAmbientModuleSymbol(moduleSymbol)) {
                        return moduleSymbol;
                    }

                    let symbolFromVariable: Symbol | undefined;
                    // First check if module was specified with "export=". If so, get the member from the resolved type
                    if (moduleSymbol && moduleSymbol.exports && moduleSymbol.exports.get(InternalSymbolName.ExportEquals)) {
                        symbolFromVariable = getPropertyOfType(getTypeOfSymbol(targetSymbol), name.escapedText);
                    }
                    else {
                        symbolFromVariable = getPropertyOfVariable(targetSymbol, name.escapedText);
                    }
                    // if symbolFromVariable is export - get its final target
                    symbolFromVariable = resolveSymbol(symbolFromVariable, dontResolveAlias);
                    let symbolFromModule = getExportOfModule(targetSymbol, name.escapedText, dontResolveAlias);
                    // If the export member we're looking for is default, and there is no real default but allowSyntheticDefaultImports is on, return the entire module as the default
                    if (!symbolFromModule && allowSyntheticDefaultImports && name.escapedText === InternalSymbolName.Default) {
                        symbolFromModule = resolveExternalModuleSymbol(moduleSymbol, dontResolveAlias) || resolveSymbol(moduleSymbol, dontResolveAlias);
                    }
                    const symbol = symbolFromModule && symbolFromVariable && symbolFromModule !== symbolFromVariable ?
                        combineValueAndTypeSymbols(symbolFromVariable, symbolFromModule) :
                        symbolFromModule || symbolFromVariable;
                    if (!symbol) {
                        const moduleName = getFullyQualifiedName(moduleSymbol);
                        const declarationName = declarationNameToString(name);
                        const suggestion = getSuggestedSymbolForNonexistentModule(name, targetSymbol);
                        if (suggestion !== undefined) {
                            const suggestionName = symbolToString(suggestion);
                            const diagnostic = error(name, Diagnostics.Module_0_has_no_exported_member_1_Did_you_mean_2, moduleName, declarationName, suggestionName);
                            if (suggestion.valueDeclaration) {
                                addRelatedInfo(diagnostic,
                                    createDiagnosticForNode(suggestion.valueDeclaration, Diagnostics._0_is_declared_here, suggestionName)
                                );
                            }
                        }
                        else {
                            error(name, Diagnostics.Module_0_has_no_exported_member_1, moduleName, declarationName);
                        }
                    }
                    return symbol;
                }
            }
        }

        function getTargetOfImportSpecifier(node: ImportSpecifier, dontResolveAlias: boolean): Symbol | undefined {
            return getExternalModuleMember(node.parent.parent.parent, node, dontResolveAlias);
        }

        function getTargetOfNamespaceExportDeclaration(node: NamespaceExportDeclaration, dontResolveAlias: boolean): Symbol {
            return resolveExternalModuleSymbol(node.parent.symbol, dontResolveAlias);
        }

        function getTargetOfExportSpecifier(node: ExportSpecifier, meaning: SymbolFlags, dontResolveAlias?: boolean) {
            return node.parent.parent.moduleSpecifier ?
                getExternalModuleMember(node.parent.parent, node, dontResolveAlias) :
                resolveEntityName(node.propertyName || node.name, meaning, /*ignoreErrors*/ false, dontResolveAlias);
        }

        function getTargetOfExportAssignment(node: ExportAssignment | BinaryExpression, dontResolveAlias: boolean): Symbol | undefined {
            const expression = (isExportAssignment(node) ? node.expression : node.right) as EntityNameExpression | ClassExpression;
            if (isClassExpression(expression)) {
                return checkExpression(expression).symbol;
            }
            const aliasLike = resolveEntityName(expression, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace, /*ignoreErrors*/ true, dontResolveAlias);
            if (aliasLike) {
                return aliasLike;
            }
            checkExpression(expression);
            return getNodeLinks(expression).resolvedSymbol;
        }

        function getTargetOfAliasDeclaration(node: Declaration, dontRecursivelyResolve = false): Symbol | undefined {
            switch (node.kind) {
                case SyntaxKind.ImportEqualsDeclaration:
                    return getTargetOfImportEqualsDeclaration(<ImportEqualsDeclaration>node, dontRecursivelyResolve);
                case SyntaxKind.ImportClause:
                    return getTargetOfImportClause(<ImportClause>node, dontRecursivelyResolve);
                case SyntaxKind.NamespaceImport:
                    return getTargetOfNamespaceImport(<NamespaceImport>node, dontRecursivelyResolve);
                case SyntaxKind.ImportSpecifier:
                    return getTargetOfImportSpecifier(<ImportSpecifier>node, dontRecursivelyResolve);
                case SyntaxKind.ExportSpecifier:
                    return getTargetOfExportSpecifier(<ExportSpecifier>node, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace, dontRecursivelyResolve);
                case SyntaxKind.ExportAssignment:
                case SyntaxKind.BinaryExpression:
                    return getTargetOfExportAssignment((<ExportAssignment | BinaryExpression>node), dontRecursivelyResolve);
                case SyntaxKind.NamespaceExportDeclaration:
                    return getTargetOfNamespaceExportDeclaration(<NamespaceExportDeclaration>node, dontRecursivelyResolve);
                default:
                    return Debug.fail();
            }
        }

        /**
         * Indicates that a symbol is an alias that does not merge with a local declaration.
         * OR Is a JSContainer which may merge an alias with a local declaration
         */
        function isNonLocalAlias(symbol: Symbol | undefined, excludes = SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace): symbol is Symbol {
            if (!symbol) return false;
            return (symbol.flags & (SymbolFlags.Alias | excludes)) === SymbolFlags.Alias || !!(symbol.flags & SymbolFlags.Alias && symbol.flags & SymbolFlags.Assignment);
        }

        function resolveSymbol(symbol: Symbol, dontResolveAlias?: boolean): Symbol;
        function resolveSymbol(symbol: Symbol | undefined, dontResolveAlias?: boolean): Symbol | undefined;
        function resolveSymbol(symbol: Symbol | undefined, dontResolveAlias?: boolean): Symbol | undefined {
            return !dontResolveAlias && isNonLocalAlias(symbol) ? resolveAlias(symbol) : symbol;
        }

        function resolveAlias(symbol: Symbol): Symbol {
            Debug.assert((symbol.flags & SymbolFlags.Alias) !== 0, "Should only get Alias here.");
            const links = getSymbolLinks(symbol);
            if (!links.target) {
                links.target = resolvingSymbol;
                const node = getDeclarationOfAliasSymbol(symbol);
                if (!node) return Debug.fail();
                const target = getTargetOfAliasDeclaration(node);
                if (links.target === resolvingSymbol) {
                    links.target = target || unknownSymbol;
                }
                else {
                    error(node, Diagnostics.Circular_definition_of_import_alias_0, symbolToString(symbol));
                }
            }
            else if (links.target === resolvingSymbol) {
                links.target = unknownSymbol;
            }
            return links.target;
        }

        function markExportAsReferenced(node: ImportEqualsDeclaration | ExportAssignment | ExportSpecifier) {
            const symbol = getSymbolOfNode(node);
            const target = resolveAlias(symbol);
            if (target) {
                const markAlias = target === unknownSymbol ||
                    ((target.flags & SymbolFlags.Value) && !isConstEnumOrConstEnumOnlyModule(target));

                if (markAlias) {
                    markAliasSymbolAsReferenced(symbol);
                }
            }
        }

        // When an alias symbol is referenced, we need to mark the entity it references as referenced and in turn repeat that until
        // we reach a non-alias or an exported entity (which is always considered referenced). We do this by checking the target of
        // the alias as an expression (which recursively takes us back here if the target references another alias).
        function markAliasSymbolAsReferenced(symbol: Symbol) {
            const links = getSymbolLinks(symbol);
            if (!links.referenced) {
                links.referenced = true;
                const node = getDeclarationOfAliasSymbol(symbol);
                if (!node) return Debug.fail();
                if (node.kind === SyntaxKind.ExportAssignment) {
                    // export default <symbol>
                    checkExpressionCached((<ExportAssignment>node).expression);
                }
                else if (node.kind === SyntaxKind.ExportSpecifier) {
                    // export { <symbol> } or export { <symbol> as foo }
                    checkExpressionCached((<ExportSpecifier>node).propertyName || (<ExportSpecifier>node).name);
                }
                else if (isInternalModuleImportEqualsDeclaration(node)) {
                    // import foo = <symbol>
                    checkExpressionCached(<Expression>node.moduleReference);
                }
            }
        }

        // This function is only for imports with entity names
        function getSymbolOfPartOfRightHandSideOfImportEquals(entityName: EntityName, dontResolveAlias?: boolean): Symbol | undefined {
            // There are three things we might try to look for. In the following examples,
            // the search term is enclosed in |...|:
            //
            //     import a = |b|; // Namespace
            //     import a = |b.c|; // Value, type, namespace
            //     import a = |b.c|.d; // Namespace
            if (entityName.kind === SyntaxKind.Identifier && isRightSideOfQualifiedNameOrPropertyAccess(entityName)) {
                entityName = <QualifiedName>entityName.parent;
            }
            // Check for case 1 and 3 in the above example
            if (entityName.kind === SyntaxKind.Identifier || entityName.parent.kind === SyntaxKind.QualifiedName) {
                return resolveEntityName(entityName, SymbolFlags.Namespace, /*ignoreErrors*/ false, dontResolveAlias);
            }
            else {
                // Case 2 in above example
                // entityName.kind could be a QualifiedName or a Missing identifier
                Debug.assert(entityName.parent.kind === SyntaxKind.ImportEqualsDeclaration);
                return resolveEntityName(entityName, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace, /*ignoreErrors*/ false, dontResolveAlias);
            }
        }

        function getFullyQualifiedName(symbol: Symbol): string {
            return symbol.parent ? getFullyQualifiedName(symbol.parent) + "." + symbolToString(symbol) : symbolToString(symbol);
        }

        /**
         * Resolves a qualified name and any involved aliases.
         */
        function resolveEntityName(name: EntityNameOrEntityNameExpression, meaning: SymbolFlags, ignoreErrors?: boolean, dontResolveAlias?: boolean, location?: Node): Symbol | undefined {
            if (nodeIsMissing(name)) {
                return undefined;
            }

            const namespaceMeaning = SymbolFlags.Namespace | (isInJSFile(name) ? meaning & SymbolFlags.Value : 0);
            let symbol: Symbol | undefined;
            if (name.kind === SyntaxKind.Identifier) {
                const message = meaning === namespaceMeaning ? Diagnostics.Cannot_find_namespace_0 : getCannotFindNameDiagnosticForName(getFirstIdentifier(name).escapedText);
                const symbolFromJSPrototype = isInJSFile(name) ? resolveEntityNameFromAssignmentDeclaration(name, meaning) : undefined;
                symbol = resolveName(location || name, name.escapedText, meaning, ignoreErrors || symbolFromJSPrototype ? undefined : message, name, /*isUse*/ true);
                if (!symbol) {
                    return symbolFromJSPrototype;
                }
            }
            else if (name.kind === SyntaxKind.QualifiedName || name.kind === SyntaxKind.PropertyAccessExpression) {
                const left = name.kind === SyntaxKind.QualifiedName ? name.left : name.expression;
                const right = name.kind === SyntaxKind.QualifiedName ? name.right : name.name;
                let namespace = resolveEntityName(left, namespaceMeaning, ignoreErrors, /*dontResolveAlias*/ false, location);
                if (!namespace || nodeIsMissing(right)) {
                    return undefined;
                }
                else if (namespace === unknownSymbol) {
                    return namespace;
                }
                if (isInJSFile(name)) {
                    if (namespace.valueDeclaration &&
                        isVariableDeclaration(namespace.valueDeclaration) &&
                        namespace.valueDeclaration.initializer &&
                        isCommonJsRequire(namespace.valueDeclaration.initializer)) {
                        const moduleName = (namespace.valueDeclaration.initializer as CallExpression).arguments[0] as StringLiteral;
                        const moduleSym = resolveExternalModuleName(moduleName, moduleName);
                        if (moduleSym) {
                            const resolvedModuleSymbol = resolveExternalModuleSymbol(moduleSym);
                            if (resolvedModuleSymbol) {
                                namespace = resolvedModuleSymbol;
                            }
                        }
                    }
                }
                symbol = getSymbol(getExportsOfSymbol(namespace), right.escapedText, meaning);
                if (!symbol) {
                    if (!ignoreErrors) {
                        error(right, Diagnostics.Namespace_0_has_no_exported_member_1, getFullyQualifiedName(namespace), declarationNameToString(right));
                    }
                    return undefined;
                }
            }
            else {
                throw Debug.assertNever(name, "Unknown entity name kind.");
            }
            Debug.assert((getCheckFlags(symbol) & CheckFlags.Instantiated) === 0, "Should never get an instantiated symbol here.");
            return (symbol.flags & meaning) || dontResolveAlias ? symbol : resolveAlias(symbol);
        }

        /**
         * 1. For prototype-property methods like `A.prototype.m = function () ...`, try to resolve names in the scope of `A` too.
         * Note that prototype-property assignment to locations outside the current file (eg globals) doesn't work, so
         * name resolution won't work either.
         * 2. For property assignments like `{ x: function f () { } }`, try to resolve names in the scope of `f` too.
         */
        function resolveEntityNameFromAssignmentDeclaration(name: Identifier, meaning: SymbolFlags) {
            if (isJSDocTypeReference(name.parent)) {
                const secondaryLocation = getAssignmentDeclarationLocation(name.parent);
                if (secondaryLocation) {
                    return resolveName(secondaryLocation, name.escapedText, meaning, /*nameNotFoundMessage*/ undefined, name, /*isUse*/ true);
                }
            }
        }

        function getAssignmentDeclarationLocation(node: TypeReferenceNode): Node | undefined {
            const typeAlias = findAncestor(node, node => !(isJSDocNode(node) || node.flags & NodeFlags.JSDoc) ? "quit" : isJSDocTypeAlias(node));
            if (typeAlias) {
                return;
            }
            const host = getJSDocHost(node);
            if (isExpressionStatement(host) &&
                isBinaryExpression(host.expression) &&
                getAssignmentDeclarationKind(host.expression) === AssignmentDeclarationKind.PrototypeProperty) {
                // X.prototype.m = /** @param {K} p */ function () { } <-- look for K on X's declaration
                const symbol = getSymbolOfNode(host.expression.left);
                if (symbol) {
                    return getDeclarationOfJSPrototypeContainer(symbol);
                }
            }
            if ((isObjectLiteralMethod(host) || isPropertyAssignment(host)) &&
                isBinaryExpression(host.parent.parent) &&
                getAssignmentDeclarationKind(host.parent.parent) === AssignmentDeclarationKind.Prototype) {
                // X.prototype = { /** @param {K} p */m() { } } <-- look for K on X's declaration
                const symbol = getSymbolOfNode(host.parent.parent.left);
                if (symbol) {
                    return getDeclarationOfJSPrototypeContainer(symbol);
                }
            }
            const sig = getHostSignatureFromJSDocHost(host);
            if (sig) {
                const symbol = getSymbolOfNode(sig);
                return symbol && symbol.valueDeclaration;
            }
        }

        function getDeclarationOfJSPrototypeContainer(symbol: Symbol) {
            const decl = symbol.parent!.valueDeclaration;
            const initializer = isAssignmentDeclaration(decl) ? getAssignedExpandoInitializer(decl) :
                hasOnlyExpressionInitializer(decl) ? getDeclaredExpandoInitializer(decl) :
                undefined;
            return initializer || decl;
        }

        function resolveExternalModuleName(location: Node, moduleReferenceExpression: Expression): Symbol | undefined {
            return resolveExternalModuleNameWorker(location, moduleReferenceExpression, Diagnostics.Cannot_find_module_0);
        }

        function resolveExternalModuleNameWorker(location: Node, moduleReferenceExpression: Expression, moduleNotFoundError: DiagnosticMessage | undefined, isForAugmentation = false): Symbol | undefined {
            return isStringLiteralLike(moduleReferenceExpression)
                ? resolveExternalModule(location, moduleReferenceExpression.text, moduleNotFoundError, moduleReferenceExpression, isForAugmentation)
                : undefined;
        }

        function resolveExternalModule(location: Node, moduleReference: string, moduleNotFoundError: DiagnosticMessage | undefined, errorNode: Node, isForAugmentation = false): Symbol | undefined {
            if (moduleReference === undefined) {
                return;
            }

            if (startsWith(moduleReference, "@types/")) {
                const diag = Diagnostics.Cannot_import_type_declaration_files_Consider_importing_0_instead_of_1;
                const withoutAtTypePrefix = removePrefix(moduleReference, "@types/");
                error(errorNode, diag, withoutAtTypePrefix, moduleReference);
            }

            const ambientModule = tryFindAmbientModule(moduleReference, /*withAugmentations*/ true);
            if (ambientModule) {
                return ambientModule;
            }
            const currentSourceFile = getSourceFileOfNode(location);
            const resolvedModule = getResolvedModule(currentSourceFile, moduleReference)!; // TODO: GH#18217
            const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
            const sourceFile = resolvedModule && !resolutionDiagnostic && host.getSourceFile(resolvedModule.resolvedFileName);
            if (sourceFile) {
                if (sourceFile.symbol) {
                    if (resolvedModule.isExternalLibraryImport && !extensionIsTypeScript(resolvedModule.extension)) {
                        errorOnImplicitAnyModule(/*isError*/ false, errorNode, resolvedModule, moduleReference);
                    }
                    // merged symbol is module declaration symbol combined with all augmentations
                    return getMergedSymbol(sourceFile.symbol);
                }
                if (moduleNotFoundError) {
                    // report errors only if it was requested
                    error(errorNode, Diagnostics.File_0_is_not_a_module, sourceFile.fileName);
                }
                return undefined;
            }

            if (patternAmbientModules) {
                const pattern = findBestPatternMatch(patternAmbientModules, _ => _.pattern, moduleReference);
                if (pattern) {
                    return getMergedSymbol(pattern.symbol);
                }
            }

            // May be an untyped module. If so, ignore resolutionDiagnostic.
            if (resolvedModule && !resolutionExtensionIsTypeScriptOrJson(resolvedModule.extension) && resolutionDiagnostic === undefined || resolutionDiagnostic === Diagnostics.Could_not_find_a_declaration_file_for_module_0_1_implicitly_has_an_any_type) {
                if (isForAugmentation) {
                    const diag = Diagnostics.Invalid_module_name_in_augmentation_Module_0_resolves_to_an_untyped_module_at_1_which_cannot_be_augmented;
                    error(errorNode, diag, moduleReference, resolvedModule.resolvedFileName);
                }
                else {
                    errorOnImplicitAnyModule(/*isError*/ noImplicitAny && !!moduleNotFoundError, errorNode, resolvedModule, moduleReference);
                }
                // Failed imports and untyped modules are both treated in an untyped manner; only difference is whether we give a diagnostic first.
                return undefined;
            }

            if (moduleNotFoundError) {
                // For relative paths, see if this was possibly a projectReference redirect
                if (pathIsRelative(moduleReference)) {
                    const sourceFile = getSourceFileOfNode(location);
                    const redirects = sourceFile.redirectedReferences;
                    if (redirects) {
                        const normalizedTargetPath = getNormalizedAbsolutePath(moduleReference, getDirectoryPath(sourceFile.fileName));
                        for (const ext of [Extension.Ts, Extension.Tsx]) {
                            const probePath = normalizedTargetPath + ext;
                            if (redirects.indexOf(probePath) >= 0) {
                                error(errorNode, Diagnostics.Output_file_0_has_not_been_built_from_source_file_1, moduleReference, probePath);
                                return undefined;
                            }
                        }
                    }
                }

                if (resolutionDiagnostic) {
                    error(errorNode, resolutionDiagnostic, moduleReference, resolvedModule.resolvedFileName);
                }
                else {
                    const tsExtension = tryExtractTypeScriptExtension(moduleReference);
                    if (tsExtension) {
                        const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
                        error(errorNode, diag, tsExtension, removeExtension(moduleReference, tsExtension));
                    }
                    else if (!compilerOptions.resolveJsonModule &&
                        fileExtensionIs(moduleReference, Extension.Json) &&
                        getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.NodeJs &&
                        hasJsonModuleEmitEnabled(compilerOptions)) {
                        error(errorNode, Diagnostics.Cannot_find_module_0_Consider_using_resolveJsonModule_to_import_module_with_json_extension, moduleReference);
                    }
                    else {
                        error(errorNode, moduleNotFoundError, moduleReference);
                    }
                }
            }
            return undefined;
        }

        function errorOnImplicitAnyModule(isError: boolean, errorNode: Node, { packageId, resolvedFileName }: ResolvedModuleFull, moduleReference: string): void {
            const errorInfo = !isExternalModuleNameRelative(moduleReference) && packageId
                ? typesPackageExists(packageId.name)
                    ? chainDiagnosticMessages(
                        /*details*/ undefined,
                        Diagnostics.If_the_0_package_actually_exposes_this_module_consider_sending_a_pull_request_to_amend_https_Colon_Slash_Slashgithub_com_SlashDefinitelyTyped_SlashDefinitelyTyped_Slashtree_Slashmaster_Slashtypes_Slash_1,
                        packageId.name, mangleScopedPackageName(packageId.name))
                    : chainDiagnosticMessages(
                        /*details*/ undefined,
                        Diagnostics.Try_npm_install_types_Slash_1_if_it_exists_or_add_a_new_declaration_d_ts_file_containing_declare_module_0,
                        moduleReference,
                        mangleScopedPackageName(packageId.name))
                : undefined;
            errorOrSuggestion(isError, errorNode, chainDiagnosticMessages(
                errorInfo,
                Diagnostics.Could_not_find_a_declaration_file_for_module_0_1_implicitly_has_an_any_type,
                moduleReference,
                resolvedFileName));
        }
        function typesPackageExists(packageName: string): boolean {
            return getPackagesSet().has(getTypesPackageName(packageName));
        }

        function resolveExternalModuleSymbol(moduleSymbol: Symbol, dontResolveAlias?: boolean): Symbol;
        function resolveExternalModuleSymbol(moduleSymbol: Symbol | undefined, dontResolveAlias?: boolean): Symbol | undefined;
        function resolveExternalModuleSymbol(moduleSymbol: Symbol, dontResolveAlias?: boolean): Symbol {
            return moduleSymbol && getMergedSymbol(getCommonJsExportEquals(resolveSymbol(moduleSymbol.exports!.get(InternalSymbolName.ExportEquals), dontResolveAlias), moduleSymbol)) || moduleSymbol;
        }

        function getCommonJsExportEquals(exported: Symbol | undefined, moduleSymbol: Symbol): Symbol | undefined {
            if (!exported || moduleSymbol.exports!.size === 1) {
                return exported;
            }
            const merged = cloneSymbol(exported);
            if (merged.exports === undefined) {
                merged.flags = merged.flags | SymbolFlags.ValueModule;
                merged.exports = createSymbolTable();
            }
            moduleSymbol.exports!.forEach((s, name) => {
                if (name === InternalSymbolName.ExportEquals) return;
                merged.exports!.set(name, merged.exports!.has(name) ? mergeSymbol(merged.exports!.get(name)!, s) : s);
            });
            return merged;
        }

        // An external module with an 'export =' declaration may be referenced as an ES6 module provided the 'export ='
        // references a symbol that is at least declared as a module or a variable. The target of the 'export =' may
        // combine other declarations with the module or variable (e.g. a class/module, function/module, interface/variable).
        function resolveESModuleSymbol(moduleSymbol: Symbol | undefined, referencingLocation: Node, dontResolveAlias: boolean): Symbol | undefined {
            const symbol = resolveExternalModuleSymbol(moduleSymbol, dontResolveAlias);
            if (!dontResolveAlias && symbol) {
                if (!(symbol.flags & (SymbolFlags.Module | SymbolFlags.Variable)) && !getDeclarationOfKind(symbol, SyntaxKind.SourceFile)) {
                    error(referencingLocation, Diagnostics.Module_0_resolves_to_a_non_module_entity_and_cannot_be_imported_using_this_construct, symbolToString(moduleSymbol!));
                    return symbol;
                }
                if (compilerOptions.esModuleInterop) {
                    const referenceParent = referencingLocation.parent;
                    if (
                        (isImportDeclaration(referenceParent) && getNamespaceDeclarationNode(referenceParent)) ||
                        isImportCall(referenceParent)
                    ) {
                        const type = getTypeOfSymbol(symbol);
                        let sigs = getSignaturesOfStructuredType(type, SignatureKind.Call);
                        if (!sigs || !sigs.length) {
                            sigs = getSignaturesOfStructuredType(type, SignatureKind.Construct);
                        }
                        if (sigs && sigs.length) {
                            const moduleType = getTypeWithSyntheticDefaultImportType(type, symbol, moduleSymbol!);
                            // Create a new symbol which has the module's type less the call and construct signatures
                            const result = createSymbol(symbol.flags, symbol.escapedName);
                            result.declarations = symbol.declarations ? symbol.declarations.slice() : [];
                            result.parent = symbol.parent;
                            result.target = symbol;
                            result.originatingImport = referenceParent;
                            if (symbol.valueDeclaration) result.valueDeclaration = symbol.valueDeclaration;
                            if (symbol.constEnumOnlyModule) result.constEnumOnlyModule = true;
                            if (symbol.members) result.members = cloneMap(symbol.members);
                            if (symbol.exports) result.exports = cloneMap(symbol.exports);
                            const resolvedModuleType = resolveStructuredTypeMembers(moduleType as StructuredType); // Should already be resolved from the signature checks above
                            result.type = createAnonymousType(result, resolvedModuleType.members, emptyArray, emptyArray, resolvedModuleType.stringIndexInfo, resolvedModuleType.numberIndexInfo);
                            return result;
                        }
                    }
                }
            }
            return symbol;
        }

        function hasExportAssignmentSymbol(moduleSymbol: Symbol): boolean {
            return moduleSymbol.exports!.get(InternalSymbolName.ExportEquals) !== undefined;
        }

        function getExportsOfModuleAsArray(moduleSymbol: Symbol): Symbol[] {
            return symbolsToArray(getExportsOfModule(moduleSymbol));
        }

        function getExportsAndPropertiesOfModule(moduleSymbol: Symbol): Symbol[] {
            const exports = getExportsOfModuleAsArray(moduleSymbol);
            const exportEquals = resolveExternalModuleSymbol(moduleSymbol);
            if (exportEquals !== moduleSymbol) {
                addRange(exports, getPropertiesOfType(getTypeOfSymbol(exportEquals)));
            }
            return exports;
        }

        function tryGetMemberInModuleExports(memberName: __String, moduleSymbol: Symbol): Symbol | undefined {
            const symbolTable = getExportsOfModule(moduleSymbol);
            if (symbolTable) {
                return symbolTable.get(memberName);
            }
        }

        function tryGetMemberInModuleExportsAndProperties(memberName: __String, moduleSymbol: Symbol): Symbol | undefined {
            const symbol = tryGetMemberInModuleExports(memberName, moduleSymbol);
            if (symbol) {
                return symbol;
            }

            const exportEquals = resolveExternalModuleSymbol(moduleSymbol);
            if (exportEquals === moduleSymbol) {
                return undefined;
            }

            const type = getTypeOfSymbol(exportEquals);
            return type.flags & TypeFlags.Primitive ? undefined : getPropertyOfType(type, memberName);
        }

        function getExportsOfSymbol(symbol: Symbol): SymbolTable {
            return symbol.flags & SymbolFlags.Class ? getResolvedMembersOrExportsOfSymbol(symbol, MembersOrExportsResolutionKind.resolvedExports) :
                symbol.flags & SymbolFlags.Module ? getExportsOfModule(symbol) :
                symbol.exports || emptySymbols;
        }

        function getExportsOfModule(moduleSymbol: Symbol): SymbolTable {
            const links = getSymbolLinks(moduleSymbol);
            return links.resolvedExports || (links.resolvedExports = getExportsOfModuleWorker(moduleSymbol));
        }

        interface ExportCollisionTracker {
            specifierText: string;
            exportsWithDuplicate: ExportDeclaration[];
        }

        type ExportCollisionTrackerTable = UnderscoreEscapedMap<ExportCollisionTracker>;

        /**
         * Extends one symbol table with another while collecting information on name collisions for error message generation into the `lookupTable` argument
         * Not passing `lookupTable` and `exportNode` disables this collection, and just extends the tables
         */
        function extendExportSymbols(target: SymbolTable, source: SymbolTable | undefined, lookupTable?: ExportCollisionTrackerTable, exportNode?: ExportDeclaration) {
            if (!source) return;
            source.forEach((sourceSymbol, id) => {
                if (id === InternalSymbolName.Default) return;

                const targetSymbol = target.get(id);
                if (!targetSymbol) {
                    target.set(id, sourceSymbol);
                    if (lookupTable && exportNode) {
                        lookupTable.set(id, {
                            specifierText: getTextOfNode(exportNode.moduleSpecifier!)
                        } as ExportCollisionTracker);
                    }
                }
                else if (lookupTable && exportNode && targetSymbol && resolveSymbol(targetSymbol) !== resolveSymbol(sourceSymbol)) {
                    const collisionTracker = lookupTable.get(id)!;
                    if (!collisionTracker.exportsWithDuplicate) {
                        collisionTracker.exportsWithDuplicate = [exportNode];
                    }
                    else {
                        collisionTracker.exportsWithDuplicate.push(exportNode);
                    }
                }
            });
        }

        function getExportsOfModuleWorker(moduleSymbol: Symbol): SymbolTable {
            const visitedSymbols: Symbol[] = [];

            // A module defined by an 'export=' consists of one export that needs to be resolved
            moduleSymbol = resolveExternalModuleSymbol(moduleSymbol);

            return visit(moduleSymbol) || emptySymbols;

            // The ES6 spec permits export * declarations in a module to circularly reference the module itself. For example,
            // module 'a' can 'export * from "b"' and 'b' can 'export * from "a"' without error.
            function visit(symbol: Symbol | undefined): SymbolTable | undefined {
                if (!(symbol && symbol.exports && pushIfUnique(visitedSymbols, symbol))) {
                    return;
                }
                const symbols = cloneMap(symbol.exports);
                // All export * declarations are collected in an __export symbol by the binder
                const exportStars = symbol.exports.get(InternalSymbolName.ExportStar);
                if (exportStars) {
                    const nestedSymbols = createSymbolTable();
                    const lookupTable = createMap<ExportCollisionTracker>() as ExportCollisionTrackerTable;
                    for (const node of exportStars.declarations) {
                        const resolvedModule = resolveExternalModuleName(node, (node as ExportDeclaration).moduleSpecifier!);
                        const exportedSymbols = visit(resolvedModule);
                        extendExportSymbols(
                            nestedSymbols,
                            exportedSymbols,
                            lookupTable,
                            node as ExportDeclaration
                        );
                    }
                    lookupTable.forEach(({ exportsWithDuplicate }, id) => {
                        // It's not an error if the file with multiple `export *`s with duplicate names exports a member with that name itself
                        if (id === "export=" || !(exportsWithDuplicate && exportsWithDuplicate.length) || symbols.has(id)) {
                            return;
                        }
                        for (const node of exportsWithDuplicate) {
                            diagnostics.add(createDiagnosticForNode(
                                node,
                                Diagnostics.Module_0_has_already_exported_a_member_named_1_Consider_explicitly_re_exporting_to_resolve_the_ambiguity,
                                lookupTable.get(id)!.specifierText,
                                unescapeLeadingUnderscores(id)
                            ));
                        }
                    });
                    extendExportSymbols(symbols, nestedSymbols);
                }
                return symbols;
            }
        }

        function getMergedSymbol(symbol: Symbol): Symbol;
        function getMergedSymbol(symbol: Symbol | undefined): Symbol | undefined;
        function getMergedSymbol(symbol: Symbol | undefined): Symbol | undefined {
            let merged: Symbol;
            return symbol && symbol.mergeId && (merged = mergedSymbols[symbol.mergeId]) ? merged : symbol;
        }

        function getSymbolOfNode(node: Declaration): Symbol;
        function getSymbolOfNode(node: Node): Symbol | undefined;
        function getSymbolOfNode(node: Node): Symbol | undefined {
            return getMergedSymbol(node.symbol && getLateBoundSymbol(node.symbol));
        }

        function getParentOfSymbol(symbol: Symbol): Symbol | undefined {
            return getMergedSymbol(symbol.parent && getLateBoundSymbol(symbol.parent));
        }

        /**
         * Attempts to find the symbol corresponding to the container a symbol is in - usually this
         * is just its' `.parent`, but for locals, this value is `undefined`
         */
        function getContainersOfSymbol(symbol: Symbol, enclosingDeclaration: Node | undefined): Symbol[] | undefined {
            const container = getParentOfSymbol(symbol);
            if (container) {
                const additionalContainers = mapDefined(container.declarations, fileSymbolIfFileSymbolExportEqualsContainer);
                if (enclosingDeclaration && getAccessibleSymbolChain(container, enclosingDeclaration, SymbolFlags.Namespace, /*externalOnly*/ false)) {
                    return concatenate([container], additionalContainers); // This order expresses a preference for the real container if it is in scope
                }
                return append(additionalContainers, container);
            }
            const candidates = mapDefined(symbol.declarations, d => !isAmbientModule(d) && d.parent && hasNonGlobalAugmentationExternalModuleSymbol(d.parent) ? getSymbolOfNode(d.parent) : undefined);
            if (!length(candidates)) {
                return undefined;
            }
            return mapDefined(candidates, candidate => getAliasForSymbolInContainer(candidate, symbol) ? candidate : undefined);

            function fileSymbolIfFileSymbolExportEqualsContainer(d: Declaration) {
                const fileSymbol = getExternalModuleContainer(d);
                const exported = fileSymbol && fileSymbol.exports && fileSymbol.exports.get(InternalSymbolName.ExportEquals);
                return resolveSymbol(exported) === resolveSymbol(container) ? fileSymbol : undefined;
            }
        }

        function getAliasForSymbolInContainer(container: Symbol, symbol: Symbol) {
            if (container === getParentOfSymbol(symbol)) {
                // fast path, `symbol` is either already the alias or isn't aliased
                return symbol;
            }
            const exports = getExportsOfSymbol(container);
            const quick = exports.get(symbol.escapedName);
            if (quick && symbolRefersToTarget(quick)) {
                return quick;
            }
            return forEachEntry(exports, exported => {
                if (symbolRefersToTarget(exported)) {
                    return exported;
                }
            });

            function symbolRefersToTarget(s: Symbol) {
                if (s === symbol || resolveSymbol(s) === symbol || resolveSymbol(s) === resolveSymbol(symbol)) {
                    return s;
                }
            }
        }

        function getExportSymbolOfValueSymbolIfExported(symbol: Symbol): Symbol;
        function getExportSymbolOfValueSymbolIfExported(symbol: Symbol | undefined): Symbol | undefined;
        function getExportSymbolOfValueSymbolIfExported(symbol: Symbol | undefined): Symbol | undefined {
            return getMergedSymbol(symbol && (symbol.flags & SymbolFlags.ExportValue) !== 0 ? symbol.exportSymbol : symbol);
        }

        function symbolIsValue(symbol: Symbol): boolean {
            return !!(symbol.flags & SymbolFlags.Value || symbol.flags & SymbolFlags.Alias && resolveAlias(symbol).flags & SymbolFlags.Value);
        }

        function findConstructorDeclaration(node: ClassLikeDeclaration): ConstructorDeclaration | undefined {
            const members = node.members;
            for (const member of members) {
                if (member.kind === SyntaxKind.Constructor && nodeIsPresent((<ConstructorDeclaration>member).body)) {
                    return <ConstructorDeclaration>member;
                }
            }
        }

        function createType(flags: TypeFlags): Type {
            const result = new Type(checker, flags);
            typeCount++;
            result.id = typeCount;
            return result;
        }

        function createIntrinsicType(kind: TypeFlags, intrinsicName: string): IntrinsicType {
            const type = <IntrinsicType>createType(kind);
            type.intrinsicName = intrinsicName;
            return type;
        }

        function createBooleanType(trueFalseTypes: ReadonlyArray<Type>): IntrinsicType & UnionType {
            const type = <IntrinsicType & UnionType>getUnionType(trueFalseTypes);
            type.flags |= TypeFlags.Boolean;
            type.intrinsicName = "boolean";
            return type;
        }

        function createObjectType(objectFlags: ObjectFlags, symbol?: Symbol): ObjectType {
            const type = <ObjectType>createType(TypeFlags.Object);
            type.objectFlags = objectFlags;
            type.symbol = symbol!;
            type.members = undefined;
            type.properties = undefined;
            type.callSignatures = undefined;
            type.constructSignatures = undefined;
            type.stringIndexInfo = undefined;
            type.numberIndexInfo = undefined;
            return type;
        }

        function createTypeofType() {
            return getUnionType(arrayFrom(typeofEQFacts.keys(), getLiteralType));
        }

        // A reserved member name starts with two underscores, but the third character cannot be an underscore
        // or the @ symbol. A third underscore indicates an escaped form of an identifer that started
        // with at least two underscores. The @ character indicates that the name is denoted by a well known ES
        // Symbol instance.
        function isReservedMemberName(name: __String) {
            return (name as string).charCodeAt(0) === CharacterCodes._ &&
                (name as string).charCodeAt(1) === CharacterCodes._ &&
                (name as string).charCodeAt(2) !== CharacterCodes._ &&
                (name as string).charCodeAt(2) !== CharacterCodes.at;
        }

        function getNamedMembers(members: SymbolTable): Symbol[] {
            let result: Symbol[] | undefined;
            members.forEach((symbol, id) => {
                if (!isReservedMemberName(id) && symbolIsValue(symbol)) {
                    (result || (result = [])).push(symbol);
                }
            });
            return result || emptyArray;
        }

        function setStructuredTypeMembers(type: StructuredType, members: SymbolTable, callSignatures: ReadonlyArray<Signature>, constructSignatures: ReadonlyArray<Signature>, stringIndexInfo: IndexInfo | undefined, numberIndexInfo: IndexInfo | undefined): ResolvedType {
            (<ResolvedType>type).members = members;
            (<ResolvedType>type).properties = members === emptySymbols ? emptyArray : getNamedMembers(members);
            (<ResolvedType>type).callSignatures = callSignatures;
            (<ResolvedType>type).constructSignatures = constructSignatures;
            (<ResolvedType>type).stringIndexInfo = stringIndexInfo;
            (<ResolvedType>type).numberIndexInfo = numberIndexInfo;
            return <ResolvedType>type;
        }

        function createAnonymousType(symbol: Symbol | undefined, members: SymbolTable, callSignatures: ReadonlyArray<Signature>, constructSignatures: ReadonlyArray<Signature>, stringIndexInfo: IndexInfo | undefined, numberIndexInfo: IndexInfo | undefined): ResolvedType {
            return setStructuredTypeMembers(createObjectType(ObjectFlags.Anonymous, symbol),
                members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function forEachSymbolTableInScope<T>(enclosingDeclaration: Node | undefined, callback: (symbolTable: SymbolTable) => T): T {
            let result: T;
            for (let location = enclosingDeclaration; location; location = location.parent) {
                // Locals of a source file are not in scope (because they get merged into the global symbol table)
                if (location.locals && !isGlobalSourceFile(location)) {
                    if (result = callback(location.locals)) {
                        return result;
                    }
                }
                switch (location.kind) {
                    case SyntaxKind.SourceFile:
                        if (!isExternalOrCommonJsModule(<SourceFile>location)) {
                            break;
                        }
                        // falls through
                    case SyntaxKind.ModuleDeclaration:
                        if (result = callback(getSymbolOfNode(location as ModuleDeclaration).exports!)) {
                            return result;
                        }
                        break;
                }
            }

            return callback(globals);
        }

        function getQualifiedLeftMeaning(rightMeaning: SymbolFlags) {
            // If we are looking in value space, the parent meaning is value, other wise it is namespace
            return rightMeaning === SymbolFlags.Value ? SymbolFlags.Value : SymbolFlags.Namespace;
        }

        function getAccessibleSymbolChain(symbol: Symbol | undefined, enclosingDeclaration: Node | undefined, meaning: SymbolFlags, useOnlyExternalAliasing: boolean, visitedSymbolTablesMap: Map<SymbolTable[]> = createMap()): Symbol[] | undefined {
            if (!(symbol && !isPropertyOrMethodDeclarationSymbol(symbol))) {
                return undefined;
            }

            const id = "" + getSymbolId(symbol);
            let visitedSymbolTables = visitedSymbolTablesMap.get(id);
            if (!visitedSymbolTables) {
                visitedSymbolTablesMap.set(id, visitedSymbolTables = []);
            }
            return forEachSymbolTableInScope(enclosingDeclaration, getAccessibleSymbolChainFromSymbolTable);

            /**
             * @param {ignoreQualification} boolean Set when a symbol is being looked for through the exports of another symbol (meaning we have a route to qualify it already)
             */
            function getAccessibleSymbolChainFromSymbolTable(symbols: SymbolTable, ignoreQualification?: boolean): Symbol[] | undefined {
                if (!pushIfUnique(visitedSymbolTables!, symbols)) {
                    return undefined;
                }

                const result = trySymbolTable(symbols, ignoreQualification);
                visitedSymbolTables!.pop();
                return result;
            }

            function canQualifySymbol(symbolFromSymbolTable: Symbol, meaning: SymbolFlags) {
                // If the symbol is equivalent and doesn't need further qualification, this symbol is accessible
                return !needsQualification(symbolFromSymbolTable, enclosingDeclaration, meaning) ||
                    // If symbol needs qualification, make sure that parent is accessible, if it is then this symbol is accessible too
                    !!getAccessibleSymbolChain(symbolFromSymbolTable.parent, enclosingDeclaration, getQualifiedLeftMeaning(meaning), useOnlyExternalAliasing, visitedSymbolTablesMap);
            }

            function isAccessible(symbolFromSymbolTable: Symbol, resolvedAliasSymbol?: Symbol, ignoreQualification?: boolean) {
                return symbol === (resolvedAliasSymbol || symbolFromSymbolTable) &&
                    // if the symbolFromSymbolTable is not external module (it could be if it was determined as ambient external module and would be in globals table)
                    // and if symbolFromSymbolTable or alias resolution matches the symbol,
                    // check the symbol can be qualified, it is only then this symbol is accessible
                    !some(symbolFromSymbolTable.declarations, hasNonGlobalAugmentationExternalModuleSymbol) &&
                    (ignoreQualification || canQualifySymbol(symbolFromSymbolTable, meaning));
            }

            function trySymbolTable(symbols: SymbolTable, ignoreQualification: boolean | undefined): Symbol[] | undefined {
                // If symbol is directly available by its name in the symbol table
                if (isAccessible(symbols.get(symbol!.escapedName)!, /*resolvedAliasSymbol*/ undefined, ignoreQualification)) {
                    return [symbol!];
                }

                // Check if symbol is any of the alias
                return forEachEntry(symbols, symbolFromSymbolTable => {
                    if (symbolFromSymbolTable.flags & SymbolFlags.Alias
                        && symbolFromSymbolTable.escapedName !== InternalSymbolName.ExportEquals
                        && symbolFromSymbolTable.escapedName !== InternalSymbolName.Default
                        && !(isUMDExportSymbol(symbolFromSymbolTable) && enclosingDeclaration && isExternalModule(getSourceFileOfNode(enclosingDeclaration)))
                        // If `!useOnlyExternalAliasing`, we can use any type of alias to get the name
                        && (!useOnlyExternalAliasing || some(symbolFromSymbolTable.declarations, isExternalModuleImportEqualsDeclaration))
                        // While exports are generally considered to be in scope, export-specifier declared symbols are _not_
                        // See similar comment in `resolveName` for details
                        && (ignoreQualification || !getDeclarationOfKind(symbolFromSymbolTable, SyntaxKind.ExportSpecifier))
                    ) {

                        const resolvedImportedSymbol = resolveAlias(symbolFromSymbolTable);
                        if (isAccessible(symbolFromSymbolTable, resolvedImportedSymbol, ignoreQualification)) {
                            return [symbolFromSymbolTable];
                        }

                        // Look in the exported members, if we can find accessibleSymbolChain, symbol is accessible using this chain
                        // but only if the symbolFromSymbolTable can be qualified
                        const candidateTable = getExportsOfSymbol(resolvedImportedSymbol);
                        const accessibleSymbolsFromExports = candidateTable && getAccessibleSymbolChainFromSymbolTable(candidateTable, /*ignoreQualification*/ true);
                        if (accessibleSymbolsFromExports && canQualifySymbol(symbolFromSymbolTable, getQualifiedLeftMeaning(meaning))) {
                            return [symbolFromSymbolTable].concat(accessibleSymbolsFromExports);
                        }
                    }
                    if (symbolFromSymbolTable.escapedName === symbol!.escapedName && symbolFromSymbolTable.exportSymbol) {
                        if (isAccessible(getMergedSymbol(symbolFromSymbolTable.exportSymbol), /*aliasSymbol*/ undefined, ignoreQualification)) {
                            return [symbol!];
                        }
                    }
                });
            }
        }

        function needsQualification(symbol: Symbol, enclosingDeclaration: Node | undefined, meaning: SymbolFlags) {
            let qualify = false;
            forEachSymbolTableInScope(enclosingDeclaration, symbolTable => {
                // If symbol of this name is not available in the symbol table we are ok
                let symbolFromSymbolTable = getMergedSymbol(symbolTable.get(symbol.escapedName));
                if (!symbolFromSymbolTable) {
                    // Continue to the next symbol table
                    return false;
                }
                // If the symbol with this name is present it should refer to the symbol
                if (symbolFromSymbolTable === symbol) {
                    // No need to qualify
                    return true;
                }

                // Qualify if the symbol from symbol table has same meaning as expected
                symbolFromSymbolTable = (symbolFromSymbolTable.flags & SymbolFlags.Alias && !getDeclarationOfKind(symbolFromSymbolTable, SyntaxKind.ExportSpecifier)) ? resolveAlias(symbolFromSymbolTable) : symbolFromSymbolTable;
                if (symbolFromSymbolTable.flags & meaning) {
                    qualify = true;
                    return true;
                }

                // Continue to the next symbol table
                return false;
            });

            return qualify;
        }

        function isPropertyOrMethodDeclarationSymbol(symbol: Symbol) {
            if (symbol.declarations && symbol.declarations.length) {
                for (const declaration of symbol.declarations) {
                    switch (declaration.kind) {
                        case SyntaxKind.PropertyDeclaration:
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.GetAccessor:
                        case SyntaxKind.SetAccessor:
                            continue;
                        default:
                            return false;
                    }
                }
                return true;
            }
            return false;
        }

        function isTypeSymbolAccessible(typeSymbol: Symbol, enclosingDeclaration: Node | undefined): boolean {
            const access = isSymbolAccessible(typeSymbol, enclosingDeclaration, SymbolFlags.Type, /*shouldComputeAliasesToMakeVisible*/ false);
            return access.accessibility === SymbolAccessibility.Accessible;
        }

        function isValueSymbolAccessible(typeSymbol: Symbol, enclosingDeclaration: Node): boolean {
            const access = isSymbolAccessible(typeSymbol, enclosingDeclaration, SymbolFlags.Value, /*shouldComputeAliasesToMakeVisible*/ false);
            return access.accessibility === SymbolAccessibility.Accessible;
        }

        function isAnySymbolAccessible(symbols: Symbol[] | undefined, enclosingDeclaration: Node, initialSymbol: Symbol, meaning: SymbolFlags, shouldComputeAliasesToMakeVisible: boolean): SymbolAccessibilityResult | undefined {
            if (!length(symbols)) return;

            let hadAccessibleChain: Symbol | undefined;
            for (const symbol of symbols!) {
                // Symbol is accessible if it by itself is accessible
                const accessibleSymbolChain = getAccessibleSymbolChain(symbol, enclosingDeclaration, meaning, /*useOnlyExternalAliasing*/ false);
                if (accessibleSymbolChain) {
                    hadAccessibleChain = symbol;
                    const hasAccessibleDeclarations = hasVisibleDeclarations(accessibleSymbolChain[0], shouldComputeAliasesToMakeVisible);
                    if (hasAccessibleDeclarations) {
                        return hasAccessibleDeclarations;
                    }
                }
                else {
                    if (some(symbol.declarations, hasNonGlobalAugmentationExternalModuleSymbol)) {
                        // Any meaning of a module symbol is always accessible via an `import` type
                        return {
                            accessibility: SymbolAccessibility.Accessible
                        };
                    }
                }

                // If we haven't got the accessible symbol, it doesn't mean the symbol is actually inaccessible.
                // It could be a qualified symbol and hence verify the path
                // e.g.:
                // module m {
                //     export class c {
                //     }
                // }
                // const x: typeof m.c
                // In the above example when we start with checking if typeof m.c symbol is accessible,
                // we are going to see if c can be accessed in scope directly.
                // But it can't, hence the accessible is going to be undefined, but that doesn't mean m.c is inaccessible
                // It is accessible if the parent m is accessible because then m.c can be accessed through qualification

                let containers = getContainersOfSymbol(symbol, enclosingDeclaration);
                // If we're trying to reference some object literal in, eg `var a = { x: 1 }`, the symbol for the literal, `__object`, is distinct
                // from the symbol of the declaration it is being assigned to. Since we can use the declaration to refer to the literal, however,
                // we'd like to make that connection here - potentially causing us to paint the declararation's visibiility, and therefore the literal.
                const firstDecl: Node = first(symbol.declarations);
                if (!length(containers) && meaning & SymbolFlags.Value && firstDecl && isObjectLiteralExpression(firstDecl)) {
                    if (firstDecl.parent && isVariableDeclaration(firstDecl.parent) && firstDecl === firstDecl.parent.initializer) {
                        containers = [getSymbolOfNode(firstDecl.parent)];
                    }
                }
                const parentResult = isAnySymbolAccessible(containers, enclosingDeclaration, initialSymbol, initialSymbol === symbol ? getQualifiedLeftMeaning(meaning) : meaning, shouldComputeAliasesToMakeVisible);
                if (parentResult) {
                    return parentResult;
                }
            }

            if (hadAccessibleChain) {
                return {
                    accessibility: SymbolAccessibility.NotAccessible,
                    errorSymbolName: symbolToString(initialSymbol, enclosingDeclaration, meaning),
                    errorModuleName: hadAccessibleChain !== initialSymbol ? symbolToString(hadAccessibleChain, enclosingDeclaration, SymbolFlags.Namespace) : undefined,
                };
            }
        }

        /**
         * Check if the given symbol in given enclosing declaration is accessible and mark all associated alias to be visible if requested
         *
         * @param symbol a Symbol to check if accessible
         * @param enclosingDeclaration a Node containing reference to the symbol
         * @param meaning a SymbolFlags to check if such meaning of the symbol is accessible
         * @param shouldComputeAliasToMakeVisible a boolean value to indicate whether to return aliases to be mark visible in case the symbol is accessible
         */
        function isSymbolAccessible(symbol: Symbol | undefined, enclosingDeclaration: Node | undefined, meaning: SymbolFlags, shouldComputeAliasesToMakeVisible: boolean): SymbolAccessibilityResult {
            if (symbol && enclosingDeclaration) {
                const result = isAnySymbolAccessible([symbol], enclosingDeclaration, symbol, meaning, shouldComputeAliasesToMakeVisible);
                if (result) {
                    return result;
                }

                // This could be a symbol that is not exported in the external module
                // or it could be a symbol from different external module that is not aliased and hence cannot be named
                const symbolExternalModule = forEach(symbol.declarations, getExternalModuleContainer);
                if (symbolExternalModule) {
                    const enclosingExternalModule = getExternalModuleContainer(enclosingDeclaration);
                    if (symbolExternalModule !== enclosingExternalModule) {
                        // name from different external module that is not visible
                        return {
                            accessibility: SymbolAccessibility.CannotBeNamed,
                            errorSymbolName: symbolToString(symbol, enclosingDeclaration, meaning),
                            errorModuleName: symbolToString(symbolExternalModule)
                        };
                    }
                }

                // Just a local name that is not accessible
                return {
                    accessibility: SymbolAccessibility.NotAccessible,
                    errorSymbolName: symbolToString(symbol, enclosingDeclaration, meaning),
                };
            }

            return { accessibility: SymbolAccessibility.Accessible };
        }

        function getExternalModuleContainer(declaration: Node) {
            const node = findAncestor(declaration, hasExternalModuleSymbol);
            return node && getSymbolOfNode(node);
        }

        function hasExternalModuleSymbol(declaration: Node) {
            return isAmbientModule(declaration) || (declaration.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>declaration));
        }

        function hasNonGlobalAugmentationExternalModuleSymbol(declaration: Node) {
            return isModuleWithStringLiteralName(declaration) || (declaration.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>declaration));
        }

        function hasVisibleDeclarations(symbol: Symbol, shouldComputeAliasToMakeVisible: boolean): SymbolVisibilityResult | undefined {
            let aliasesToMakeVisible: LateVisibilityPaintedStatement[] | undefined;
            if (!every(symbol.declarations, getIsDeclarationVisible)) {
                return undefined;
            }
            return { accessibility: SymbolAccessibility.Accessible, aliasesToMakeVisible };

            function getIsDeclarationVisible(declaration: Declaration) {
                if (!isDeclarationVisible(declaration)) {
                    // Mark the unexported alias as visible if its parent is visible
                    // because these kind of aliases can be used to name types in declaration file

                    const anyImportSyntax = getAnyImportSyntax(declaration);
                    if (anyImportSyntax &&
                        !hasModifier(anyImportSyntax, ModifierFlags.Export) && // import clause without export
                        isDeclarationVisible(anyImportSyntax.parent)) {
                        return addVisibleAlias(declaration, anyImportSyntax);
                    }
                    else if (isVariableDeclaration(declaration) && isVariableStatement(declaration.parent.parent) &&
                        !hasModifier(declaration.parent.parent, ModifierFlags.Export) && // unexported variable statement
                        isDeclarationVisible(declaration.parent.parent.parent)) {
                        return addVisibleAlias(declaration, declaration.parent.parent);
                    }
                    else if (isLateVisibilityPaintedStatement(declaration) // unexported top-level statement
                        && !hasModifier(declaration, ModifierFlags.Export)
                        && isDeclarationVisible(declaration.parent)) {
                        return addVisibleAlias(declaration, declaration);
                    }

                    // Declaration is not visible
                    return false;
                }

                return true;
            }

            function addVisibleAlias(declaration: Declaration, aliasingStatement: LateVisibilityPaintedStatement) {
                // In function "buildTypeDisplay" where we decide whether to write type-alias or serialize types,
                // we want to just check if type- alias is accessible or not but we don't care about emitting those alias at that time
                // since we will do the emitting later in trackSymbol.
                if (shouldComputeAliasToMakeVisible) {
                    getNodeLinks(declaration).isVisible = true;
                    aliasesToMakeVisible = appendIfUnique(aliasesToMakeVisible, aliasingStatement);
                }
                return true;
            }
        }

        function isEntityNameVisible(entityName: EntityNameOrEntityNameExpression, enclosingDeclaration: Node): SymbolVisibilityResult {
            // get symbol of the first identifier of the entityName
            let meaning: SymbolFlags;
            if (entityName.parent.kind === SyntaxKind.TypeQuery ||
                isExpressionWithTypeArgumentsInClassExtendsClause(entityName.parent) ||
                entityName.parent.kind === SyntaxKind.ComputedPropertyName) {
                // Typeof value
                meaning = SymbolFlags.Value | SymbolFlags.ExportValue;
            }
            else if (entityName.kind === SyntaxKind.QualifiedName || entityName.kind === SyntaxKind.PropertyAccessExpression ||
                entityName.parent.kind === SyntaxKind.ImportEqualsDeclaration) {
                // Left identifier from type reference or TypeAlias
                // Entity name of the import declaration
                meaning = SymbolFlags.Namespace;
            }
            else {
                // Type Reference or TypeAlias entity = Identifier
                meaning = SymbolFlags.Type;
            }

            const firstIdentifier = getFirstIdentifier(entityName);
            const symbol = resolveName(enclosingDeclaration, firstIdentifier.escapedText, meaning, /*nodeNotFoundErrorMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ false);

            // Verify if the symbol is accessible
            return (symbol && hasVisibleDeclarations(symbol, /*shouldComputeAliasToMakeVisible*/ true)) || {
                accessibility: SymbolAccessibility.NotAccessible,
                errorSymbolName: getTextOfNode(firstIdentifier),
                errorNode: firstIdentifier
            };
        }

        function symbolToString(symbol: Symbol, enclosingDeclaration?: Node, meaning?: SymbolFlags, flags: SymbolFormatFlags = SymbolFormatFlags.AllowAnyNodeKind, writer?: EmitTextWriter): string {
            let nodeFlags = NodeBuilderFlags.IgnoreErrors;
            if (flags & SymbolFormatFlags.UseOnlyExternalAliasing) {
                nodeFlags |= NodeBuilderFlags.UseOnlyExternalAliasing;
            }
            if (flags & SymbolFormatFlags.WriteTypeParametersOrArguments) {
                nodeFlags |= NodeBuilderFlags.WriteTypeParametersInQualifiedName;
            }
            if (flags & SymbolFormatFlags.UseAliasDefinedOutsideCurrentScope) {
                nodeFlags |= NodeBuilderFlags.UseAliasDefinedOutsideCurrentScope;
            }
            const builder = flags & SymbolFormatFlags.AllowAnyNodeKind ? nodeBuilder.symbolToExpression : nodeBuilder.symbolToEntityName;
            return writer ? symbolToStringWorker(writer).getText() : usingSingleLineStringWriter(symbolToStringWorker);

            function symbolToStringWorker(writer: EmitTextWriter) {
                const entity = builder(symbol, meaning!, enclosingDeclaration, nodeFlags)!; // TODO: GH#18217
                const printer = createPrinter({ removeComments: true });
                const sourceFile = enclosingDeclaration && getSourceFileOfNode(enclosingDeclaration);
                printer.writeNode(EmitHint.Unspecified, entity, /*sourceFile*/ sourceFile, writer);
                return writer;
            }
        }

        function signatureToString(signature: Signature, enclosingDeclaration?: Node, flags = TypeFormatFlags.None, kind?: SignatureKind, writer?: EmitTextWriter): string {
            return writer ? signatureToStringWorker(writer).getText() : usingSingleLineStringWriter(signatureToStringWorker);

            function signatureToStringWorker(writer: EmitTextWriter) {
                let sigOutput: SyntaxKind;
                if (flags & TypeFormatFlags.WriteArrowStyleSignature) {
                    sigOutput = kind === SignatureKind.Construct ? SyntaxKind.ConstructorType : SyntaxKind.FunctionType;
                }
                else {
                    sigOutput = kind === SignatureKind.Construct ? SyntaxKind.ConstructSignature : SyntaxKind.CallSignature;
                }
                const sig = nodeBuilder.signatureToSignatureDeclaration(signature, sigOutput, enclosingDeclaration, toNodeBuilderFlags(flags) | NodeBuilderFlags.IgnoreErrors | NodeBuilderFlags.WriteTypeParametersInQualifiedName);
                const printer = createPrinter({ removeComments: true, omitTrailingSemicolon: true });
                const sourceFile = enclosingDeclaration && getSourceFileOfNode(enclosingDeclaration);
                printer.writeNode(EmitHint.Unspecified, sig!, /*sourceFile*/ sourceFile, writer); // TODO: GH#18217
                return writer;
            }
        }

        function typeToString(type: Type, enclosingDeclaration?: Node, flags: TypeFormatFlags = TypeFormatFlags.AllowUniqueESSymbolType | TypeFormatFlags.UseAliasDefinedOutsideCurrentScope, writer: EmitTextWriter = createTextWriter("")): string {
            const noTruncation = compilerOptions.noErrorTruncation || flags & TypeFormatFlags.NoTruncation;
            const typeNode = nodeBuilder.typeToTypeNode(type, enclosingDeclaration, toNodeBuilderFlags(flags) | NodeBuilderFlags.IgnoreErrors | (noTruncation ? NodeBuilderFlags.NoTruncation : 0), writer);
            if (typeNode === undefined) return Debug.fail("should always get typenode");
            const options = { removeComments: true };
            const printer = createPrinter(options);
            const sourceFile = enclosingDeclaration && getSourceFileOfNode(enclosingDeclaration);
            printer.writeNode(EmitHint.Unspecified, typeNode, /*sourceFile*/ sourceFile, writer);
            const result = writer.getText();

            const maxLength = noTruncation ? undefined : defaultMaximumTruncationLength * 2;
            if (maxLength && result && result.length >= maxLength) {
                return result.substr(0, maxLength - "...".length) + "...";
            }
            return result;
        }

        function toNodeBuilderFlags(flags = TypeFormatFlags.None): NodeBuilderFlags {
            return flags & TypeFormatFlags.NodeBuilderFlagsMask;
        }

        function createNodeBuilder() {
            return {
                typeToTypeNode: (type: Type, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => typeToTypeNodeHelper(type, context)),
                indexInfoToIndexSignatureDeclaration: (indexInfo: IndexInfo, kind: IndexKind, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => indexInfoToIndexSignatureDeclarationHelper(indexInfo, kind, context))!, // TODO: GH#18217
                signatureToSignatureDeclaration: (signature: Signature, kind: SyntaxKind, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => signatureToSignatureDeclarationHelper(signature, kind, context)),
                symbolToEntityName: (symbol: Symbol, meaning: SymbolFlags, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => symbolToName(symbol, context, meaning, /*expectsIdentifier*/ false)),
                symbolToExpression: (symbol: Symbol, meaning: SymbolFlags, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => symbolToExpression(symbol, context, meaning)),
                symbolToTypeParameterDeclarations: (symbol: Symbol, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => typeParametersToTypeParameterDeclarations(symbol, context)),
                symbolToParameterDeclaration: (symbol: Symbol, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => symbolToParameterDeclaration(symbol, context)),
                typeParameterToDeclaration: (parameter: TypeParameter, enclosingDeclaration?: Node, flags?: NodeBuilderFlags, tracker?: SymbolTracker) =>
                    withContext(enclosingDeclaration, flags, tracker, context => typeParameterToDeclaration(parameter, context)),
            };

            function withContext<T>(enclosingDeclaration: Node | undefined, flags: NodeBuilderFlags | undefined, tracker: SymbolTracker | undefined, cb: (context: NodeBuilderContext) => T): T | undefined {
                Debug.assert(enclosingDeclaration === undefined || (enclosingDeclaration.flags & NodeFlags.Synthesized) === 0);
                const context: NodeBuilderContext = {
                    enclosingDeclaration,
                    flags: flags || NodeBuilderFlags.None,
                    tracker: tracker && tracker.trackSymbol ? tracker : { trackSymbol: noop },
                    encounteredError: false,
                    visitedSymbols: undefined,
                    inferTypeParameters: undefined,
                    approximateLength: 0
                };
                const resultingNode = cb(context);
                return context.encounteredError ? undefined : resultingNode;
            }

            function checkTruncationLength(context: NodeBuilderContext): boolean {
                if (context.truncating) return context.truncating;
                return context.truncating = !(context.flags & NodeBuilderFlags.NoTruncation) && context.approximateLength > defaultMaximumTruncationLength;
            }

            function typeToTypeNodeHelper(type: Type, context: NodeBuilderContext): TypeNode {
                if (cancellationToken && cancellationToken.throwIfCancellationRequested) {
                    cancellationToken.throwIfCancellationRequested();
                }
                const inTypeAlias = context.flags & NodeBuilderFlags.InTypeAlias;
                context.flags &= ~NodeBuilderFlags.InTypeAlias;

                if (!type) {
                    context.encounteredError = true;
                    return undefined!; // TODO: GH#18217
                }

                if (type.flags & TypeFlags.Any) {
                    context.approximateLength += 3;
                    return createKeywordTypeNode(SyntaxKind.AnyKeyword);
                }
                if (type.flags & TypeFlags.Unknown) {
                    return createKeywordTypeNode(SyntaxKind.UnknownKeyword);
                }
                if (type.flags & TypeFlags.String) {
                    context.approximateLength += 6;
                    return createKeywordTypeNode(SyntaxKind.StringKeyword);
                }
                if (type.flags & TypeFlags.Number) {
                    context.approximateLength += 6;
                    return createKeywordTypeNode(SyntaxKind.NumberKeyword);
                }
                if (type.flags & TypeFlags.Boolean) {
                    context.approximateLength += 7;
                    return createKeywordTypeNode(SyntaxKind.BooleanKeyword);
                }
                if (type.flags & TypeFlags.EnumLiteral && !(type.flags & TypeFlags.Union)) {
                    const parentSymbol = getParentOfSymbol(type.symbol)!;
                    const parentName = symbolToName(parentSymbol, context, SymbolFlags.Type, /*expectsIdentifier*/ false);
                    const enumLiteralName = getDeclaredTypeOfSymbol(parentSymbol) === type ? parentName : createQualifiedName(parentName, symbolName(type.symbol));
                    context.approximateLength += symbolName(type.symbol).length;
                    return createTypeReferenceNode(enumLiteralName, /*typeArguments*/ undefined);
                }
                if (type.flags & TypeFlags.EnumLike) {
                    const name = symbolToName(type.symbol, context, SymbolFlags.Type, /*expectsIdentifier*/ false);
                    context.approximateLength += symbolName(type.symbol).length;
                    return createTypeReferenceNode(name, /*typeArguments*/ undefined);
                }
                if (type.flags & TypeFlags.StringLiteral) {
                    context.approximateLength += ((<StringLiteralType>type).value.length + 2);
                    return createLiteralTypeNode(setEmitFlags(createLiteral((<StringLiteralType>type).value), EmitFlags.NoAsciiEscaping));
                }
                if (type.flags & TypeFlags.NumberLiteral) {
                    context.approximateLength += (("" + (<NumberLiteralType>type).value).length);
                    return createLiteralTypeNode((createLiteral((<NumberLiteralType>type).value)));
                }
                if (type.flags & TypeFlags.BooleanLiteral) {
                    context.approximateLength += (<IntrinsicType>type).intrinsicName.length;
                    return (<IntrinsicType>type).intrinsicName === "true" ? createTrue() : createFalse();
                }
                if (type.flags & TypeFlags.UniqueESSymbol) {
                    if (!(context.flags & NodeBuilderFlags.AllowUniqueESSymbolType)) {
                        if (isValueSymbolAccessible(type.symbol, context.enclosingDeclaration!)) {
                            context.approximateLength += 6;
                            return symbolToTypeNode(type.symbol, context, SymbolFlags.Value);
                        }
                        if (context.tracker.reportInaccessibleUniqueSymbolError) {
                            context.tracker.reportInaccessibleUniqueSymbolError();
                        }
                    }
                    context.approximateLength += 13;
                    return createTypeOperatorNode(SyntaxKind.UniqueKeyword, createKeywordTypeNode(SyntaxKind.SymbolKeyword));
                }
                if (type.flags & TypeFlags.Void) {
                    context.approximateLength += 4;
                    return createKeywordTypeNode(SyntaxKind.VoidKeyword);
                }
                if (type.flags & TypeFlags.Undefined) {
                    context.approximateLength += 9;
                    return createKeywordTypeNode(SyntaxKind.UndefinedKeyword);
                }
                if (type.flags & TypeFlags.Null) {
                    context.approximateLength += 4;
                    return createKeywordTypeNode(SyntaxKind.NullKeyword);
                }
                if (type.flags & TypeFlags.Never) {
                    context.approximateLength += 5;
                    return createKeywordTypeNode(SyntaxKind.NeverKeyword);
                }
                if (type.flags & TypeFlags.ESSymbol) {
                    context.approximateLength += 6;
                    return createKeywordTypeNode(SyntaxKind.SymbolKeyword);
                }
                if (type.flags & TypeFlags.NonPrimitive) {
                    context.approximateLength += 6;
                    return createKeywordTypeNode(SyntaxKind.ObjectKeyword);
                }
                if (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType) {
                    if (context.flags & NodeBuilderFlags.InObjectTypeLiteral) {
                        if (!context.encounteredError && !(context.flags & NodeBuilderFlags.AllowThisInObjectLiteral)) {
                            context.encounteredError = true;
                        }
                        if (context.tracker.reportInaccessibleThisError) {
                            context.tracker.reportInaccessibleThisError();
                        }
                    }
                    context.approximateLength += 4;
                    return createThis();
                }

                const objectFlags = getObjectFlags(type);

                if (objectFlags & ObjectFlags.Reference) {
                    Debug.assert(!!(type.flags & TypeFlags.Object));
                    return typeReferenceToTypeNode(<TypeReference>type);
                }
                if (type.flags & TypeFlags.TypeParameter || objectFlags & ObjectFlags.ClassOrInterface) {
                    if (type.flags & TypeFlags.TypeParameter && contains(context.inferTypeParameters, type)) {
                        context.approximateLength += (symbolName(type.symbol).length + 6);
                        return createInferTypeNode(typeParameterToDeclarationWithConstraint(type as TypeParameter, context, /*constraintNode*/ undefined));
                    }
                    if (context.flags & NodeBuilderFlags.GenerateNamesForShadowedTypeParams &&
                        type.flags & TypeFlags.TypeParameter &&
                        length(type.symbol.declarations) &&
                        isTypeParameterDeclaration(type.symbol.declarations[0]) &&
                        typeParameterShadowsNameInScope(type, context) &&
                        !isTypeSymbolAccessible(type.symbol, context.enclosingDeclaration)) {
                        const name = (type.symbol.declarations[0] as TypeParameterDeclaration).name;
                        context.approximateLength += idText(name).length;
                        return createTypeReferenceNode(getGeneratedNameForNode(name, GeneratedIdentifierFlags.Optimistic | GeneratedIdentifierFlags.ReservedInNestedScopes), /*typeArguments*/ undefined);
                    }
                    // Ignore constraint/default when creating a usage (as opposed to declaration) of a type parameter.
                    return type.symbol
                        ? symbolToTypeNode(type.symbol, context, SymbolFlags.Type)
                        : createTypeReferenceNode(createIdentifier("?"), /*typeArguments*/ undefined);
                }
                if (!inTypeAlias && type.aliasSymbol && (context.flags & NodeBuilderFlags.UseAliasDefinedOutsideCurrentScope || isTypeSymbolAccessible(type.aliasSymbol, context.enclosingDeclaration))) {
                    const typeArgumentNodes = mapToTypeNodes(type.aliasTypeArguments, context);
                    if (isReservedMemberName(type.aliasSymbol.escapedName) && !(type.aliasSymbol.flags & SymbolFlags.Class)) return createTypeReferenceNode(createIdentifier(""), typeArgumentNodes);
                    return symbolToTypeNode(type.aliasSymbol, context, SymbolFlags.Type, typeArgumentNodes);
                }
                if (type.flags & (TypeFlags.Union | TypeFlags.Intersection)) {
                    const types = type.flags & TypeFlags.Union ? formatUnionTypes((<UnionType>type).types) : (<IntersectionType>type).types;
                    const typeNodes = mapToTypeNodes(types, context, /*isBareList*/ true);
                    if (typeNodes && typeNodes.length > 0) {
                        const unionOrIntersectionTypeNode = createUnionOrIntersectionTypeNode(type.flags & TypeFlags.Union ? SyntaxKind.UnionType : SyntaxKind.IntersectionType, typeNodes);
                        return unionOrIntersectionTypeNode;
                    }
                    else {
                        if (!context.encounteredError && !(context.flags & NodeBuilderFlags.AllowEmptyUnionOrIntersection)) {
                            context.encounteredError = true;
                        }
                        return undefined!; // TODO: GH#18217
                    }
                }
                if (objectFlags & (ObjectFlags.Anonymous | ObjectFlags.Mapped)) {
                    Debug.assert(!!(type.flags & TypeFlags.Object));
                    // The type is an object literal type.
                    return createAnonymousTypeNode(<ObjectType>type);
                }
                if (type.flags & TypeFlags.Index) {
                    const indexedType = (<IndexType>type).type;
                    context.approximateLength += 6;
                    const indexTypeNode = typeToTypeNodeHelper(indexedType, context);
                    return createTypeOperatorNode(indexTypeNode);
                }
                if (type.flags & TypeFlags.IndexedAccess) {
                    const objectTypeNode = typeToTypeNodeHelper((<IndexedAccessType>type).objectType, context);
                    const indexTypeNode = typeToTypeNodeHelper((<IndexedAccessType>type).indexType, context);
                    context.approximateLength += 2;
                    return createIndexedAccessTypeNode(objectTypeNode, indexTypeNode);
                }
                if (type.flags & TypeFlags.Conditional) {
                    const checkTypeNode = typeToTypeNodeHelper((<ConditionalType>type).checkType, context);
                    const saveInferTypeParameters = context.inferTypeParameters;
                    context.inferTypeParameters = (<ConditionalType>type).root.inferTypeParameters;
                    const extendsTypeNode = typeToTypeNodeHelper((<ConditionalType>type).extendsType, context);
                    context.inferTypeParameters = saveInferTypeParameters;
                    const trueTypeNode = typeToTypeNodeHelper(getTrueTypeFromConditionalType(<ConditionalType>type), context);
                    const falseTypeNode = typeToTypeNodeHelper(getFalseTypeFromConditionalType(<ConditionalType>type), context);
                    context.approximateLength += 15;
                    return createConditionalTypeNode(checkTypeNode, extendsTypeNode, trueTypeNode, falseTypeNode);
                }
                if (type.flags & TypeFlags.Substitution) {
                    return typeToTypeNodeHelper((<SubstitutionType>type).typeVariable, context);
                }

                return Debug.fail("Should be unreachable.");

                function createMappedTypeNodeFromType(type: MappedType) {
                    Debug.assert(!!(type.flags & TypeFlags.Object));
                    const readonlyToken = type.declaration.readonlyToken ? <ReadonlyToken | PlusToken | MinusToken>createToken(type.declaration.readonlyToken.kind) : undefined;
                    const questionToken = type.declaration.questionToken ? <QuestionToken | PlusToken | MinusToken>createToken(type.declaration.questionToken.kind) : undefined;
                    let appropriateConstraintTypeNode: TypeNode;
                    if (isMappedTypeWithKeyofConstraintDeclaration(type)) {
                        // We have a { [P in keyof T]: X }
                        // We do this to ensure we retain the toplevel keyof-ness of the type which may be lost due to keyof distribution during `getConstraintTypeFromMappedType`
                        appropriateConstraintTypeNode = createTypeOperatorNode(typeToTypeNodeHelper(getModifiersTypeFromMappedType(type), context));
                    }
                    else {
                        appropriateConstraintTypeNode = typeToTypeNodeHelper(getConstraintTypeFromMappedType(type), context);
                    }
                    const typeParameterNode = typeParameterToDeclarationWithConstraint(getTypeParameterFromMappedType(type), context, appropriateConstraintTypeNode);
                    const templateTypeNode = typeToTypeNodeHelper(getTemplateTypeFromMappedType(type), context);
                    const mappedTypeNode = createMappedTypeNode(readonlyToken, typeParameterNode, questionToken, templateTypeNode);
                    context.approximateLength += 10;
                    return setEmitFlags(mappedTypeNode, EmitFlags.SingleLine);
                }

                function createAnonymousTypeNode(type: ObjectType): TypeNode {
                    const symbol = type.symbol;
                    let id: string;
                    if (symbol) {
                        const isConstructorObject = getObjectFlags(type) & ObjectFlags.Anonymous && type.symbol && type.symbol.flags & SymbolFlags.Class;
                        id = (isConstructorObject ? "+" : "") + getSymbolId(symbol);
                        if (isJavascriptConstructor(symbol.valueDeclaration)) {
                            // Instance and static types share the same symbol; only add 'typeof' for the static side.
                            const isInstanceType = type === getInferredClassType(symbol) ? SymbolFlags.Type : SymbolFlags.Value;
                            return symbolToTypeNode(symbol, context, isInstanceType);
                        }
                        // Always use 'typeof T' for type of class, enum, and module objects
                        else if (symbol.flags & SymbolFlags.Class && !getBaseTypeVariableOfClass(symbol) && !(symbol.valueDeclaration.kind === SyntaxKind.ClassExpression && context.flags & NodeBuilderFlags.WriteClassExpressionAsTypeLiteral) ||
                            symbol.flags & (SymbolFlags.Enum | SymbolFlags.ValueModule) ||
                            shouldWriteTypeOfFunctionSymbol()) {
                            return symbolToTypeNode(symbol, context, SymbolFlags.Value);
                        }
                        else if (context.visitedSymbols && context.visitedSymbols.has(id)) {
                            // If type is an anonymous type literal in a type alias declaration, use type alias name
                            const typeAlias = getTypeAliasForTypeLiteral(type);
                            if (typeAlias) {
                                // The specified symbol flags need to be reinterpreted as type flags
                                return symbolToTypeNode(typeAlias, context, SymbolFlags.Type);
                            }
                            else {
                                context.approximateLength += 3;
                                return createKeywordTypeNode(SyntaxKind.AnyKeyword);
                            }
                        }
                        else {
                            // Since instantiations of the same anonymous type have the same symbol, tracking symbols instead
                            // of types allows us to catch circular references to instantiations of the same anonymous type
                            if (!context.visitedSymbols) {
                                context.visitedSymbols = createMap<true>();
                            }

                            context.visitedSymbols.set(id, true);
                            const result = createTypeNodeFromObjectType(type);
                            context.visitedSymbols.delete(id);
                            return result;
                        }
                    }
                    else {
                        // Anonymous types without a symbol are never circular.
                        return createTypeNodeFromObjectType(type);
                    }
                    function shouldWriteTypeOfFunctionSymbol() {
                        const isStaticMethodSymbol = !!(symbol.flags & SymbolFlags.Method) &&  // typeof static method
                            some(symbol.declarations, declaration => hasModifier(declaration, ModifierFlags.Static));
                        const isNonLocalFunctionSymbol = !!(symbol.flags & SymbolFlags.Function) &&
                            (symbol.parent || // is exported function symbol
                                forEach(symbol.declarations, declaration =>
                                    declaration.parent.kind === SyntaxKind.SourceFile || declaration.parent.kind === SyntaxKind.ModuleBlock));
                        if (isStaticMethodSymbol || isNonLocalFunctionSymbol) {
                            // typeof is allowed only for static/non local functions
                            return (!!(context.flags & NodeBuilderFlags.UseTypeOfFunction) || (context.visitedSymbols && context.visitedSymbols.has(id))) && // it is type of the symbol uses itself recursively
                                (!(context.flags & NodeBuilderFlags.UseStructuralFallback) || isValueSymbolAccessible(symbol, context.enclosingDeclaration!)); // TODO: GH#18217 // And the build is going to succeed without visibility error or there is no structural fallback allowed
                        }
                    }
                }

                function createTypeNodeFromObjectType(type: ObjectType): TypeNode {
                    if (isGenericMappedType(type)) {
                        return createMappedTypeNodeFromType(type);
                    }

                    const resolved = resolveStructuredTypeMembers(type);
                    if (!resolved.properties.length && !resolved.stringIndexInfo && !resolved.numberIndexInfo) {
                        if (!resolved.callSignatures.length && !resolved.constructSignatures.length) {
                            context.approximateLength += 2;
                            return setEmitFlags(createTypeLiteralNode(/*members*/ undefined), EmitFlags.SingleLine);
                        }

                        if (resolved.callSignatures.length === 1 && !resolved.constructSignatures.length) {
                            const signature = resolved.callSignatures[0];
                            const signatureNode = <FunctionTypeNode>signatureToSignatureDeclarationHelper(signature, SyntaxKind.FunctionType, context);
                            return signatureNode;

                        }

                        if (resolved.constructSignatures.length === 1 && !resolved.callSignatures.length) {
                            const signature = resolved.constructSignatures[0];
                            const signatureNode = <ConstructorTypeNode>signatureToSignatureDeclarationHelper(signature, SyntaxKind.ConstructorType, context);
                            return signatureNode;
                        }
                    }

                    const savedFlags = context.flags;
                    context.flags |= NodeBuilderFlags.InObjectTypeLiteral;
                    const members = createTypeNodesFromResolvedType(resolved);
                    context.flags = savedFlags;
                    const typeLiteralNode = createTypeLiteralNode(members);
                    context.approximateLength += 2;
                    return setEmitFlags(typeLiteralNode, (context.flags & NodeBuilderFlags.MultilineObjectLiterals) ? 0 : EmitFlags.SingleLine);
                }

                function typeReferenceToTypeNode(type: TypeReference) {
                    const typeArguments: ReadonlyArray<Type> = type.typeArguments || emptyArray;
                    if (type.target === globalArrayType) {
                        if (context.flags & NodeBuilderFlags.WriteArrayAsGenericType) {
                            const typeArgumentNode = typeToTypeNodeHelper(typeArguments[0], context);
                            return createTypeReferenceNode("Array", [typeArgumentNode]);
                        }

                        const elementType = typeToTypeNodeHelper(typeArguments[0], context);
                        return createArrayTypeNode(elementType);
                    }
                    else if (type.target.objectFlags & ObjectFlags.Tuple) {
                        if (typeArguments.length > 0) {
                            const arity = getTypeReferenceArity(type);
                            const tupleConstituentNodes = mapToTypeNodes(typeArguments.slice(0, arity), context);
                            const hasRestElement = (<TupleType>type.target).hasRestElement;
                            if (tupleConstituentNodes && tupleConstituentNodes.length > 0) {
                                for (let i = (<TupleType>type.target).minLength; i < arity; i++) {
                                    tupleConstituentNodes[i] = hasRestElement && i === arity - 1 ?
                                        createRestTypeNode(createArrayTypeNode(tupleConstituentNodes[i])) :
                                        createOptionalTypeNode(tupleConstituentNodes[i]);
                                }
                                return createTupleTypeNode(tupleConstituentNodes);
                            }
                        }
                        if (context.encounteredError || (context.flags & NodeBuilderFlags.AllowEmptyTuple)) {
                            return createTupleTypeNode([]);
                        }
                        context.encounteredError = true;
                        return undefined!; // TODO: GH#18217
                    }
                    else if (context.flags & NodeBuilderFlags.WriteClassExpressionAsTypeLiteral &&
                        type.symbol.valueDeclaration &&
                        isClassLike(type.symbol.valueDeclaration) &&
                        !isValueSymbolAccessible(type.symbol, context.enclosingDeclaration!)
                    ) {
                        return createAnonymousTypeNode(type);
                    }
                    else {
                        const outerTypeParameters = type.target.outerTypeParameters;
                        let i = 0;
                        let resultType: TypeReferenceNode | ImportTypeNode | undefined;
                        if (outerTypeParameters) {
                            const length = outerTypeParameters.length;
                            while (i < length) {
                                // Find group of type arguments for type parameters with the same declaring container.
                                const start = i;
                                const parent = getParentSymbolOfTypeParameter(outerTypeParameters[i])!;
                                do {
                                    i++;
                                } while (i < length && getParentSymbolOfTypeParameter(outerTypeParameters[i]) === parent);
                                // When type parameters are their own type arguments for the whole group (i.e. we have
                                // the default outer type arguments), we don't show the group.
                                if (!rangeEquals(outerTypeParameters, typeArguments, start, i)) {
                                    const typeArgumentSlice = mapToTypeNodes(typeArguments.slice(start, i), context);
                                    const flags = context.flags;
                                    context.flags |= NodeBuilderFlags.ForbidIndexedAccessSymbolReferences;
                                    const ref = symbolToTypeNode(parent, context, SymbolFlags.Type, typeArgumentSlice) as TypeReferenceNode | ImportTypeNode;
                                    context.flags = flags;
                                    resultType = !resultType ? ref : appendReferenceToType(resultType, ref as TypeReferenceNode);
                                }
                            }
                        }
                        let typeArgumentNodes: ReadonlyArray<TypeNode> | undefined;
                        if (typeArguments.length > 0) {
                            const typeParameterCount = (type.target.typeParameters || emptyArray).length;
                            typeArgumentNodes = mapToTypeNodes(typeArguments.slice(i, typeParameterCount), context);
                        }
                        const flags = context.flags;
                        context.flags |= NodeBuilderFlags.ForbidIndexedAccessSymbolReferences;
                        const finalRef = symbolToTypeNode(type.symbol, context, SymbolFlags.Type, typeArgumentNodes);
                        context.flags = flags;
                        return !resultType ? finalRef : appendReferenceToType(resultType, finalRef as TypeReferenceNode);
                    }
                }


                function appendReferenceToType(root: TypeReferenceNode | ImportTypeNode, ref: TypeReferenceNode): TypeReferenceNode | ImportTypeNode {
                    if (isImportTypeNode(root)) {
                        // first shift type arguments
                        const innerParams = root.typeArguments;
                        if (root.qualifier) {
                            (isIdentifier(root.qualifier) ? root.qualifier : root.qualifier.right).typeArguments = innerParams;
                        }
                        root.typeArguments = ref.typeArguments;
                        // then move qualifiers
                        const ids = getAccessStack(ref);
                        for (const id of ids) {
                            root.qualifier = root.qualifier ? createQualifiedName(root.qualifier, id) : id;
                        }
                        return root;
                    }
                    else {
                        // first shift type arguments
                        const innerParams = root.typeArguments;
                        (isIdentifier(root.typeName) ? root.typeName : root.typeName.right).typeArguments = innerParams;
                        root.typeArguments = ref.typeArguments;
                        // then move qualifiers
                        const ids = getAccessStack(ref);
                        for (const id of ids) {
                            root.typeName = createQualifiedName(root.typeName, id);
                        }
                        return root;
                    }
                }

                function getAccessStack(ref: TypeReferenceNode): Identifier[] {
                    let state = ref.typeName;
                    const ids = [];
                    while (!isIdentifier(state)) {
                        ids.unshift(state.right);
                        state = state.left;
                    }
                    ids.unshift(state);
                    return ids;
                }

                function createTypeNodesFromResolvedType(resolvedType: ResolvedType): TypeElement[] | undefined {
                    if (checkTruncationLength(context)) {
                        return [createPropertySignature(/*modifiers*/ undefined, "...", /*questionToken*/ undefined, /*type*/ undefined, /*initializer*/ undefined)];
                    }
                    const typeElements: TypeElement[] = [];
                    for (const signature of resolvedType.callSignatures) {
                        typeElements.push(<CallSignatureDeclaration>signatureToSignatureDeclarationHelper(signature, SyntaxKind.CallSignature, context));
                    }
                    for (const signature of resolvedType.constructSignatures) {
                        typeElements.push(<ConstructSignatureDeclaration>signatureToSignatureDeclarationHelper(signature, SyntaxKind.ConstructSignature, context));
                    }
                    if (resolvedType.stringIndexInfo) {
                        const indexInfo = resolvedType.objectFlags & ObjectFlags.ReverseMapped ?
                            createIndexInfo(anyType, resolvedType.stringIndexInfo.isReadonly, resolvedType.stringIndexInfo.declaration) :
                            resolvedType.stringIndexInfo;
                        typeElements.push(indexInfoToIndexSignatureDeclarationHelper(indexInfo, IndexKind.String, context));
                    }
                    if (resolvedType.numberIndexInfo) {
                        typeElements.push(indexInfoToIndexSignatureDeclarationHelper(resolvedType.numberIndexInfo, IndexKind.Number, context));
                    }

                    const properties = resolvedType.properties;
                    if (!properties) {
                        return typeElements;
                    }

                    let i = 0;
                    for (const propertySymbol of properties) {
                        i++;
                        if (context.flags & NodeBuilderFlags.WriteClassExpressionAsTypeLiteral) {
                            if (propertySymbol.flags & SymbolFlags.Prototype) {
                                continue;
                            }
                            if (getDeclarationModifierFlagsFromSymbol(propertySymbol) & (ModifierFlags.Private | ModifierFlags.Protected) && context.tracker.reportPrivateInBaseOfClassExpression) {
                                context.tracker.reportPrivateInBaseOfClassExpression(unescapeLeadingUnderscores(propertySymbol.escapedName));
                            }
                        }
                        if (checkTruncationLength(context) && (i + 2 < properties.length - 1)) {
                            typeElements.push(createPropertySignature(/*modifiers*/ undefined, `... ${properties.length - i} more ...`, /*questionToken*/ undefined, /*type*/ undefined, /*initializer*/ undefined));
                            addPropertyToElementList(properties[properties.length - 1], context, typeElements);
                            break;
                        }
                        addPropertyToElementList(propertySymbol, context, typeElements);

                    }
                    return typeElements.length ? typeElements : undefined;
                }
            }

            function addPropertyToElementList(propertySymbol: Symbol, context: NodeBuilderContext, typeElements: TypeElement[]) {
                const propertyType = getCheckFlags(propertySymbol) & CheckFlags.ReverseMapped && context.flags & NodeBuilderFlags.InReverseMappedType ?
                    anyType : getTypeOfSymbol(propertySymbol);
                const saveEnclosingDeclaration = context.enclosingDeclaration;
                context.enclosingDeclaration = undefined;
                if (context.tracker.trackSymbol && getCheckFlags(propertySymbol) & CheckFlags.Late) {
                    const decl = first(propertySymbol.declarations);
                    if (hasLateBindableName(decl)) {
                        trackComputedName(decl.name, saveEnclosingDeclaration, context);
                    }
                }
                const propertyName = symbolToName(propertySymbol, context, SymbolFlags.Value, /*expectsIdentifier*/ true);
                context.approximateLength += (symbolName(propertySymbol).length + 1);
                context.enclosingDeclaration = saveEnclosingDeclaration;
                const optionalToken = propertySymbol.flags & SymbolFlags.Optional ? createToken(SyntaxKind.QuestionToken) : undefined;
                if (propertySymbol.flags & (SymbolFlags.Function | SymbolFlags.Method) && !getPropertiesOfObjectType(propertyType).length) {
                    const signatures = getSignaturesOfType(propertyType, SignatureKind.Call);
                    for (const signature of signatures) {
                        const methodDeclaration = <MethodSignature>signatureToSignatureDeclarationHelper(signature, SyntaxKind.MethodSignature, context);
                        methodDeclaration.name = propertyName;
                        methodDeclaration.questionToken = optionalToken;
                        if (propertySymbol.valueDeclaration) {
                            // Copy comments to node for declaration emit
                            setCommentRange(methodDeclaration, propertySymbol.valueDeclaration);
                        }
                        typeElements.push(methodDeclaration);
                    }
                }
                else {
                    const savedFlags = context.flags;
                    context.flags |= !!(getCheckFlags(propertySymbol) & CheckFlags.ReverseMapped) ? NodeBuilderFlags.InReverseMappedType : 0;
                    const propertyTypeNode = propertyType ? typeToTypeNodeHelper(propertyType, context) : createKeywordTypeNode(SyntaxKind.AnyKeyword);
                    context.flags = savedFlags;

                    const modifiers = isReadonlySymbol(propertySymbol) ? [createToken(SyntaxKind.ReadonlyKeyword)] : undefined;
                    if (modifiers) {
                        context.approximateLength += 9;
                    }
                    const propertySignature = createPropertySignature(
                        modifiers,
                        propertyName,
                        optionalToken,
                        propertyTypeNode,
                    /*initializer*/ undefined);
                    if (propertySymbol.valueDeclaration) {
                        // Copy comments to node for declaration emit
                        setCommentRange(propertySignature, propertySymbol.valueDeclaration);
                    }
                    typeElements.push(propertySignature);
                }
            }

            function mapToTypeNodes(types: ReadonlyArray<Type> | undefined, context: NodeBuilderContext, isBareList?: boolean): TypeNode[] | undefined {
                if (some(types)) {
                    if (checkTruncationLength(context)) {
                        if (!isBareList) {
                            return [createTypeReferenceNode("...", /*typeArguments*/ undefined)];
                        }
                        else if (types.length > 2) {
                            return [
                                typeToTypeNodeHelper(types[0], context),
                                createTypeReferenceNode(`... ${types.length - 2} more ...`, /*typeArguments*/ undefined),
                                typeToTypeNodeHelper(types[types.length - 1], context)
                            ];
                        }
                    }
                    const result = [];
                    let i = 0;
                    for (const type of types) {
                        i++;
                        if (checkTruncationLength(context) && (i + 2 < types.length - 1)) {
                            result.push(createTypeReferenceNode(`... ${types.length - i} more ...`, /*typeArguments*/ undefined));
                            const typeNode = typeToTypeNodeHelper(types[types.length - 1], context);
                            if (typeNode) {
                                result.push(typeNode);
                            }
                            break;
                        }
                        context.approximateLength += 2; // Account for whitespace + separator
                        const typeNode = typeToTypeNodeHelper(type, context);
                        if (typeNode) {
                            result.push(typeNode);
                        }
                    }

                    return result;
                }
            }

            function indexInfoToIndexSignatureDeclarationHelper(indexInfo: IndexInfo, kind: IndexKind, context: NodeBuilderContext): IndexSignatureDeclaration {
                const name = getNameFromIndexInfo(indexInfo) || "x";
                const indexerTypeNode = createKeywordTypeNode(kind === IndexKind.String ? SyntaxKind.StringKeyword : SyntaxKind.NumberKeyword);

                const indexingParameter = createParameter(
                    /*decorators*/ undefined,
                    /*modifiers*/ undefined,
                    /*dotDotDotToken*/ undefined,
                    name,
                    /*questionToken*/ undefined,
                    indexerTypeNode,
                    /*initializer*/ undefined);
                const typeNode = typeToTypeNodeHelper(indexInfo.type || anyType, context);
                if (!indexInfo.type && !(context.flags & NodeBuilderFlags.AllowEmptyIndexInfoType)) {
                    context.encounteredError = true;
                }
                context.approximateLength += (name.length + 4);
                return createIndexSignature(
                    /*decorators*/ undefined,
                    indexInfo.isReadonly ? [createToken(SyntaxKind.ReadonlyKeyword)] : undefined,
                    [indexingParameter],
                    typeNode);
            }

            function signatureToSignatureDeclarationHelper(signature: Signature, kind: SyntaxKind, context: NodeBuilderContext): SignatureDeclaration {
                let typeParameters: TypeParameterDeclaration[] | undefined;
                let typeArguments: TypeNode[] | undefined;
                if (context.flags & NodeBuilderFlags.WriteTypeArgumentsOfSignature && signature.target && signature.mapper && signature.target.typeParameters) {
                    typeArguments = signature.target.typeParameters.map(parameter => typeToTypeNodeHelper(instantiateType(parameter, signature.mapper!), context));
                }
                else {
                    typeParameters = signature.typeParameters && signature.typeParameters.map(parameter => typeParameterToDeclaration(parameter, context));
                }

                const parameters = getExpandedParameters(signature).map(parameter => symbolToParameterDeclaration(parameter, context, kind === SyntaxKind.Constructor));
                if (signature.thisParameter) {
                    const thisParameter = symbolToParameterDeclaration(signature.thisParameter, context);
                    parameters.unshift(thisParameter);
                }

                let returnTypeNode: TypeNode | undefined;
                const typePredicate = getTypePredicateOfSignature(signature);
                if (typePredicate) {
                    const parameterName = typePredicate.kind === TypePredicateKind.Identifier ?
                        setEmitFlags(createIdentifier(typePredicate.parameterName), EmitFlags.NoAsciiEscaping) :
                        createThisTypeNode();
                    const typeNode = typeToTypeNodeHelper(typePredicate.type, context);
                    returnTypeNode = createTypePredicateNode(parameterName, typeNode);
                }
                else {
                    const returnType = getReturnTypeOfSignature(signature);
                    returnTypeNode = returnType && typeToTypeNodeHelper(returnType, context);
                }
                if (context.flags & NodeBuilderFlags.SuppressAnyReturnType) {
                    if (returnTypeNode && returnTypeNode.kind === SyntaxKind.AnyKeyword) {
                        returnTypeNode = undefined;
                    }
                }
                else if (!returnTypeNode) {
                    returnTypeNode = createKeywordTypeNode(SyntaxKind.AnyKeyword);
                }
                context.approximateLength += 3; // Usually a signature contributes a few more characters than this, but 3 is the minimum
                return createSignatureDeclaration(kind, typeParameters, parameters, returnTypeNode, typeArguments);
            }

            function typeParameterShadowsNameInScope(type: TypeParameter, context: NodeBuilderContext) {
                return !!resolveName(context.enclosingDeclaration, type.symbol.escapedName, SymbolFlags.Type, /*nameNotFoundArg*/ undefined, type.symbol.escapedName, /*isUse*/ false);
            }

            function typeParameterToDeclarationWithConstraint(type: TypeParameter, context: NodeBuilderContext, constraintNode: TypeNode | undefined): TypeParameterDeclaration {
                const savedContextFlags = context.flags;
                context.flags &= ~NodeBuilderFlags.WriteTypeParametersInQualifiedName; // Avoids potential infinite loop when building for a claimspace with a generic
                const shouldUseGeneratedName =
                    context.flags & NodeBuilderFlags.GenerateNamesForShadowedTypeParams &&
                    type.symbol.declarations[0] &&
                    isTypeParameterDeclaration(type.symbol.declarations[0]) &&
                    typeParameterShadowsNameInScope(type, context);
                const name = shouldUseGeneratedName
                    ? getGeneratedNameForNode((type.symbol.declarations[0] as TypeParameterDeclaration).name, GeneratedIdentifierFlags.Optimistic | GeneratedIdentifierFlags.ReservedInNestedScopes)
                    : symbolToName(type.symbol, context, SymbolFlags.Type, /*expectsIdentifier*/ true);
                const defaultParameter = getDefaultFromTypeParameter(type);
                const defaultParameterNode = defaultParameter && typeToTypeNodeHelper(defaultParameter, context);
                context.flags = savedContextFlags;
                return createTypeParameterDeclaration(name, constraintNode, defaultParameterNode);
            }

            function typeParameterToDeclaration(type: TypeParameter, context: NodeBuilderContext, constraint = getConstraintOfTypeParameter(type)): TypeParameterDeclaration {
                const constraintNode = constraint && typeToTypeNodeHelper(constraint, context);
                return typeParameterToDeclarationWithConstraint(type, context, constraintNode);
            }

            function symbolToParameterDeclaration(parameterSymbol: Symbol, context: NodeBuilderContext, preserveModifierFlags?: boolean): ParameterDeclaration {
                let parameterDeclaration: ParameterDeclaration | JSDocParameterTag | undefined = getDeclarationOfKind<ParameterDeclaration>(parameterSymbol, SyntaxKind.Parameter);
                if (!parameterDeclaration && !isTransientSymbol(parameterSymbol)) {
                    parameterDeclaration = getDeclarationOfKind<JSDocParameterTag>(parameterSymbol, SyntaxKind.JSDocParameterTag);
                }

                let parameterType = getTypeOfSymbol(parameterSymbol);
                if (parameterDeclaration && isRequiredInitializedParameter(parameterDeclaration)) {
                    parameterType = getOptionalType(parameterType);
                }
                const parameterTypeNode = typeToTypeNodeHelper(parameterType, context);

                const modifiers = !(context.flags & NodeBuilderFlags.OmitParameterModifiers) && preserveModifierFlags && parameterDeclaration && parameterDeclaration.modifiers ? parameterDeclaration.modifiers.map(getSynthesizedClone) : undefined;
                const isRest = parameterDeclaration && isRestParameter(parameterDeclaration) || getCheckFlags(parameterSymbol) & CheckFlags.RestParameter;
                const dotDotDotToken = isRest ? createToken(SyntaxKind.DotDotDotToken) : undefined;
                const name = parameterDeclaration
                    ? parameterDeclaration.name ?
                            parameterDeclaration.name.kind === SyntaxKind.Identifier ? setEmitFlags(getSynthesizedClone(parameterDeclaration.name), EmitFlags.NoAsciiEscaping) :
                            parameterDeclaration.name.kind === SyntaxKind.QualifiedName ? setEmitFlags(getSynthesizedClone(parameterDeclaration.name.right), EmitFlags.NoAsciiEscaping) :
                            cloneBindingName(parameterDeclaration.name) :
                        symbolName(parameterSymbol)
                    : symbolName(parameterSymbol);
                const isOptional = parameterDeclaration && isOptionalParameter(parameterDeclaration) || getCheckFlags(parameterSymbol) & CheckFlags.OptionalParameter;
                const questionToken = isOptional ? createToken(SyntaxKind.QuestionToken) : undefined;
                const parameterNode = createParameter(
                    /*decorators*/ undefined,
                    modifiers,
                    dotDotDotToken,
                    name,
                    questionToken,
                    parameterTypeNode,
                    /*initializer*/ undefined);
                context.approximateLength += symbolName(parameterSymbol).length + 3;
                return parameterNode;

                function cloneBindingName(node: BindingName): BindingName {
                    return <BindingName>elideInitializerAndSetEmitFlags(node);
                    function elideInitializerAndSetEmitFlags(node: Node): Node {
                        if (context.tracker.trackSymbol && isComputedPropertyName(node) && isLateBindableName(node)) {
                            trackComputedName(node, context.enclosingDeclaration, context);
                        }
                        const visited = visitEachChild(node, elideInitializerAndSetEmitFlags, nullTransformationContext, /*nodesVisitor*/ undefined, elideInitializerAndSetEmitFlags)!;
                        const clone = nodeIsSynthesized(visited) ? visited : getSynthesizedClone(visited);
                        if (clone.kind === SyntaxKind.BindingElement) {
                            (<BindingElement>clone).initializer = undefined;
                        }
                        return setEmitFlags(clone, EmitFlags.SingleLine | EmitFlags.NoAsciiEscaping);
                    }
                }
            }

            function trackComputedName(node: LateBoundName, enclosingDeclaration: Node | undefined, context: NodeBuilderContext) {
                if (!context.tracker.trackSymbol) return;
                // get symbol of the first identifier of the entityName
                const firstIdentifier = getFirstIdentifier(node.expression);
                const name = resolveName(firstIdentifier, firstIdentifier.escapedText, SymbolFlags.Value | SymbolFlags.ExportValue, /*nodeNotFoundErrorMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ true);
                if (name) {
                    context.tracker.trackSymbol(name, enclosingDeclaration, SymbolFlags.Value);
                }
            }

            function lookupSymbolChain(symbol: Symbol, context: NodeBuilderContext, meaning: SymbolFlags, yieldModuleSymbol?: boolean) {
                context.tracker.trackSymbol!(symbol, context.enclosingDeclaration, meaning); // TODO: GH#18217
                // Try to get qualified name if the symbol is not a type parameter and there is an enclosing declaration.
                let chain: Symbol[];
                const isTypeParameter = symbol.flags & SymbolFlags.TypeParameter;
                if (!isTypeParameter && (context.enclosingDeclaration || context.flags & NodeBuilderFlags.UseFullyQualifiedType)) {
                    chain = Debug.assertDefined(getSymbolChain(symbol, meaning, /*endOfChain*/ true));
                    Debug.assert(chain && chain.length > 0);
                }
                else {
                    chain = [symbol];
                }
                return chain;

                /** @param endOfChain Set to false for recursive calls; non-recursive calls should always output something. */
                function getSymbolChain(symbol: Symbol, meaning: SymbolFlags, endOfChain: boolean): Symbol[] | undefined {
                    let accessibleSymbolChain = getAccessibleSymbolChain(symbol, context.enclosingDeclaration, meaning, !!(context.flags & NodeBuilderFlags.UseOnlyExternalAliasing));

                    if (!accessibleSymbolChain ||
                        needsQualification(accessibleSymbolChain[0], context.enclosingDeclaration, accessibleSymbolChain.length === 1 ? meaning : getQualifiedLeftMeaning(meaning))) {

                        // Go up and add our parent.
                        const parents = getContainersOfSymbol(accessibleSymbolChain ? accessibleSymbolChain[0] : symbol, context.enclosingDeclaration);
                        if (length(parents)) {
                            for (const parent of parents!) {
                                const parentChain = getSymbolChain(parent, getQualifiedLeftMeaning(meaning), /*endOfChain*/ false);
                                if (parentChain) {
                                    accessibleSymbolChain = parentChain.concat(accessibleSymbolChain || [getAliasForSymbolInContainer(parent, symbol) || symbol]);
                                    break;
                                }
                            }
                        }
                    }

                    if (accessibleSymbolChain) {
                        return accessibleSymbolChain;
                    }
                    if (
                        // If this is the last part of outputting the symbol, always output. The cases apply only to parent symbols.
                        endOfChain ||
                        // If a parent symbol is an anonymous type, don't write it.
                        !(symbol.flags & (SymbolFlags.TypeLiteral | SymbolFlags.ObjectLiteral))) {
                        // If a parent symbol is an external module, don't write it. (We prefer just `x` vs `"foo/bar".x`.)
                        if (!endOfChain && !yieldModuleSymbol && !!forEach(symbol.declarations, hasNonGlobalAugmentationExternalModuleSymbol)) {
                            return;
                        }
                        return [symbol];
                    }
                }
            }

            function typeParametersToTypeParameterDeclarations(symbol: Symbol, context: NodeBuilderContext) {
                let typeParameterNodes: NodeArray<TypeParameterDeclaration> | undefined;
                const targetSymbol = getTargetSymbol(symbol);
                if (targetSymbol.flags & (SymbolFlags.Class | SymbolFlags.Interface | SymbolFlags.TypeAlias)) {
                    typeParameterNodes = createNodeArray(map(getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol), tp => typeParameterToDeclaration(tp, context)));
                }
                return typeParameterNodes;
            }

            function lookupTypeParameterNodes(chain: Symbol[], index: number, context: NodeBuilderContext) {
                Debug.assert(chain && 0 <= index && index < chain.length);
                const symbol = chain[index];
                let typeParameterNodes: ReadonlyArray<TypeNode> | ReadonlyArray<TypeParameterDeclaration> | undefined;
                if (context.flags & NodeBuilderFlags.WriteTypeParametersInQualifiedName && index < (chain.length - 1)) {
                    const parentSymbol = symbol;
                    const nextSymbol = chain[index + 1];
                    if (getCheckFlags(nextSymbol) & CheckFlags.Instantiated) {
                        const params = getTypeParametersOfClassOrInterface(
                            parentSymbol.flags & SymbolFlags.Alias ? resolveAlias(parentSymbol) : parentSymbol
                        );
                        typeParameterNodes = mapToTypeNodes(map(params, (nextSymbol as TransientSymbol).mapper!), context);
                    }
                    else {
                        typeParameterNodes = typeParametersToTypeParameterDeclarations(symbol, context);
                    }
                }
                return typeParameterNodes;
            }

            /**
             * Given A[B][C][D], finds A[B]
             */
            function getTopmostIndexedAccessType(top: IndexedAccessTypeNode): IndexedAccessTypeNode {
                if (isIndexedAccessTypeNode(top.objectType)) {
                    return getTopmostIndexedAccessType(top.objectType);
                }
                return top;
            }

            function getSpecifierForModuleSymbol(symbol: Symbol, context: NodeBuilderContext) {
                const file = getDeclarationOfKind<SourceFile>(symbol, SyntaxKind.SourceFile);
                if (file && file.moduleName !== undefined) {
                    // Use the amd name if it is available
                    return file.moduleName;
                }
                if (!file) {
                    if (context.tracker.trackReferencedAmbientModule) {
                        const ambientDecls = filter(symbol.declarations, isAmbientModule);
                        if (length(ambientDecls)) {
                            for (const decl of ambientDecls) {
                                context.tracker.trackReferencedAmbientModule(decl, symbol);
                            }
                        }
                    }
                    if (ambientModuleSymbolRegex.test(symbol.escapedName as string)) {
                        return (symbol.escapedName as string).substring(1, (symbol.escapedName as string).length - 1);
                    }
                }
                if (!context.enclosingDeclaration || !context.tracker.moduleResolverHost) {
                    // If there's no context declaration, we can't lookup a non-ambient specifier, so we just use the symbol name
                    if (ambientModuleSymbolRegex.test(symbol.escapedName as string)) {
                        return (symbol.escapedName as string).substring(1, (symbol.escapedName as string).length - 1);
                    }
                    return getSourceFileOfNode(getNonAugmentationDeclaration(symbol)!).fileName; // A resolver may not be provided for baselines and errors - in those cases we use the fileName in full
                }
                const contextFile = getSourceFileOfNode(getOriginalNode(context.enclosingDeclaration));
                const links = getSymbolLinks(symbol);
                let specifier = links.specifierCache && links.specifierCache.get(contextFile.path);
                if (!specifier) {
                    const isBundle = (compilerOptions.out || compilerOptions.outFile);
                    // For declaration bundles, we need to generate absolute paths relative to the common source dir for imports,
                    // just like how the declaration emitter does for the ambient module declarations - we can easily accomplish this
                    // using the `baseUrl` compiler option (which we would otherwise never use in declaration emit) and a non-relative
                    // specifier preference
                    const { moduleResolverHost } = context.tracker;
                    const specifierCompilerOptions = isBundle ? { ...compilerOptions, baseUrl: moduleResolverHost.getCommonSourceDirectory() } : compilerOptions;
                    specifier = first(moduleSpecifiers.getModuleSpecifiers(
                        symbol,
                        specifierCompilerOptions,
                        contextFile,
                        moduleResolverHost,
                        host.getSourceFiles(),
                        { importModuleSpecifierPreference: isBundle ? "non-relative" : "relative" },
                        host.redirectTargetsMap,
                    ));
                    links.specifierCache = links.specifierCache || createMap();
                    links.specifierCache.set(contextFile.path, specifier);
                }
                return specifier;
            }

            function symbolToTypeNode(symbol: Symbol, context: NodeBuilderContext, meaning: SymbolFlags, overrideTypeArguments?: ReadonlyArray<TypeNode>): TypeNode {
                const chain = lookupSymbolChain(symbol, context, meaning, !(context.flags & NodeBuilderFlags.UseAliasDefinedOutsideCurrentScope)); // If we're using aliases outside the current scope, dont bother with the module

                const isTypeOf = meaning === SymbolFlags.Value;
                if (some(chain[0].declarations, hasNonGlobalAugmentationExternalModuleSymbol)) {
                    // module is root, must use `ImportTypeNode`
                    const nonRootParts = chain.length > 1 ? createAccessFromSymbolChain(chain, chain.length - 1, 1) : undefined;
                    const typeParameterNodes = overrideTypeArguments || lookupTypeParameterNodes(chain, 0, context);
                    const specifier = getSpecifierForModuleSymbol(chain[0], context);
                    const lit = createLiteralTypeNode(createLiteral(specifier));
                    if (context.tracker.trackExternalModuleSymbolOfImportTypeNode) context.tracker.trackExternalModuleSymbolOfImportTypeNode(chain[0]);
                    context.approximateLength += specifier.length + 10; // specifier + import("")
                    if (!nonRootParts || isEntityName(nonRootParts)) {
                        if (nonRootParts) {
                            const lastId = isIdentifier(nonRootParts) ? nonRootParts : nonRootParts.right;
                            lastId.typeArguments = undefined;
                        }
                        return createImportTypeNode(lit, nonRootParts as EntityName, typeParameterNodes as ReadonlyArray<TypeNode>, isTypeOf);
                    }
                    else {
                        const splitNode = getTopmostIndexedAccessType(nonRootParts);
                        const qualifier = (splitNode.objectType as TypeReferenceNode).typeName;
                        return createIndexedAccessTypeNode(createImportTypeNode(lit, qualifier, typeParameterNodes as ReadonlyArray<TypeNode>, isTypeOf), splitNode.indexType);
                    }
                }

                const entityName = createAccessFromSymbolChain(chain, chain.length - 1, 0);
                if (isIndexedAccessTypeNode(entityName)) {
                    return entityName; // Indexed accesses can never be `typeof`
                }
                if (isTypeOf) {
                    return createTypeQueryNode(entityName);
                }
                else {
                    const lastId = isIdentifier(entityName) ? entityName : entityName.right;
                    const lastTypeArgs = lastId.typeArguments;
                    lastId.typeArguments = undefined;
                    return createTypeReferenceNode(entityName, lastTypeArgs as NodeArray<TypeNode>);
                }

                function createAccessFromSymbolChain(chain: Symbol[], index: number, stopper: number): EntityName | IndexedAccessTypeNode {
                    const typeParameterNodes = index === (chain.length - 1) ? overrideTypeArguments : lookupTypeParameterNodes(chain, index, context);
                    const symbol = chain[index];

                    if (index === 0) {
                       context.flags |= NodeBuilderFlags.InInitialEntityName;
                    }
                    const symbolName = getNameOfSymbolAsWritten(symbol, context);
                    context.approximateLength += symbolName.length + 1;
                    if (index === 0) {
                        context.flags ^= NodeBuilderFlags.InInitialEntityName;
                    }

                    const parent = chain[index - 1];
                    if (!(context.flags & NodeBuilderFlags.ForbidIndexedAccessSymbolReferences) && parent && getMembersOfSymbol(parent) && getMembersOfSymbol(parent).get(symbol.escapedName) === symbol) {
                        // Should use an indexed access
                        const LHS = createAccessFromSymbolChain(chain, index - 1, stopper);
                        if (isIndexedAccessTypeNode(LHS)) {
                            return createIndexedAccessTypeNode(LHS, createLiteralTypeNode(createLiteral(symbolName)));
                        }
                        else {
                            return createIndexedAccessTypeNode(createTypeReferenceNode(LHS, typeParameterNodes as ReadonlyArray<TypeNode>), createLiteralTypeNode(createLiteral(symbolName)));
                        }
                    }

                    const identifier = setEmitFlags(createIdentifier(symbolName, typeParameterNodes), EmitFlags.NoAsciiEscaping);
                    identifier.symbol = symbol;

                    if (index > stopper) {
                        const LHS = createAccessFromSymbolChain(chain, index - 1, stopper);
                        if (!isEntityName(LHS)) {
                            return Debug.fail("Impossible construct - an export of an indexed access cannot be reachable");
                        }
                        return createQualifiedName(LHS, identifier);
                    }
                    return identifier;
                }
            }

            function symbolToName(symbol: Symbol, context: NodeBuilderContext, meaning: SymbolFlags, expectsIdentifier: true): Identifier;
            function symbolToName(symbol: Symbol, context: NodeBuilderContext, meaning: SymbolFlags, expectsIdentifier: false): EntityName;
            function symbolToName(symbol: Symbol, context: NodeBuilderContext, meaning: SymbolFlags, expectsIdentifier: boolean): EntityName {
                const chain = lookupSymbolChain(symbol, context, meaning);

                if (expectsIdentifier && chain.length !== 1
                    && !context.encounteredError
                    && !(context.flags & NodeBuilderFlags.AllowQualifedNameInPlaceOfIdentifier)) {
                    context.encounteredError = true;
                }
                return createEntityNameFromSymbolChain(chain, chain.length - 1);

                function createEntityNameFromSymbolChain(chain: Symbol[], index: number): EntityName {
                    const typeParameterNodes = lookupTypeParameterNodes(chain, index, context);
                    const symbol = chain[index];

                    if (index === 0) {
                        context.flags |= NodeBuilderFlags.InInitialEntityName;
                    }
                    const symbolName = getNameOfSymbolAsWritten(symbol, context);
                    if (index === 0) {
                        context.flags ^= NodeBuilderFlags.InInitialEntityName;
                    }

                    const identifier = setEmitFlags(createIdentifier(symbolName, typeParameterNodes), EmitFlags.NoAsciiEscaping);
                    identifier.symbol = symbol;

                    return index > 0 ? createQualifiedName(createEntityNameFromSymbolChain(chain, index - 1), identifier) : identifier;
                }
            }

            function symbolToExpression(symbol: Symbol, context: NodeBuilderContext, meaning: SymbolFlags) {
                const chain = lookupSymbolChain(symbol, context, meaning);

                return createExpressionFromSymbolChain(chain, chain.length - 1);

                function createExpressionFromSymbolChain(chain: Symbol[], index: number): Expression {
                    const typeParameterNodes = lookupTypeParameterNodes(chain, index, context);
                    const symbol = chain[index];

                    if (index === 0) {
                        context.flags |= NodeBuilderFlags.InInitialEntityName;
                    }
                    let symbolName = getNameOfSymbolAsWritten(symbol, context);
                    if (index === 0) {
                        context.flags ^= NodeBuilderFlags.InInitialEntityName;
                    }
                    let firstChar = symbolName.charCodeAt(0);
                    const canUsePropertyAccess = isIdentifierStart(firstChar, languageVersion);
                    if (index === 0 || canUsePropertyAccess) {
                        const identifier = setEmitFlags(createIdentifier(symbolName, typeParameterNodes), EmitFlags.NoAsciiEscaping);
                        identifier.symbol = symbol;

                        return index > 0 ? createPropertyAccess(createExpressionFromSymbolChain(chain, index - 1), identifier) : identifier;
                    }
                    else {
                        if (firstChar === CharacterCodes.openBracket) {
                            symbolName = symbolName.substring(1, symbolName.length - 1);
                            firstChar = symbolName.charCodeAt(0);
                        }
                        let expression: Expression | undefined;
                        if (isSingleOrDoubleQuote(firstChar)) {
                            expression = createLiteral(symbolName.substring(1, symbolName.length - 1).replace(/\\./g, s => s.substring(1)));
                            (expression as StringLiteral).singleQuote = firstChar === CharacterCodes.singleQuote;
                        }
                        else if (("" + +symbolName) === symbolName) {
                            expression = createLiteral(+symbolName);
                        }
                        if (!expression) {
                            expression = setEmitFlags(createIdentifier(symbolName, typeParameterNodes), EmitFlags.NoAsciiEscaping);
                            expression.symbol = symbol;
                        }
                        return createElementAccess(createExpressionFromSymbolChain(chain, index - 1), expression);
                    }
                }
            }
        }

        function typePredicateToString(typePredicate: TypePredicate, enclosingDeclaration?: Node, flags: TypeFormatFlags = TypeFormatFlags.UseAliasDefinedOutsideCurrentScope, writer?: EmitTextWriter): string {
            return writer ? typePredicateToStringWorker(writer).getText() : usingSingleLineStringWriter(typePredicateToStringWorker);

            function typePredicateToStringWorker(writer: EmitTextWriter) {
                const predicate = createTypePredicateNode(
                    typePredicate.kind === TypePredicateKind.Identifier ? createIdentifier(typePredicate.parameterName) : createThisTypeNode(),
                    nodeBuilder.typeToTypeNode(typePredicate.type, enclosingDeclaration, toNodeBuilderFlags(flags) | NodeBuilderFlags.IgnoreErrors | NodeBuilderFlags.WriteTypeParametersInQualifiedName)!, // TODO: GH#18217
                );
                const printer = createPrinter({ removeComments: true });
                const sourceFile = enclosingDeclaration && getSourceFileOfNode(enclosingDeclaration);
                printer.writeNode(EmitHint.Unspecified, predicate, /*sourceFile*/ sourceFile, writer);
                return writer;
            }
        }

        function formatUnionTypes(types: ReadonlyArray<Type>): Type[] {
            const result: Type[] = [];
            let flags: TypeFlags = 0;
            for (let i = 0; i < types.length; i++) {
                const t = types[i];
                flags |= t.flags;
                if (!(t.flags & TypeFlags.Nullable)) {
                    if (t.flags & (TypeFlags.BooleanLiteral | TypeFlags.EnumLiteral)) {
                        const baseType = t.flags & TypeFlags.BooleanLiteral ? booleanType : getBaseTypeOfEnumLiteralType(<LiteralType>t);
                        if (baseType.flags & TypeFlags.Union) {
                            const count = (<UnionType>baseType).types.length;
                            if (i + count <= types.length && types[i + count - 1] === (<UnionType>baseType).types[count - 1]) {
                                result.push(baseType);
                                i += count - 1;
                                continue;
                            }
                        }
                    }
                    result.push(t);
                }
            }
            if (flags & TypeFlags.Null) result.push(nullType);
            if (flags & TypeFlags.Undefined) result.push(undefinedType);
            return result || types;
        }

        function visibilityToString(flags: ModifierFlags): string | undefined {
            if (flags === ModifierFlags.Private) {
                return "private";
            }
            if (flags === ModifierFlags.Protected) {
                return "protected";
            }
            return "public";
        }

        function getTypeAliasForTypeLiteral(type: Type): Symbol | undefined {
            if (type.symbol && type.symbol.flags & SymbolFlags.TypeLiteral) {
                const node = findAncestor(type.symbol.declarations[0].parent, n => n.kind !== SyntaxKind.ParenthesizedType)!;
                if (node.kind === SyntaxKind.TypeAliasDeclaration) {
                    return getSymbolOfNode(node);
                }
            }
            return undefined;
        }

        function isTopLevelInExternalModuleAugmentation(node: Node): boolean {
            return node && node.parent &&
                node.parent.kind === SyntaxKind.ModuleBlock &&
                isExternalModuleAugmentation(node.parent.parent);
        }

        interface NodeBuilderContext {
            enclosingDeclaration: Node | undefined;
            flags: NodeBuilderFlags;
            tracker: SymbolTracker;

            // State
            encounteredError: boolean;
            visitedSymbols: Map<true> | undefined;
            inferTypeParameters: TypeParameter[] | undefined;
            approximateLength: number;
            truncating?: boolean;
        }

        function isDefaultBindingContext(location: Node) {
            return location.kind === SyntaxKind.SourceFile || isAmbientModule(location);
        }

        /**
         * Gets a human-readable name for a symbol.
         * Should *not* be used for the right-hand side of a `.` -- use `symbolName(symbol)` for that instead.
         *
         * Unlike `symbolName(symbol)`, this will include quotes if the name is from a string literal.
         * It will also use a representation of a number as written instead of a decimal form, e.g. `0o11` instead of `9`.
         */
        function getNameOfSymbolAsWritten(symbol: Symbol, context?: NodeBuilderContext): string {
            if (context && symbol.escapedName === InternalSymbolName.Default && !(context.flags & NodeBuilderFlags.UseAliasDefinedOutsideCurrentScope) &&
                // If it's not the first part of an entity name, it must print as `default`
                (!(context.flags & NodeBuilderFlags.InInitialEntityName) ||
                // if the symbol is synthesized, it will only be referenced externally it must print as `default`
                !symbol.declarations ||
                // if not in the same binding context (source file, module declaration), it must print as `default`
                (context.enclosingDeclaration && findAncestor(symbol.declarations[0], isDefaultBindingContext) !== findAncestor(context.enclosingDeclaration, isDefaultBindingContext)))) {
                return "default";
            }
            if (symbol.declarations && symbol.declarations.length) {
                const declaration = symbol.declarations[0];
                const name = getNameOfDeclaration(declaration);
                if (name) {
                    return declarationNameToString(name);
                }
                if (declaration.parent && declaration.parent.kind === SyntaxKind.VariableDeclaration) {
                    return declarationNameToString((<VariableDeclaration>declaration.parent).name);
                }
                switch (declaration.kind) {
                    case SyntaxKind.ClassExpression:
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.ArrowFunction:
                        if (context && !context.encounteredError && !(context.flags & NodeBuilderFlags.AllowAnonymousIdentifier)) {
                            context.encounteredError = true;
                        }
                        return declaration.kind === SyntaxKind.ClassExpression ? "(Anonymous class)" : "(Anonymous function)";
                }
            }
            const nameType = symbol.nameType;
            if (nameType) {
                if (nameType.flags & TypeFlags.StringLiteral && !isIdentifierText((<StringLiteralType>nameType).value, compilerOptions.target)) {
                    return `"${escapeString((<StringLiteralType>nameType).value, CharacterCodes.doubleQuote)}"`;
                }
                if (nameType && nameType.flags & TypeFlags.UniqueESSymbol) {
                    return `[${getNameOfSymbolAsWritten((<UniqueESSymbolType>nameType).symbol, context)}]`;
                }
            }
            return symbolName(symbol);
        }

        function isDeclarationVisible(node: Node): boolean {
            if (node) {
                const links = getNodeLinks(node);
                if (links.isVisible === undefined) {
                    links.isVisible = !!determineIfDeclarationIsVisible();
                }
                return links.isVisible;
            }

            return false;

            function determineIfDeclarationIsVisible() {
                switch (node.kind) {
                    case SyntaxKind.JSDocCallbackTag:
                    case SyntaxKind.JSDocTypedefTag:
                        // Top-level jsdoc type aliases are considered exported
                        // First parent is comment node, second is hosting declaration or token; we only care about those tokens or declarations whose parent is a source file
                        return !!(node.parent && node.parent.parent && node.parent.parent.parent && isSourceFile(node.parent.parent.parent));
                    case SyntaxKind.BindingElement:
                        return isDeclarationVisible(node.parent.parent);
                    case SyntaxKind.VariableDeclaration:
                        if (isBindingPattern((node as VariableDeclaration).name) &&
                            !((node as VariableDeclaration).name as BindingPattern).elements.length) {
                            // If the binding pattern is empty, this variable declaration is not visible
                            return false;
                        }
                        // falls through
                    case SyntaxKind.ModuleDeclaration:
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.TypeAliasDeclaration:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.EnumDeclaration:
                    case SyntaxKind.ImportEqualsDeclaration:
                        // external module augmentation is always visible
                        if (isExternalModuleAugmentation(node)) {
                            return true;
                        }
                        const parent = getDeclarationContainer(node);
                        // If the node is not exported or it is not ambient module element (except import declaration)
                        if (!(getCombinedModifierFlags(node as Declaration) & ModifierFlags.Export) &&
                            !(node.kind !== SyntaxKind.ImportEqualsDeclaration && parent.kind !== SyntaxKind.SourceFile && parent.flags & NodeFlags.Ambient)) {
                            return isGlobalSourceFile(parent);
                        }
                        // Exported members/ambient module elements (exception import declaration) are visible if parent is visible
                        return isDeclarationVisible(parent);

                    case SyntaxKind.PropertyDeclaration:
                    case SyntaxKind.PropertySignature:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                        if (hasModifier(node, ModifierFlags.Private | ModifierFlags.Protected)) {
                            // Private/protected properties/methods are not visible
                            return false;
                        }
                        // Public properties/methods are visible if its parents are visible, so:
                        // falls through

                    case SyntaxKind.Constructor:
                    case SyntaxKind.ConstructSignature:
                    case SyntaxKind.CallSignature:
                    case SyntaxKind.IndexSignature:
                    case SyntaxKind.Parameter:
                    case SyntaxKind.ModuleBlock:
                    case SyntaxKind.FunctionType:
                    case SyntaxKind.ConstructorType:
                    case SyntaxKind.TypeLiteral:
                    case SyntaxKind.TypeReference:
                    case SyntaxKind.ArrayType:
                    case SyntaxKind.TupleType:
                    case SyntaxKind.UnionType:
                    case SyntaxKind.IntersectionType:
                    case SyntaxKind.ParenthesizedType:
                        return isDeclarationVisible(node.parent);

                    // Default binding, import specifier and namespace import is visible
                    // only on demand so by default it is not visible
                    case SyntaxKind.ImportClause:
                    case SyntaxKind.NamespaceImport:
                    case SyntaxKind.ImportSpecifier:
                        return false;

                    // Type parameters are always visible
                    case SyntaxKind.TypeParameter:
                    // Source file and namespace export are always visible
                    case SyntaxKind.SourceFile:
                    case SyntaxKind.NamespaceExportDeclaration:
                        return true;

                    // Export assignments do not create name bindings outside the module
                    case SyntaxKind.ExportAssignment:
                        return false;

                    default:
                        return false;
                }
            }
        }

        function collectLinkedAliases(node: Identifier, setVisibility?: boolean): Node[] | undefined {
            let exportSymbol: Symbol | undefined;
            if (node.parent && node.parent.kind === SyntaxKind.ExportAssignment) {
                exportSymbol = resolveName(node, node.escapedText, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias, /*nameNotFoundMessage*/ undefined, node, /*isUse*/ false);
            }
            else if (node.parent.kind === SyntaxKind.ExportSpecifier) {
                exportSymbol = getTargetOfExportSpecifier(<ExportSpecifier>node.parent, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias);
            }
            let result: Node[] | undefined;
            if (exportSymbol) {
                buildVisibleNodeList(exportSymbol.declarations);
            }
            return result;

            function buildVisibleNodeList(declarations: Declaration[]) {
                forEach(declarations, declaration => {
                    const resultNode = getAnyImportSyntax(declaration) || declaration;
                    if (setVisibility) {
                        getNodeLinks(declaration).isVisible = true;
                    }
                    else {
                        result = result || [];
                        pushIfUnique(result, resultNode);
                    }

                    if (isInternalModuleImportEqualsDeclaration(declaration)) {
                        // Add the referenced top container visible
                        const internalModuleReference = <Identifier | QualifiedName>declaration.moduleReference;
                        const firstIdentifier = getFirstIdentifier(internalModuleReference);
                        const importSymbol = resolveName(declaration, firstIdentifier.escapedText, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace,
                            undefined, undefined, /*isUse*/ false);
                        if (importSymbol) {
                            buildVisibleNodeList(importSymbol.declarations);
                        }
                    }
                });
            }
        }

        /**
         * Push an entry on the type resolution stack. If an entry with the given target and the given property name
         * is already on the stack, and no entries in between already have a type, then a circularity has occurred.
         * In this case, the result values of the existing entry and all entries pushed after it are changed to false,
         * and the value false is returned. Otherwise, the new entry is just pushed onto the stack, and true is returned.
         * In order to see if the same query has already been done before, the target object and the propertyName both
         * must match the one passed in.
         *
         * @param target The symbol, type, or signature whose type is being queried
         * @param propertyName The property name that should be used to query the target for its type
         */
        function pushTypeResolution(target: TypeSystemEntity, propertyName: TypeSystemPropertyName): boolean {
            const resolutionCycleStartIndex = findResolutionCycleStartIndex(target, propertyName);
            if (resolutionCycleStartIndex >= 0) {
                // A cycle was found
                const { length } = resolutionTargets;
                for (let i = resolutionCycleStartIndex; i < length; i++) {
                    resolutionResults[i] = false;
                }
                return false;
            }
            resolutionTargets.push(target);
            resolutionResults.push(/*items*/ true);
            resolutionPropertyNames.push(propertyName);
            return true;
        }

        function findResolutionCycleStartIndex(target: TypeSystemEntity, propertyName: TypeSystemPropertyName): number {
            for (let i = resolutionTargets.length - 1; i >= 0; i--) {
                if (hasType(resolutionTargets[i], resolutionPropertyNames[i])) {
                    return -1;
                }
                if (resolutionTargets[i] === target && resolutionPropertyNames[i] === propertyName) {
                    return i;
                }
            }
            return -1;
        }

        function hasType(target: TypeSystemEntity, propertyName: TypeSystemPropertyName): boolean {
            switch (propertyName) {
                case TypeSystemPropertyName.Type:
                    return !!getSymbolLinks(<Symbol>target).type;
                case TypeSystemPropertyName.DeclaredType:
                    return !!getSymbolLinks(<Symbol>target).declaredType;
                case TypeSystemPropertyName.ResolvedBaseConstructorType:
                    return !!(<InterfaceType>target).resolvedBaseConstructorType;
                case TypeSystemPropertyName.ResolvedReturnType:
                    return !!(<Signature>target).resolvedReturnType;
                case TypeSystemPropertyName.ImmediateBaseConstraint:
                    return !!(<Type>target).immediateBaseConstraint;
            }
            return Debug.assertNever(propertyName);
        }

        // Pop an entry from the type resolution stack and return its associated result value. The result value will
        // be true if no circularities were detected, or false if a circularity was found.
        function popTypeResolution(): boolean {
            resolutionTargets.pop();
            resolutionPropertyNames.pop();
            return resolutionResults.pop()!;
        }

        function getDeclarationContainer(node: Node): Node {
           return findAncestor(getRootDeclaration(node), node => {
                switch (node.kind) {
                    case SyntaxKind.VariableDeclaration:
                    case SyntaxKind.VariableDeclarationList:
                    case SyntaxKind.ImportSpecifier:
                    case SyntaxKind.NamedImports:
                    case SyntaxKind.NamespaceImport:
                    case SyntaxKind.ImportClause:
                        return false;
                    default:
                        return true;
                }
            })!.parent;
        }

        function getTypeOfPrototypeProperty(prototype: Symbol): Type {
            // TypeScript 1.0 spec (April 2014): 8.4
            // Every class automatically contains a static property member named 'prototype',
            // the type of which is an instantiation of the class type with type Any supplied as a type argument for each type parameter.
            // It is an error to explicitly declare a static property member with the name 'prototype'.
            const classType = <InterfaceType>getDeclaredTypeOfSymbol(getParentOfSymbol(prototype)!);
            return classType.typeParameters ? createTypeReference(<GenericType>classType, map(classType.typeParameters, _ => anyType)) : classType;
        }

        // Return the type of the given property in the given type, or undefined if no such property exists
        function getTypeOfPropertyOfType(type: Type, name: __String): Type | undefined {
            const prop = getPropertyOfType(type, name);
            return prop ? getTypeOfSymbol(prop) : undefined;
        }

        function isTypeAny(type: Type | undefined) {
            return type && (type.flags & TypeFlags.Any) !== 0;
        }

        // Return the type of a binding element parent. We check SymbolLinks first to see if a type has been
        // assigned by contextual typing.
        function getTypeForBindingElementParent(node: BindingElementGrandparent) {
            const symbol = getSymbolOfNode(node);
            return symbol && getSymbolLinks(symbol).type || getTypeForVariableLikeDeclaration(node, /*includeOptionality*/ false);
        }

        function isComputedNonLiteralName(name: PropertyName): boolean {
            return name.kind === SyntaxKind.ComputedPropertyName && !isStringOrNumericLiteralLike(name.expression);
        }

        function getRestType(source: Type, properties: PropertyName[], symbol: Symbol | undefined): Type {
            source = filterType(source, t => !(t.flags & TypeFlags.Nullable));
            if (source.flags & TypeFlags.Never) {
                return emptyObjectType;
            }

            if (source.flags & TypeFlags.Union) {
                return mapType(source, t => getRestType(t, properties, symbol));
            }

            const members = createSymbolTable();
            const names = createUnderscoreEscapedMap<true>();
            for (const name of properties) {
                names.set(getTextOfPropertyName(name), true);
            }
            for (const prop of getPropertiesOfType(source)) {
                if (!names.has(prop.escapedName)
                    && !(getDeclarationModifierFlagsFromSymbol(prop) & (ModifierFlags.Private | ModifierFlags.Protected))
                    && isSpreadableProperty(prop)) {
                    members.set(prop.escapedName, getNonReadonlySymbol(prop));
                }
            }
            const stringIndexInfo = getIndexInfoOfType(source, IndexKind.String);
            const numberIndexInfo = getIndexInfoOfType(source, IndexKind.Number);
            return createAnonymousType(symbol, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
        }

        /** Return the inferred type for a binding element */
        function getTypeForBindingElement(declaration: BindingElement): Type | undefined {
            const pattern = declaration.parent;
            let parentType = getTypeForBindingElementParent(pattern.parent);
            // If parent has the unknown (error) type, then so does this binding element
            if (parentType === errorType) {
                return errorType;
            }
            // If no type was specified or inferred for parent,
            // infer from the initializer of the binding element if one is present.
            // Otherwise, go with the undefined type of the parent.
            if (!parentType) {
                return declaration.initializer ? checkDeclarationInitializer(declaration) : parentType;
            }
            if (isTypeAny(parentType)) {
                return parentType;
            }

            let type: Type | undefined;
            if (pattern.kind === SyntaxKind.ObjectBindingPattern) {
                if (declaration.dotDotDotToken) {
                    if (parentType.flags & TypeFlags.Unknown || !isValidSpreadType(parentType)) {
                        error(declaration, Diagnostics.Rest_types_may_only_be_created_from_object_types);
                        return errorType;
                    }
                    const literalMembers: PropertyName[] = [];
                    for (const element of pattern.elements) {
                        if (!element.dotDotDotToken) {
                            literalMembers.push(element.propertyName || element.name as Identifier);
                        }
                    }
                    type = getRestType(parentType, literalMembers, declaration.symbol);
                }
                else {
                    // Use explicitly specified property name ({ p: xxx } form), or otherwise the implied name ({ p } form)
                    const name = declaration.propertyName || <Identifier>declaration.name;
                    const isLate = isLateBindableName(name);
                    const isWellKnown = isComputedPropertyName(name) && isWellKnownSymbolSyntactically(name.expression);
                    if (!isLate && !isWellKnown && isComputedNonLiteralName(name)) {
                        const exprType = checkExpression((name as ComputedPropertyName).expression);
                        if (isTypeAssignableToKind(exprType, TypeFlags.ESSymbolLike)) {
                            if (noImplicitAny) {
                                error(declaration, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(exprType), typeToString(parentType));
                            }
                            return anyType;
                        }
                        const indexerType = isTypeAssignableToKind(exprType, TypeFlags.NumberLike) && getIndexTypeOfType(parentType, IndexKind.Number) || getIndexTypeOfType(parentType, IndexKind.String);
                        if (!indexerType && noImplicitAny && !compilerOptions.suppressImplicitAnyIndexErrors) {
                            if (getIndexTypeOfType(parentType, IndexKind.Number)) {
                                error(declaration, Diagnostics.Element_implicitly_has_an_any_type_because_index_expression_is_not_of_type_number);
                            }
                            else {
                                error(declaration, Diagnostics.Element_implicitly_has_an_any_type_because_type_0_has_no_index_signature, typeToString(parentType));
                            }
                        }
                        return indexerType || anyType;
                    }

                    // Use type of the specified property, or otherwise, for a numeric name, the type of the numeric index signature,
                    // or otherwise the type of the string index signature.
                    const nameType = isLate ? checkComputedPropertyName(name as ComputedPropertyName) as LiteralType | UniqueESSymbolType : undefined;
                    const text = isLate ? getLateBoundNameFromType(nameType!) :
                        isWellKnown ? getPropertyNameForKnownSymbolName(idText(((name as ComputedPropertyName).expression as PropertyAccessExpression).name)) :
                        getTextOfPropertyName(name);

                    // Relax null check on ambient destructuring parameters, since the parameters have no implementation and are just documentation
                    if (strictNullChecks && declaration.flags & NodeFlags.Ambient && isParameterDeclaration(declaration)) {
                        parentType = getNonNullableType(parentType);
                    }
                    if (isLate && nameType && !getPropertyOfType(parentType, text) && isTypeAssignableToKind(nameType, TypeFlags.ESSymbolLike)) {
                        if (noImplicitAny) {
                            error(declaration, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(nameType), typeToString(parentType));
                        }
                        return anyType;
                    }
                    const declaredType = getConstraintForLocation(getTypeOfPropertyOfType(parentType, text), declaration.name);
                    type = declaredType && getFlowTypeOfReference(declaration, declaredType) ||
                        isNumericLiteralName(text) && getIndexTypeOfType(parentType, IndexKind.Number) ||
                        getIndexTypeOfType(parentType, IndexKind.String);
                    if (!type) {
                        error(name, Diagnostics.Type_0_has_no_property_1_and_no_string_index_signature, typeToString(parentType), declarationNameToString(name));
                        return errorType;
                    }
                }
            }
            else {
                // This elementType will be used if the specific property corresponding to this index is not
                // present (aka the tuple element property). This call also checks that the parentType is in
                // fact an iterable or array (depending on target language).
                const elementType = checkIteratedTypeOrElementType(parentType, pattern, /*allowStringInput*/ false, /*allowAsyncIterables*/ false);
                const index = pattern.elements.indexOf(declaration);
                if (declaration.dotDotDotToken) {
                    // If the parent is a tuple type, the rest element has an array type with a union of the
                    // remaining tuple element types. Otherwise, the rest element has an array type with same
                    // element type as the parent type.
                    type = isTupleType(parentType) ?
                        getArrayLiteralType((parentType.typeArguments || emptyArray).slice(index, getTypeReferenceArity(parentType))) :
                        createArrayType(elementType);
                }
                else {
                    // Use specific property type when parent is a tuple or numeric index type when parent is an array
                    const index = pattern.elements.indexOf(declaration);
                    type = isTupleLikeType(parentType) ?
                        getTupleElementType(parentType, index) || declaration.initializer && checkDeclarationInitializer(declaration) :
                        elementType;
                    if (!type) {
                        if (isTupleType(parentType)) {
                            error(declaration, Diagnostics.Tuple_type_0_with_length_1_cannot_be_assigned_to_tuple_with_length_2, typeToString(parentType), getTypeReferenceArity(<TypeReference>parentType), pattern.elements.length);
                        }
                        else {
                            error(declaration, Diagnostics.Type_0_has_no_property_1, typeToString(parentType), "" + index);
                        }
                        return errorType;
                    }
                }
            }
            // In strict null checking mode, if a default value of a non-undefined type is specified, remove
            // undefined from the final type.
            if (strictNullChecks && declaration.initializer && !(getFalsyFlags(checkExpressionCached(declaration.initializer)) & TypeFlags.Undefined)) {
                type = getTypeWithFacts(type, TypeFacts.NEUndefined);
            }
            return declaration.initializer && !getEffectiveTypeAnnotationNode(walkUpBindingElementsAndPatterns(declaration)) ?
                getUnionType([type, checkExpressionCached(declaration.initializer)], UnionReduction.Subtype) :
                type;
        }

        function getTypeForDeclarationFromJSDocComment(declaration: Node) {
            const jsdocType = getJSDocType(declaration);
            if (jsdocType) {
                return getTypeFromTypeNode(jsdocType);
            }
            return undefined;
        }

        function isNullOrUndefined(node: Expression) {
            const expr = skipParentheses(node);
            return expr.kind === SyntaxKind.NullKeyword || expr.kind === SyntaxKind.Identifier && getResolvedSymbol(<Identifier>expr) === undefinedSymbol;
        }

        function isEmptyArrayLiteral(node: Expression) {
            const expr = skipParentheses(node);
            return expr.kind === SyntaxKind.ArrayLiteralExpression && (<ArrayLiteralExpression>expr).elements.length === 0;
        }

        function addOptionality(type: Type, optional = true): Type {
            return strictNullChecks && optional ? getOptionalType(type) : type;
        }

        // Return the inferred type for a variable, parameter, or property declaration
        function getTypeForVariableLikeDeclaration(declaration: ParameterDeclaration | PropertyDeclaration | PropertySignature | VariableDeclaration | BindingElement, includeOptionality: boolean): Type | undefined {
            // A variable declared in a for..in statement is of type string, or of type keyof T when the
            // right hand expression is of a type parameter type.
            if (isVariableDeclaration(declaration) && declaration.parent.parent.kind === SyntaxKind.ForInStatement) {
                const indexType = getIndexType(checkNonNullExpression(declaration.parent.parent.expression));
                return indexType.flags & (TypeFlags.TypeParameter | TypeFlags.Index) ? getExtractStringType(indexType) : stringType;
            }

            if (isVariableDeclaration(declaration) && declaration.parent.parent.kind === SyntaxKind.ForOfStatement) {
                // checkRightHandSideOfForOf will return undefined if the for-of expression type was
                // missing properties/signatures required to get its iteratedType (like
                // [Symbol.iterator] or next). This may be because we accessed properties from anyType,
                // or it may have led to an error inside getElementTypeOfIterable.
                const forOfStatement = declaration.parent.parent;
                return checkRightHandSideOfForOf(forOfStatement.expression, forOfStatement.awaitModifier) || anyType;
            }

            if (isBindingPattern(declaration.parent)) {
                return getTypeForBindingElement(<BindingElement>declaration);
            }

            const isOptional = includeOptionality && (
                isParameter(declaration) && isJSDocOptionalParameter(declaration)
                || !isBindingElement(declaration) && !isVariableDeclaration(declaration) && !!declaration.questionToken);

            // Use type from type annotation if one is present
            const declaredType = tryGetTypeFromEffectiveTypeNode(declaration);
            if (declaredType) {
                return addOptionality(declaredType, isOptional);
            }

            if ((noImplicitAny || isInJSFile(declaration)) &&
                declaration.kind === SyntaxKind.VariableDeclaration && !isBindingPattern(declaration.name) &&
                !(getCombinedModifierFlags(declaration) & ModifierFlags.Export) && !(declaration.flags & NodeFlags.Ambient)) {
                // If --noImplicitAny is on or the declaration is in a Javascript file,
                // use control flow tracked 'any' type for non-ambient, non-exported var or let variables with no
                // initializer or a 'null' or 'undefined' initializer.
                if (!(getCombinedNodeFlags(declaration) & NodeFlags.Const) && (!declaration.initializer || isNullOrUndefined(declaration.initializer))) {
                    return autoType;
                }
                // Use control flow tracked 'any[]' type for non-ambient, non-exported variables with an empty array
                // literal initializer.
                if (declaration.initializer && isEmptyArrayLiteral(declaration.initializer)) {
                    return autoArrayType;
                }
            }

            if (declaration.kind === SyntaxKind.Parameter) {
                const func = <FunctionLikeDeclaration>declaration.parent;
                // For a parameter of a set accessor, use the type of the get accessor if one is present
                if (func.kind === SyntaxKind.SetAccessor && !hasNonBindableDynamicName(func)) {
                    const getter = getDeclarationOfKind<AccessorDeclaration>(getSymbolOfNode(declaration.parent), SyntaxKind.GetAccessor);
                    if (getter) {
                        const getterSignature = getSignatureFromDeclaration(getter);
                        const thisParameter = getAccessorThisParameter(func as AccessorDeclaration);
                        if (thisParameter && declaration === thisParameter) {
                            // Use the type from the *getter*
                            Debug.assert(!thisParameter.type);
                            return getTypeOfSymbol(getterSignature.thisParameter!);
                        }
                        return getReturnTypeOfSignature(getterSignature);
                    }
                }
                if (isInJSFile(declaration)) {
                    const typeTag = getJSDocType(func);
                    if (typeTag && isFunctionTypeNode(typeTag)) {
                        return getTypeAtPosition(getSignatureFromDeclaration(typeTag), func.parameters.indexOf(declaration));
                    }
                }
                // Use contextual parameter type if one is available
                const type = declaration.symbol.escapedName === InternalSymbolName.This ? getContextualThisParameterType(func) : getContextuallyTypedParameterType(declaration);
                if (type) {
                    return addOptionality(type, isOptional);
                }
            }
            else if (isInJSFile(declaration)) {
                const containerObjectType = getJSContainerObjectType(declaration, getSymbolOfNode(declaration), getDeclaredExpandoInitializer(declaration));
                if (containerObjectType) {
                    return containerObjectType;
                }
            }

            // Use the type of the initializer expression if one is present
            if (declaration.initializer) {
                const type = checkDeclarationInitializer(declaration);
                return addOptionality(type, isOptional);
            }

            if (isJsxAttribute(declaration)) {
                // if JSX attribute doesn't have initializer, by default the attribute will have boolean value of true.
                // I.e <Elem attr /> is sugar for <Elem attr={true} />
                return trueType;
            }

            // If the declaration specifies a binding pattern, use the type implied by the binding pattern
            if (isBindingPattern(declaration.name)) {
                return getTypeFromBindingPattern(declaration.name, /*includePatternInType*/ false, /*reportErrors*/ true);
            }

            // No type specified and nothing can be inferred
            return undefined;
        }

        function getWidenedTypeFromAssignmentDeclaration(symbol: Symbol, resolvedSymbol?: Symbol) {
            // function/class/{} initializers are themselves containers, so they won't merge in the same way as other initializers
            const container = getAssignedExpandoInitializer(symbol.valueDeclaration);
            if (container) {
                const tag = getJSDocTypeTag(container);
                if (tag && tag.typeExpression) {
                    return getTypeFromTypeNode(tag.typeExpression);
                }
                const containerObjectType = getJSContainerObjectType(symbol.valueDeclaration, symbol, container);
                return containerObjectType || getWidenedLiteralType(checkExpressionCached(container));
            }
            let definedInConstructor = false;
            let definedInMethod = false;
            let jsdocType: Type | undefined;
            let types: Type[] | undefined;
            for (const declaration of symbol.declarations) {
                const expression = isBinaryExpression(declaration) ? declaration :
                    isPropertyAccessExpression(declaration) ? isBinaryExpression(declaration.parent) ? declaration.parent : declaration :
                        undefined;
                if (!expression) {
                    return errorType;
                }

                const kind = isPropertyAccessExpression(expression) ? getAssignmentDeclarationPropertyAccessKind(expression) : getAssignmentDeclarationKind(expression);
                if (kind === AssignmentDeclarationKind.ThisProperty) {
                    if (isDeclarationInConstructor(expression)) {
                        definedInConstructor = true;
                    }
                    else {
                        definedInMethod = true;
                    }
                }
                jsdocType = getJSDocTypeFromAssignmentDeclaration(jsdocType, expression, symbol, declaration);
                if (!jsdocType) {
                    (types || (types = [])).push(isBinaryExpression(expression) ? getInitializerTypeFromAssignmentDeclaration(symbol, resolvedSymbol, expression, kind) : neverType);
                }
            }
            let type = jsdocType;
            if (!type) {
                let constructorTypes = definedInConstructor ? getConstructorDefinedThisAssignmentTypes(types!, symbol.declarations) : undefined;
                // use only the constructor types unless they were only assigned null | undefined (including widening variants)
                if (definedInMethod) {
                    const propType = getTypeOfAssignmentDeclarationPropertyOfBaseType(symbol);
                    if (propType) {
                        (constructorTypes || (constructorTypes = [])).push(propType);
                        definedInConstructor = true;
                    }
                }
                const sourceTypes = some(constructorTypes, t => !!(t.flags & ~(TypeFlags.Nullable | TypeFlags.ContainsWideningType))) ? constructorTypes : types; // TODO: GH#18217
                type = getUnionType(sourceTypes!, UnionReduction.Subtype);
            }
            const widened = getWidenedType(addOptionality(type, definedInMethod && !definedInConstructor));
            if (filterType(widened, t => !!(t.flags & ~TypeFlags.Nullable)) === neverType) {
                if (noImplicitAny) {
                    reportImplicitAnyError(symbol.valueDeclaration, anyType);
                }
                return anyType;
            }
            return widened;
        }

        function getJSContainerObjectType(decl: Node, symbol: Symbol, init: Expression | undefined): Type | undefined {
            if (!isInJSFile(decl) || !init || !isObjectLiteralExpression(init) || init.properties.length) {
                return undefined;
            }
            const exports = createSymbolTable();
            while (isBinaryExpression(decl) || isPropertyAccessExpression(decl)) {
                const s = getSymbolOfNode(decl);
                if (s && hasEntries(s.exports)) {
                    mergeSymbolTable(exports, s.exports);
                }
                decl = isBinaryExpression(decl) ? decl.parent : decl.parent.parent;
            }
            const s = getSymbolOfNode(decl);
            if (s && hasEntries(s.exports)) {
                mergeSymbolTable(exports, s.exports);
            }
            const type = createAnonymousType(symbol, exports, emptyArray, emptyArray, undefined, undefined);
            type.objectFlags |= ObjectFlags.JSLiteral;
            return type;
        }

        function getJSDocTypeFromAssignmentDeclaration(declaredType: Type | undefined, expression: Expression, _symbol: Symbol, declaration: Declaration) {
            const typeNode = getJSDocType(expression.parent);
            if (typeNode) {
                const type = getWidenedType(getTypeFromTypeNode(typeNode));
                if (!declaredType) {
                    return type;
                }
                else if (declaredType !== errorType && type !== errorType && !isTypeIdenticalTo(declaredType, type)) {
                    errorNextVariableOrPropertyDeclarationMustHaveSameType(declaredType, declaration, type);
                }
            }
            return declaredType;
        }

        /** If we don't have an explicit JSDoc type, get the type from the initializer. */
        function getInitializerTypeFromAssignmentDeclaration(symbol: Symbol, resolvedSymbol: Symbol | undefined, expression: BinaryExpression, kind: AssignmentDeclarationKind) {
            const type = resolvedSymbol ? getTypeOfSymbol(resolvedSymbol) : getWidenedLiteralType(checkExpressionCached(expression.right));
            if (type.flags & TypeFlags.Object &&
                kind === AssignmentDeclarationKind.ModuleExports &&
                symbol.escapedName === InternalSymbolName.ExportEquals) {
                const exportedType = resolveStructuredTypeMembers(type as ObjectType);
                const members = createSymbolTable();
                copyEntries(exportedType.members, members);
                if (resolvedSymbol && !resolvedSymbol.exports) {
                    resolvedSymbol.exports = createSymbolTable();
                }
                (resolvedSymbol || symbol).exports!.forEach((s, name) => {
                    if (members.has(name)) {
                        const exportedMember = exportedType.members.get(name)!;
                        const union = createSymbol(s.flags | exportedMember.flags, name);
                        union.type = getUnionType([getTypeOfSymbol(s), getTypeOfSymbol(exportedMember)]);
                        members.set(name, union);
                    }
                    else {
                        members.set(name, s);
                    }
                });
                const result = createAnonymousType(
                    exportedType.symbol,
                    members,
                    exportedType.callSignatures,
                    exportedType.constructSignatures,
                    exportedType.stringIndexInfo,
                    exportedType.numberIndexInfo);
                result.objectFlags |= (getObjectFlags(type) & ObjectFlags.JSLiteral); // Propagate JSLiteral flag
                return result;
            }
            if (isEmptyArrayLiteralType(type)) {
                if (noImplicitAny) {
                    reportImplicitAnyError(expression, anyArrayType);
                }
                return anyArrayType;
            }
            return type;
        }

        function isDeclarationInConstructor(expression: Expression) {
            const thisContainer = getThisContainer(expression, /*includeArrowFunctions*/ false);
            // Properties defined in a constructor (or base constructor, or javascript constructor function) don't get undefined added.
            // Function expressions that are assigned to the prototype count as methods.
            return thisContainer.kind === SyntaxKind.Constructor ||
                thisContainer.kind === SyntaxKind.FunctionDeclaration ||
                (thisContainer.kind === SyntaxKind.FunctionExpression && !isPrototypePropertyAssignment(thisContainer.parent));
        }

        function getConstructorDefinedThisAssignmentTypes(types: Type[], declarations: Declaration[]): Type[] | undefined {
            Debug.assert(types.length === declarations.length);
            return types.filter((_, i) => {
                const declaration = declarations[i];
                const expression = isBinaryExpression(declaration) ? declaration :
                    isBinaryExpression(declaration.parent) ? declaration.parent : undefined;
                return expression && isDeclarationInConstructor(expression);
            });
        }

        /** check for definition in base class if any declaration is in a class */
        function getTypeOfAssignmentDeclarationPropertyOfBaseType(property: Symbol) {
            const parentDeclaration = forEach(property.declarations, d => {
                const parent = getThisContainer(d, /*includeArrowFunctions*/ false).parent;
                return isClassLike(parent) && parent;
            });
            if (parentDeclaration) {
                const classType = getDeclaredTypeOfSymbol(getSymbolOfNode(parentDeclaration)) as InterfaceType;
                const baseClassType = classType && getBaseTypes(classType)[0];
                if (baseClassType) {
                    return getTypeOfPropertyOfType(baseClassType, property.escapedName);
                }
            }
        }

        // Return the type implied by a binding pattern element. This is the type of the initializer of the element if
        // one is present. Otherwise, if the element is itself a binding pattern, it is the type implied by the binding
        // pattern. Otherwise, it is the type any.
        function getTypeFromBindingElement(element: BindingElement, includePatternInType?: boolean, reportErrors?: boolean): Type {
            if (element.initializer) {
                return addOptionality(checkDeclarationInitializer(element));
            }
            if (isBindingPattern(element.name)) {
                return getTypeFromBindingPattern(element.name, includePatternInType, reportErrors);
            }
            if (reportErrors && noImplicitAny && !declarationBelongsToPrivateAmbientMember(element)) {
                reportImplicitAnyError(element, anyType);
            }
            return anyType;
        }

        // Return the type implied by an object binding pattern
        function getTypeFromObjectBindingPattern(pattern: ObjectBindingPattern, includePatternInType: boolean, reportErrors: boolean): Type {
            const members = createSymbolTable();
            let stringIndexInfo: IndexInfo | undefined;
            let objectFlags = ObjectFlags.ObjectLiteral;
            forEach(pattern.elements, e => {
                const name = e.propertyName || <Identifier>e.name;
                if (isComputedNonLiteralName(name)) {
                    // do not include computed properties in the implied type
                    objectFlags |= ObjectFlags.ObjectLiteralPatternWithComputedProperties;
                    return;
                }
                if (e.dotDotDotToken) {
                    stringIndexInfo = createIndexInfo(anyType, /*isReadonly*/ false);
                    return;
                }

                const text = getTextOfPropertyName(name);
                const flags = SymbolFlags.Property | (e.initializer ? SymbolFlags.Optional : 0);
                const symbol = createSymbol(flags, text);
                symbol.type = getTypeFromBindingElement(e, includePatternInType, reportErrors);
                symbol.bindingElement = e;
                members.set(symbol.escapedName, symbol);
            });
            const result = createAnonymousType(undefined, members, emptyArray, emptyArray, stringIndexInfo, undefined);
            result.flags |= TypeFlags.ContainsObjectLiteral;
            result.objectFlags |= objectFlags;
            if (includePatternInType) {
                result.pattern = pattern;
            }
            return result;
        }

        // Return the type implied by an array binding pattern
        function getTypeFromArrayBindingPattern(pattern: BindingPattern, includePatternInType: boolean, reportErrors: boolean): Type {
            const elements = pattern.elements;
            const lastElement = lastOrUndefined(elements);
            const hasRestElement = !!(lastElement && lastElement.kind === SyntaxKind.BindingElement && lastElement.dotDotDotToken);
            if (elements.length === 0 || elements.length === 1 && hasRestElement) {
                return languageVersion >= ScriptTarget.ES2015 ? createIterableType(anyType) : anyArrayType;
            }
            const elementTypes = map(elements, e => isOmittedExpression(e) ? anyType : getTypeFromBindingElement(e, includePatternInType, reportErrors));
            const minLength = findLastIndex(elements, e => !isOmittedExpression(e) && !hasDefaultValue(e), elements.length - (hasRestElement ? 2 : 1)) + 1;
            let result = <TypeReference>createTupleType(elementTypes, minLength, hasRestElement);
            if (includePatternInType) {
                result = cloneTypeReference(result);
                result.pattern = pattern;
            }
            return result;
        }

        // Return the type implied by a binding pattern. This is the type implied purely by the binding pattern itself
        // and without regard to its context (i.e. without regard any type annotation or initializer associated with the
        // declaration in which the binding pattern is contained). For example, the implied type of [x, y] is [any, any]
        // and the implied type of { x, y: z = 1 } is { x: any; y: number; }. The type implied by a binding pattern is
        // used as the contextual type of an initializer associated with the binding pattern. Also, for a destructuring
        // parameter with no type annotation or initializer, the type implied by the binding pattern becomes the type of
        // the parameter.
        function getTypeFromBindingPattern(pattern: BindingPattern, includePatternInType = false, reportErrors = false): Type {
            return pattern.kind === SyntaxKind.ObjectBindingPattern
                ? getTypeFromObjectBindingPattern(pattern, includePatternInType, reportErrors)
                : getTypeFromArrayBindingPattern(pattern, includePatternInType, reportErrors);
        }

        // Return the type associated with a variable, parameter, or property declaration. In the simple case this is the type
        // specified in a type annotation or inferred from an initializer. However, in the case of a destructuring declaration it
        // is a bit more involved. For example:
        //
        //   var [x, s = ""] = [1, "one"];
        //
        // Here, the array literal [1, "one"] is contextually typed by the type [any, string], which is the implied type of the
        // binding pattern [x, s = ""]. Because the contextual type is a tuple type, the resulting type of [1, "one"] is the
        // tuple type [number, string]. Thus, the type inferred for 'x' is number and the type inferred for 's' is string.
        function getWidenedTypeForVariableLikeDeclaration(declaration: ParameterDeclaration | PropertyDeclaration | PropertySignature | VariableDeclaration | BindingElement, reportErrors?: boolean): Type {
            return widenTypeForVariableLikeDeclaration(getTypeForVariableLikeDeclaration(declaration, /*includeOptionality*/ true), declaration, reportErrors);
        }

        function widenTypeForVariableLikeDeclaration(type: Type | undefined, declaration: any, reportErrors?: boolean) {
            if (type) {
                if (reportErrors) {
                    reportErrorsFromWidening(declaration, type);
                }

                // always widen a 'unique symbol' type if the type was created for a different declaration.
                if (type.flags & TypeFlags.UniqueESSymbol && (isBindingElement(declaration) || !declaration.type) && type.symbol !== getSymbolOfNode(declaration)) {
                    type = esSymbolType;
                }

                return getWidenedType(type);
            }

            // Rest parameters default to type any[], other parameters default to type any
            type = isParameter(declaration) && declaration.dotDotDotToken ? anyArrayType : anyType;

            // Report implicit any errors unless this is a private property within an ambient declaration
            if (reportErrors && noImplicitAny) {
                if (!declarationBelongsToPrivateAmbientMember(declaration)) {
                    reportImplicitAnyError(declaration, type);
                }
            }
            return type;
        }

        function declarationBelongsToPrivateAmbientMember(declaration: VariableLikeDeclaration) {
            const root = getRootDeclaration(declaration);
            const memberDeclaration = root.kind === SyntaxKind.Parameter ? root.parent : root;
            return isPrivateWithinAmbient(memberDeclaration);
        }

        function tryGetTypeFromEffectiveTypeNode(declaration: Declaration) {
            const typeNode = getEffectiveTypeAnnotationNode(declaration);
            if (typeNode) {
                return getTypeFromTypeNode(typeNode);
            }
        }

        function getTypeOfVariableOrParameterOrProperty(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            return links.type || (links.type = getTypeOfVariableOrParameterOrPropertyWorker(symbol));
        }

        function getTypeOfVariableOrParameterOrPropertyWorker(symbol: Symbol) {
            // Handle prototype property
            if (symbol.flags & SymbolFlags.Prototype) {
                return getTypeOfPrototypeProperty(symbol);
            }
            // CommonsJS require and module both have type any.
            if (symbol === requireSymbol) {
                return anyType;
            }
            if (symbol.flags & SymbolFlags.ModuleExports) {
                const fileSymbol = getSymbolOfNode(getSourceFileOfNode(symbol.valueDeclaration));
                const members = createSymbolTable();
                members.set("exports" as __String, fileSymbol);
                return createAnonymousType(symbol, members, emptyArray, emptyArray, undefined, undefined);
            }
            // Handle catch clause variables
            const declaration = symbol.valueDeclaration;
            if (isCatchClauseVariableDeclarationOrBindingElement(declaration)) {
                return anyType;
            }
            // Handle export default expressions
            if (isSourceFile(declaration)) {
                const jsonSourceFile = cast(declaration, isJsonSourceFile);
                if (!jsonSourceFile.statements.length) {
                    return emptyObjectType;
                }
                const type = getWidenedLiteralType(checkExpression(jsonSourceFile.statements[0].expression));
                if (type.flags & TypeFlags.Object) {
                    return getRegularTypeOfObjectLiteral(type);
                }
                return type;
            }
            if (declaration.kind === SyntaxKind.ExportAssignment) {
                return checkExpression((<ExportAssignment>declaration).expression);
            }

            // Handle variable, parameter or property
            if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
                return errorType;
            }
            let type: Type | undefined;
            if (isInJSFile(declaration) &&
                (isBinaryExpression(declaration) || isPropertyAccessExpression(declaration) && isBinaryExpression(declaration.parent))) {
                type = getWidenedTypeFromAssignmentDeclaration(symbol);
            }
            else if (isJSDocPropertyLikeTag(declaration)
                || isPropertyAccessExpression(declaration)
                || isIdentifier(declaration)
                || isClassDeclaration(declaration)
                || isFunctionDeclaration(declaration)
                || (isMethodDeclaration(declaration) && !isObjectLiteralMethod(declaration))
                || isMethodSignature(declaration)) {
                // Symbol is property of some kind that is merged with something - should use `getTypeOfFuncClassEnumModule` and not `getTypeOfVariableOrParameterOrProperty`
                if (symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.Enum | SymbolFlags.ValueModule)) {
                    return getTypeOfFuncClassEnumModule(symbol);
                }
                type = isBinaryExpression(declaration.parent) ?
                    getWidenedTypeFromAssignmentDeclaration(symbol) :
                    tryGetTypeFromEffectiveTypeNode(declaration) || anyType;
            }
            else if (isPropertyAssignment(declaration)) {
                type = tryGetTypeFromEffectiveTypeNode(declaration) || checkPropertyAssignment(declaration);
            }
            else if (isJsxAttribute(declaration)) {
                type = tryGetTypeFromEffectiveTypeNode(declaration) || checkJsxAttribute(declaration);
            }
            else if (isShorthandPropertyAssignment(declaration)) {
                type = tryGetTypeFromEffectiveTypeNode(declaration) || checkExpressionForMutableLocation(declaration.name, CheckMode.Normal);
            }
            else if (isObjectLiteralMethod(declaration)) {
                type = tryGetTypeFromEffectiveTypeNode(declaration) || checkObjectLiteralMethod(declaration, CheckMode.Normal);
            }
            else if (isParameter(declaration)
                     || isPropertyDeclaration(declaration)
                     || isPropertySignature(declaration)
                     || isVariableDeclaration(declaration)
                     || isBindingElement(declaration)) {
                type = getWidenedTypeForVariableLikeDeclaration(declaration, /*includeOptionality*/ true);
            }
            else {
                return Debug.fail("Unhandled declaration kind! " + Debug.showSyntaxKind(declaration) + " for " + Debug.showSymbol(symbol));
            }

            if (!popTypeResolution()) {
                type = reportCircularityError(symbol);
            }
            return type;
        }

        function getAnnotatedAccessorTypeNode(accessor: AccessorDeclaration | undefined): TypeNode | undefined {
            if (accessor) {
                if (accessor.kind === SyntaxKind.GetAccessor) {
                    const getterTypeAnnotation = getEffectiveReturnTypeNode(accessor);
                    return getterTypeAnnotation;
                }
                else {
                    const setterTypeAnnotation = getEffectiveSetAccessorTypeAnnotationNode(accessor);
                    return setterTypeAnnotation;
                }
            }
            return undefined;
        }

        function getAnnotatedAccessorType(accessor: AccessorDeclaration | undefined): Type | undefined {
            const node = getAnnotatedAccessorTypeNode(accessor);
            return node && getTypeFromTypeNode(node);
        }

        function getAnnotatedAccessorThisParameter(accessor: AccessorDeclaration): Symbol | undefined {
            const parameter = getAccessorThisParameter(accessor);
            return parameter && parameter.symbol;
        }

        function getThisTypeOfDeclaration(declaration: SignatureDeclaration): Type | undefined {
            return getThisTypeOfSignature(getSignatureFromDeclaration(declaration));
        }

        function getTypeOfAccessors(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            return links.type || (links.type = getTypeOfAccessorsWorker(symbol));
        }

        function getTypeOfAccessorsWorker(symbol: Symbol): Type {
            const getter = getDeclarationOfKind<AccessorDeclaration>(symbol, SyntaxKind.GetAccessor);
            const setter = getDeclarationOfKind<AccessorDeclaration>(symbol, SyntaxKind.SetAccessor);

            if (getter && isInJSFile(getter)) {
                const jsDocType = getTypeForDeclarationFromJSDocComment(getter);
                if (jsDocType) {
                    return jsDocType;
                }
            }

            if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
                return errorType;
            }

            let type: Type;

            // First try to see if the user specified a return type on the get-accessor.
            const getterReturnType = getAnnotatedAccessorType(getter);
            if (getterReturnType) {
                type = getterReturnType;
            }
            else {
                // If the user didn't specify a return type, try to use the set-accessor's parameter type.
                const setterParameterType = getAnnotatedAccessorType(setter);
                if (setterParameterType) {
                    type = setterParameterType;
                }
                else {
                    // If there are no specified types, try to infer it from the body of the get accessor if it exists.
                    if (getter && getter.body) {
                        type = getReturnTypeFromBody(getter);
                    }
                    // Otherwise, fall back to 'any'.
                    else {
                        if (noImplicitAny) {
                            if (setter) {
                                error(setter, Diagnostics.Property_0_implicitly_has_type_any_because_its_set_accessor_lacks_a_parameter_type_annotation, symbolToString(symbol));
                            }
                            else {
                                Debug.assert(!!getter, "there must existed getter as we are current checking either setter or getter in this function");
                                error(getter, Diagnostics.Property_0_implicitly_has_type_any_because_its_get_accessor_lacks_a_return_type_annotation, symbolToString(symbol));
                            }
                        }
                        type = anyType;
                    }
                }
            }
            if (!popTypeResolution()) {
                type = anyType;
                if (noImplicitAny) {
                    const getter = getDeclarationOfKind<AccessorDeclaration>(symbol, SyntaxKind.GetAccessor);
                    error(getter, Diagnostics._0_implicitly_has_return_type_any_because_it_does_not_have_a_return_type_annotation_and_is_referenced_directly_or_indirectly_in_one_of_its_return_expressions, symbolToString(symbol));
                }
            }
            return type;
        }

        function getBaseTypeVariableOfClass(symbol: Symbol) {
            const baseConstructorType = getBaseConstructorTypeOfClass(getDeclaredTypeOfClassOrInterface(symbol));
            return baseConstructorType.flags & TypeFlags.TypeVariable ? baseConstructorType : undefined;
        }

        function getTypeOfFuncClassEnumModule(symbol: Symbol): Type {
            let links = getSymbolLinks(symbol);
            const originalLinks = links;
            if (!links.type) {
                const jsDeclaration = getDeclarationOfExpando(symbol.valueDeclaration);
                if (jsDeclaration) {
                    const jsSymbol = getSymbolOfNode(jsDeclaration);
                    if (jsSymbol && (hasEntries(jsSymbol.exports) || hasEntries(jsSymbol.members))) {
                        symbol = cloneSymbol(symbol);
                        // note:we overwrite links because we just cloned the symbol
                        links = symbol as TransientSymbol;
                        if (hasEntries(jsSymbol.exports)) {
                            symbol.exports = symbol.exports || createSymbolTable();
                            mergeSymbolTable(symbol.exports, jsSymbol.exports);
                        }
                        if (hasEntries(jsSymbol.members)) {
                            symbol.members = symbol.members || createSymbolTable();
                            mergeSymbolTable(symbol.members, jsSymbol.members);
                        }
                    }
                }
                originalLinks.type = links.type = getTypeOfFuncClassEnumModuleWorker(symbol);
            }
            return links.type;
        }

        function getTypeOfFuncClassEnumModuleWorker(symbol: Symbol): Type {
            const declaration = symbol.valueDeclaration;
            if (symbol.flags & SymbolFlags.Module && isShorthandAmbientModuleSymbol(symbol)) {
                return anyType;
            }
            else if (declaration.kind === SyntaxKind.BinaryExpression ||
                     declaration.kind === SyntaxKind.PropertyAccessExpression && declaration.parent.kind === SyntaxKind.BinaryExpression) {
                return getWidenedTypeFromAssignmentDeclaration(symbol);
            }
            else if (symbol.flags & SymbolFlags.ValueModule && declaration && isSourceFile(declaration) && declaration.commonJsModuleIndicator) {
                const resolvedModule = resolveExternalModuleSymbol(symbol);
                if (resolvedModule !== symbol) {
                    if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
                        return errorType;
                    }
                    const exportEquals = getMergedSymbol(symbol.exports!.get(InternalSymbolName.ExportEquals)!);
                    const type = getWidenedTypeFromAssignmentDeclaration(exportEquals, exportEquals === resolvedModule ? undefined : resolvedModule);
                    if (!popTypeResolution()) {
                        return reportCircularityError(symbol);
                    }
                    return type;
                }
            }
            const type = createObjectType(ObjectFlags.Anonymous, symbol);
            if (symbol.flags & SymbolFlags.Class) {
                const baseTypeVariable = getBaseTypeVariableOfClass(symbol);
                return baseTypeVariable ? getIntersectionType([type, baseTypeVariable]) : type;
            }
            else {
                return strictNullChecks && symbol.flags & SymbolFlags.Optional ? getOptionalType(type) : type;
            }
        }

        function getTypeOfEnumMember(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            return links.type || (links.type = getDeclaredTypeOfEnumMember(symbol));
        }

        function getTypeOfAlias(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                const targetSymbol = resolveAlias(symbol);

                // It only makes sense to get the type of a value symbol. If the result of resolving
                // the alias is not a value, then it has no type. To get the type associated with a
                // type symbol, call getDeclaredTypeOfSymbol.
                // This check is important because without it, a call to getTypeOfSymbol could end
                // up recursively calling getTypeOfAlias, causing a stack overflow.
                links.type = targetSymbol.flags & SymbolFlags.Value
                    ? getTypeOfSymbol(targetSymbol)
                    : errorType;
            }
            return links.type;
        }

        function getTypeOfInstantiatedSymbol(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
                    return links.type = errorType;
                }
                let type = instantiateType(getTypeOfSymbol(links.target!), links.mapper!);
                if (!popTypeResolution()) {
                    type = reportCircularityError(symbol);
                }
                links.type = type;
            }
            return links.type;
        }

        function reportCircularityError(symbol: Symbol) {
            // Check if variable has type annotation that circularly references the variable itself
            if (getEffectiveTypeAnnotationNode(<VariableLikeDeclaration>symbol.valueDeclaration)) {
                error(symbol.valueDeclaration, Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_type_annotation,
                    symbolToString(symbol));
                return errorType;
            }
            // Otherwise variable has initializer that circularly references the variable itself
            if (noImplicitAny) {
                error(symbol.valueDeclaration, Diagnostics._0_implicitly_has_type_any_because_it_does_not_have_a_type_annotation_and_is_referenced_directly_or_indirectly_in_its_own_initializer,
                    symbolToString(symbol));
            }
            return anyType;
        }

        function getTypeOfSymbol(symbol: Symbol): Type {
            if (getCheckFlags(symbol) & CheckFlags.Instantiated) {
                return getTypeOfInstantiatedSymbol(symbol);
            }
            if (getCheckFlags(symbol) & CheckFlags.ReverseMapped) {
                return getTypeOfReverseMappedSymbol(symbol as ReverseMappedSymbol);
            }
            if (symbol.flags & (SymbolFlags.Variable | SymbolFlags.Property)) {
                return getTypeOfVariableOrParameterOrProperty(symbol);
            }
            if (symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.Enum | SymbolFlags.ValueModule)) {
                return getTypeOfFuncClassEnumModule(symbol);
            }
            if (symbol.flags & SymbolFlags.EnumMember) {
                return getTypeOfEnumMember(symbol);
            }
            if (symbol.flags & SymbolFlags.Accessor) {
                return getTypeOfAccessors(symbol);
            }
            if (symbol.flags & SymbolFlags.Alias) {
                return getTypeOfAlias(symbol);
            }
            return errorType;
        }

        function isReferenceToType(type: Type, target: Type) {
            return type !== undefined
                && target !== undefined
                && (getObjectFlags(type) & ObjectFlags.Reference) !== 0
                && (<TypeReference>type).target === target;
        }

        function getTargetType(type: Type): Type {
            return getObjectFlags(type) & ObjectFlags.Reference ? (<TypeReference>type).target : type;
        }

        // TODO: GH#18217 If `checkBase` is undefined, we should not call this because this will always return false.
        function hasBaseType(type: Type, checkBase: Type | undefined) {
            return check(type);
            function check(type: Type): boolean {
                if (getObjectFlags(type) & (ObjectFlags.ClassOrInterface | ObjectFlags.Reference)) {
                    const target = <InterfaceType>getTargetType(type);
                    return target === checkBase || some(getBaseTypes(target), check);
                }
                else if (type.flags & TypeFlags.Intersection) {
                    return some((<IntersectionType>type).types, check);
                }
                return false;
            }
        }

        // Appends the type parameters given by a list of declarations to a set of type parameters and returns the resulting set.
        // The function allocates a new array if the input type parameter set is undefined, but otherwise it modifies the set
        // in-place and returns the same array.
        function appendTypeParameters(typeParameters: TypeParameter[] | undefined, declarations: ReadonlyArray<TypeParameterDeclaration>): TypeParameter[] | undefined {
            for (const declaration of declarations) {
                typeParameters = appendIfUnique(typeParameters, getDeclaredTypeOfTypeParameter(getSymbolOfNode(declaration)));
            }
            return typeParameters;
        }

        // Return the outer type parameters of a node or undefined if the node has no outer type parameters.
        function getOuterTypeParameters(node: Node, includeThisTypes?: boolean): TypeParameter[] | undefined {
            while (true) {
                node = node.parent; // TODO: GH#18217 Use SourceFile kind check instead
                if (!node) {
                    return undefined;
                }
                switch (node.kind) {
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.ClassExpression:
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.CallSignature:
                    case SyntaxKind.ConstructSignature:
                    case SyntaxKind.MethodSignature:
                    case SyntaxKind.FunctionType:
                    case SyntaxKind.ConstructorType:
                    case SyntaxKind.JSDocFunctionType:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.ArrowFunction:
                    case SyntaxKind.TypeAliasDeclaration:
                    case SyntaxKind.JSDocTemplateTag:
                    case SyntaxKind.JSDocTypedefTag:
                    case SyntaxKind.JSDocCallbackTag:
                    case SyntaxKind.MappedType:
                    case SyntaxKind.ConditionalType:
                        const outerTypeParameters = getOuterTypeParameters(node, includeThisTypes);
                        if (node.kind === SyntaxKind.MappedType) {
                            return append(outerTypeParameters, getDeclaredTypeOfTypeParameter(getSymbolOfNode((<MappedTypeNode>node).typeParameter)));
                        }
                        else if (node.kind === SyntaxKind.ConditionalType) {
                            return concatenate(outerTypeParameters, getInferTypeParameters(<ConditionalTypeNode>node));
                        }
                        const outerAndOwnTypeParameters = appendTypeParameters(outerTypeParameters, getEffectiveTypeParameterDeclarations(<DeclarationWithTypeParameters>node));
                        const thisType = includeThisTypes &&
                            (node.kind === SyntaxKind.ClassDeclaration || node.kind === SyntaxKind.ClassExpression || node.kind === SyntaxKind.InterfaceDeclaration) &&
                            getDeclaredTypeOfClassOrInterface(getSymbolOfNode(node as ClassLikeDeclaration | InterfaceDeclaration)).thisType;
                        return thisType ? append(outerAndOwnTypeParameters, thisType) : outerAndOwnTypeParameters;
                }
            }
        }

        // The outer type parameters are those defined by enclosing generic classes, methods, or functions.
        function getOuterTypeParametersOfClassOrInterface(symbol: Symbol): TypeParameter[] | undefined {
            const declaration = symbol.flags & SymbolFlags.Class ? symbol.valueDeclaration : getDeclarationOfKind(symbol, SyntaxKind.InterfaceDeclaration)!;
            return getOuterTypeParameters(declaration);
        }

        // The local type parameters are the combined set of type parameters from all declarations of the class,
        // interface, or type alias.
        function getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol: Symbol): TypeParameter[] | undefined {
            let result: TypeParameter[] | undefined;
            for (const node of symbol.declarations) {
                if (node.kind === SyntaxKind.InterfaceDeclaration ||
                    node.kind === SyntaxKind.ClassDeclaration ||
                    node.kind === SyntaxKind.ClassExpression ||
                    isTypeAlias(node)) {
                    const declaration = <InterfaceDeclaration | TypeAliasDeclaration | JSDocTypedefTag | JSDocCallbackTag>node;
                    result = appendTypeParameters(result, getEffectiveTypeParameterDeclarations(declaration));
                }
            }
            return result;
        }

        // The full set of type parameters for a generic class or interface type consists of its outer type parameters plus
        // its locally declared type parameters.
        function getTypeParametersOfClassOrInterface(symbol: Symbol): TypeParameter[] | undefined {
            return concatenate(getOuterTypeParametersOfClassOrInterface(symbol), getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol));
        }

        // A type is a mixin constructor if it has a single construct signature taking no type parameters and a single
        // rest parameter of type any[].
        function isMixinConstructorType(type: Type) {
            const signatures = getSignaturesOfType(type, SignatureKind.Construct);
            if (signatures.length === 1) {
                const s = signatures[0];
                return !s.typeParameters && s.parameters.length === 1 && s.hasRestParameter && getTypeOfParameter(s.parameters[0]) === anyArrayType;
            }
            return false;
        }

        function isConstructorType(type: Type): boolean {
            if (isValidBaseType(type) && getSignaturesOfType(type, SignatureKind.Construct).length > 0) {
                return true;
            }
            if (type.flags & TypeFlags.TypeVariable) {
                const constraint = getBaseConstraintOfType(type);
                return !!constraint && isValidBaseType(constraint) && isMixinConstructorType(constraint);
            }
            return isJavascriptConstructorType(type);
        }

        function getBaseTypeNodeOfClass(type: InterfaceType): ExpressionWithTypeArguments | undefined {
            return getEffectiveBaseTypeNode(type.symbol.valueDeclaration as ClassLikeDeclaration);
        }

        function getConstructorsForTypeArguments(type: Type, typeArgumentNodes: ReadonlyArray<TypeNode> | undefined, location: Node): ReadonlyArray<Signature> {
            const typeArgCount = length(typeArgumentNodes);
            const isJavascript = isInJSFile(location);
            if (isJavascriptConstructorType(type) && !typeArgCount) {
                return getSignaturesOfType(type, SignatureKind.Call);
            }
            return filter(getSignaturesOfType(type, SignatureKind.Construct),
                sig => (isJavascript || typeArgCount >= getMinTypeArgumentCount(sig.typeParameters)) && typeArgCount <= length(sig.typeParameters));
        }

        function getInstantiatedConstructorsForTypeArguments(type: Type, typeArgumentNodes: ReadonlyArray<TypeNode> | undefined, location: Node): ReadonlyArray<Signature> {
            const signatures = getConstructorsForTypeArguments(type, typeArgumentNodes, location);
            const typeArguments = map(typeArgumentNodes, getTypeFromTypeNode);
            return sameMap<Signature>(signatures, sig => some(sig.typeParameters) ? getSignatureInstantiation(sig, typeArguments, isInJSFile(location)) : sig);
        }

        /**
         * The base constructor of a class can resolve to
         * * undefinedType if the class has no extends clause,
         * * unknownType if an error occurred during resolution of the extends expression,
         * * nullType if the extends expression is the null value,
         * * anyType if the extends expression has type any, or
         * * an object type with at least one construct signature.
         */
        function getBaseConstructorTypeOfClass(type: InterfaceType): Type {
            if (!type.resolvedBaseConstructorType) {
                const decl = <ClassLikeDeclaration>type.symbol.valueDeclaration;
                const extended = getEffectiveBaseTypeNode(decl);
                const baseTypeNode = getBaseTypeNodeOfClass(type);
                if (!baseTypeNode) {
                    return type.resolvedBaseConstructorType = undefinedType;
                }
                if (!pushTypeResolution(type, TypeSystemPropertyName.ResolvedBaseConstructorType)) {
                    return errorType;
                }
                const baseConstructorType = checkExpression(baseTypeNode.expression);
                if (extended && baseTypeNode !== extended) {
                    Debug.assert(!extended.typeArguments); // Because this is in a JS file, and baseTypeNode is in an @extends tag
                    checkExpression(extended.expression);
                }
                if (baseConstructorType.flags & (TypeFlags.Object | TypeFlags.Intersection)) {
                    // Resolving the members of a class requires us to resolve the base class of that class.
                    // We force resolution here such that we catch circularities now.
                    resolveStructuredTypeMembers(<ObjectType>baseConstructorType);
                }
                if (!popTypeResolution()) {
                    error(type.symbol.valueDeclaration, Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_base_expression, symbolToString(type.symbol));
                    return type.resolvedBaseConstructorType = errorType;
                }
                if (!(baseConstructorType.flags & TypeFlags.Any) && baseConstructorType !== nullWideningType && !isConstructorType(baseConstructorType)) {
                    error(baseTypeNode.expression, Diagnostics.Type_0_is_not_a_constructor_function_type, typeToString(baseConstructorType));
                    return type.resolvedBaseConstructorType = errorType;
                }
                type.resolvedBaseConstructorType = baseConstructorType;
            }
            return type.resolvedBaseConstructorType;
        }

        function getBaseTypes(type: InterfaceType): BaseType[] {
            if (!type.resolvedBaseTypes) {
                if (type.objectFlags & ObjectFlags.Tuple) {
                    type.resolvedBaseTypes = [createArrayType(getUnionType(type.typeParameters || emptyArray))];
                }
                else if (type.symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                    if (type.symbol.flags & SymbolFlags.Class) {
                        resolveBaseTypesOfClass(type);
                    }
                    if (type.symbol.flags & SymbolFlags.Interface) {
                        resolveBaseTypesOfInterface(type);
                    }
                }
                else {
                    Debug.fail("type must be class or interface");
                }
            }
            return type.resolvedBaseTypes;
        }

        function resolveBaseTypesOfClass(type: InterfaceType) {
            type.resolvedBaseTypes = resolvingEmptyArray;
            const baseConstructorType = getApparentType(getBaseConstructorTypeOfClass(type));
            if (!(baseConstructorType.flags & (TypeFlags.Object | TypeFlags.Intersection | TypeFlags.Any))) {
                return type.resolvedBaseTypes = emptyArray;
            }
            const baseTypeNode = getBaseTypeNodeOfClass(type)!;
            const typeArgs = typeArgumentsFromTypeReferenceNode(baseTypeNode);
            let baseType: Type;
            const originalBaseType = baseConstructorType && baseConstructorType.symbol ? getDeclaredTypeOfSymbol(baseConstructorType.symbol) : undefined;
            if (baseConstructorType.symbol && baseConstructorType.symbol.flags & SymbolFlags.Class &&
                areAllOuterTypeParametersApplied(originalBaseType!)) {
                // When base constructor type is a class with no captured type arguments we know that the constructors all have the same type parameters as the
                // class and all return the instance type of the class. There is no need for further checks and we can apply the
                // type arguments in the same manner as a type reference to get the same error reporting experience.
                baseType = getTypeFromClassOrInterfaceReference(baseTypeNode, baseConstructorType.symbol, typeArgs);
            }
            else if (baseConstructorType.flags & TypeFlags.Any) {
                baseType = baseConstructorType;
            }
            else if (isJavascriptConstructorType(baseConstructorType) && !baseTypeNode.typeArguments) {
                baseType = getJavascriptClassType(baseConstructorType.symbol) || anyType;
            }
            else {
                // The class derives from a "class-like" constructor function, check that we have at least one construct signature
                // with a matching number of type parameters and use the return type of the first instantiated signature. Elsewhere
                // we check that all instantiated signatures return the same type.
                const constructors = getInstantiatedConstructorsForTypeArguments(baseConstructorType, baseTypeNode.typeArguments, baseTypeNode);
                if (!constructors.length) {
                    error(baseTypeNode.expression, Diagnostics.No_base_constructor_has_the_specified_number_of_type_arguments);
                    return type.resolvedBaseTypes = emptyArray;
                }
                baseType = getReturnTypeOfSignature(constructors[0]);
            }

            if (baseType === errorType) {
                return type.resolvedBaseTypes = emptyArray;
            }
            if (!isValidBaseType(baseType)) {
                error(baseTypeNode.expression, Diagnostics.Base_constructor_return_type_0_is_not_a_class_or_interface_type, typeToString(baseType));
                return type.resolvedBaseTypes = emptyArray;
            }
            if (type === baseType || hasBaseType(baseType, type)) {
                error(type.symbol.valueDeclaration, Diagnostics.Type_0_recursively_references_itself_as_a_base_type,
                    typeToString(type, /*enclosingDeclaration*/ undefined, TypeFormatFlags.WriteArrayAsGenericType));
                return type.resolvedBaseTypes = emptyArray;
            }
            if (type.resolvedBaseTypes === resolvingEmptyArray) {
                // Circular reference, likely through instantiation of default parameters
                // (otherwise there'd be an error from hasBaseType) - this is fine, but `.members` should be reset
                // as `getIndexedAccessType` via `instantiateType` via `getTypeFromClassOrInterfaceReference` forces a
                // partial instantiation of the members without the base types fully resolved
                type.members = undefined;
            }
            return type.resolvedBaseTypes = [baseType];
        }

        function areAllOuterTypeParametersApplied(type: Type): boolean { // TODO: GH#18217 Shouldn't this take an InterfaceType?
            // An unapplied type parameter has its symbol still the same as the matching argument symbol.
            // Since parameters are applied outer-to-inner, only the last outer parameter needs to be checked.
            const outerTypeParameters = (<InterfaceType>type).outerTypeParameters;
            if (outerTypeParameters) {
                const last = outerTypeParameters.length - 1;
                const typeArguments = (<TypeReference>type).typeArguments!;
                return outerTypeParameters[last].symbol !== typeArguments[last].symbol;
            }
            return true;
        }

        // A valid base type is `any`, any non-generic object type or intersection of non-generic
        // object types.
        function isValidBaseType(type: Type): type is BaseType {
            return !!(type.flags & (TypeFlags.Object | TypeFlags.NonPrimitive | TypeFlags.Any)) && !isGenericMappedType(type) ||
                !!(type.flags & TypeFlags.Intersection) && every((<IntersectionType>type).types, isValidBaseType);
        }

        function resolveBaseTypesOfInterface(type: InterfaceType): void {
            type.resolvedBaseTypes = type.resolvedBaseTypes || emptyArray;
            for (const declaration of type.symbol.declarations) {
                if (declaration.kind === SyntaxKind.InterfaceDeclaration && getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration)) {
                    for (const node of getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration)!) {
                        const baseType = getTypeFromTypeNode(node);
                        if (baseType !== errorType) {
                            if (isValidBaseType(baseType)) {
                                if (type !== baseType && !hasBaseType(baseType, type)) {
                                    if (type.resolvedBaseTypes === emptyArray) {
                                        type.resolvedBaseTypes = [<ObjectType>baseType];
                                    }
                                    else {
                                        type.resolvedBaseTypes.push(baseType);
                                    }
                                }
                                else {
                                    error(declaration, Diagnostics.Type_0_recursively_references_itself_as_a_base_type, typeToString(type, /*enclosingDeclaration*/ undefined, TypeFormatFlags.WriteArrayAsGenericType));
                                }
                            }
                            else {
                                error(node, Diagnostics.An_interface_may_only_extend_a_class_or_another_interface);
                            }
                        }
                    }
                }
            }
        }

        /**
         * Returns true if the interface given by the symbol is free of "this" references.
         *
         * Specifically, the result is true if the interface itself contains no references
         * to "this" in its body, if all base types are interfaces,
         * and if none of the base interfaces have a "this" type.
         */
        function isThislessInterface(symbol: Symbol): boolean {
            for (const declaration of symbol.declarations) {
                if (declaration.kind === SyntaxKind.InterfaceDeclaration) {
                    if (declaration.flags & NodeFlags.ContainsThis) {
                        return false;
                    }
                    const baseTypeNodes = getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration);
                    if (baseTypeNodes) {
                        for (const node of baseTypeNodes) {
                            if (isEntityNameExpression(node.expression)) {
                                const baseSymbol = resolveEntityName(node.expression, SymbolFlags.Type, /*ignoreErrors*/ true);
                                if (!baseSymbol || !(baseSymbol.flags & SymbolFlags.Interface) || getDeclaredTypeOfClassOrInterface(baseSymbol).thisType) {
                                    return false;
                                }
                            }
                        }
                    }
                }
            }
            return true;
        }

        function getDeclaredTypeOfClassOrInterface(symbol: Symbol): InterfaceType {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const kind = symbol.flags & SymbolFlags.Class ? ObjectFlags.Class : ObjectFlags.Interface;
                const type = links.declaredType = <InterfaceType>createObjectType(kind, symbol);
                const outerTypeParameters = getOuterTypeParametersOfClassOrInterface(symbol);
                const localTypeParameters = getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol);
                // A class or interface is generic if it has type parameters or a "this" type. We always give classes a "this" type
                // because it is not feasible to analyze all members to determine if the "this" type escapes the class (in particular,
                // property types inferred from initializers and method return types inferred from return statements are very hard
                // to exhaustively analyze). We give interfaces a "this" type if we can't definitely determine that they are free of
                // "this" references.
                if (outerTypeParameters || localTypeParameters || kind === ObjectFlags.Class || !isThislessInterface(symbol)) {
                    type.objectFlags |= ObjectFlags.Reference;
                    type.typeParameters = concatenate(outerTypeParameters, localTypeParameters);
                    type.outerTypeParameters = outerTypeParameters;
                    type.localTypeParameters = localTypeParameters;
                    (<GenericType>type).instantiations = createMap<TypeReference>();
                    (<GenericType>type).instantiations.set(getTypeListId(type.typeParameters), <GenericType>type);
                    (<GenericType>type).target = <GenericType>type;
                    (<GenericType>type).typeArguments = type.typeParameters;
                    type.thisType = <TypeParameter>createType(TypeFlags.TypeParameter);
                    type.thisType.isThisType = true;
                    type.thisType.symbol = symbol;
                    type.thisType.constraint = type;
                }
            }
            return <InterfaceType>links.declaredType;
        }

        function getDeclaredTypeOfTypeAlias(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                // Note that we use the links object as the target here because the symbol object is used as the unique
                // identity for resolution of the 'type' property in SymbolLinks.
                if (!pushTypeResolution(symbol, TypeSystemPropertyName.DeclaredType)) {
                    return errorType;
                }

                const declaration = <JSDocTypedefTag | JSDocCallbackTag | TypeAliasDeclaration>find(symbol.declarations, d =>
                    isJSDocTypeAlias(d) || d.kind === SyntaxKind.TypeAliasDeclaration);
                const typeNode = isJSDocTypeAlias(declaration) ? declaration.typeExpression : declaration.type;
                // If typeNode is missing, we will error in checkJSDocTypedefTag.
                let type = typeNode ? getTypeFromTypeNode(typeNode) : errorType;

                if (popTypeResolution()) {
                    const typeParameters = getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol);
                    if (typeParameters) {
                        // Initialize the instantiation cache for generic type aliases. The declared type corresponds to
                        // an instantiation of the type alias with the type parameters supplied as type arguments.
                        links.typeParameters = typeParameters;
                        links.instantiations = createMap<Type>();
                        links.instantiations.set(getTypeListId(typeParameters), type);
                    }
                }
                else {
                    type = errorType;
                    error(declaration.name, Diagnostics.Type_alias_0_circularly_references_itself, symbolToString(symbol));
                }
                links.declaredType = type;
            }
            return links.declaredType;
        }

        function isStringConcatExpression(expr: Node): boolean {
            if (expr.kind === SyntaxKind.StringLiteral) {
                return true;
            }
            else if (expr.kind === SyntaxKind.BinaryExpression) {
                return isStringConcatExpression((<BinaryExpression>expr).left) && isStringConcatExpression((<BinaryExpression>expr).right);
            }
            return false;
        }

        function isLiteralEnumMember(member: EnumMember) {
            const expr = member.initializer;
            if (!expr) {
                return !(member.flags & NodeFlags.Ambient);
            }
            switch (expr.kind) {
                case SyntaxKind.StringLiteral:
                case SyntaxKind.NumericLiteral:
                    return true;
                case SyntaxKind.PrefixUnaryExpression:
                    return (<PrefixUnaryExpression>expr).operator === SyntaxKind.MinusToken &&
                        (<PrefixUnaryExpression>expr).operand.kind === SyntaxKind.NumericLiteral;
                case SyntaxKind.Identifier:
                    return nodeIsMissing(expr) || !!getSymbolOfNode(member.parent).exports!.get((<Identifier>expr).escapedText);
                case SyntaxKind.BinaryExpression:
                    return isStringConcatExpression(expr);
                default:
                    return false;
            }
        }

        function getEnumKind(symbol: Symbol): EnumKind {
            const links = getSymbolLinks(symbol);
            if (links.enumKind !== undefined) {
                return links.enumKind;
            }
            let hasNonLiteralMember = false;
            for (const declaration of symbol.declarations) {
                if (declaration.kind === SyntaxKind.EnumDeclaration) {
                    for (const member of (<EnumDeclaration>declaration).members) {
                        if (member.initializer && member.initializer.kind === SyntaxKind.StringLiteral) {
                            return links.enumKind = EnumKind.Literal;
                        }
                        if (!isLiteralEnumMember(member)) {
                            hasNonLiteralMember = true;
                        }
                    }
                }
            }
            return links.enumKind = hasNonLiteralMember ? EnumKind.Numeric : EnumKind.Literal;
        }

        function getBaseTypeOfEnumLiteralType(type: Type) {
            return type.flags & TypeFlags.EnumLiteral && !(type.flags & TypeFlags.Union) ? getDeclaredTypeOfSymbol(getParentOfSymbol(type.symbol)!) : type;
        }

        function getDeclaredTypeOfEnum(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (links.declaredType) {
                return links.declaredType;
            }
            if (getEnumKind(symbol) === EnumKind.Literal) {
                enumCount++;
                const memberTypeList: Type[] = [];
                for (const declaration of symbol.declarations) {
                    if (declaration.kind === SyntaxKind.EnumDeclaration) {
                        for (const member of (<EnumDeclaration>declaration).members) {
                            const memberType = getFreshTypeOfLiteralType(getLiteralType(getEnumMemberValue(member)!, enumCount, getSymbolOfNode(member))); // TODO: GH#18217
                            getSymbolLinks(getSymbolOfNode(member)).declaredType = memberType;
                            memberTypeList.push(getRegularTypeOfLiteralType(memberType));
                        }
                    }
                }
                if (memberTypeList.length) {
                    const enumType = getUnionType(memberTypeList, UnionReduction.Literal, symbol, /*aliasTypeArguments*/ undefined);
                    if (enumType.flags & TypeFlags.Union) {
                        enumType.flags |= TypeFlags.EnumLiteral;
                        enumType.symbol = symbol;
                    }
                    return links.declaredType = enumType;
                }
            }
            const enumType = createType(TypeFlags.Enum);
            enumType.symbol = symbol;
            return links.declaredType = enumType;
        }

        function getDeclaredTypeOfEnumMember(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const enumType = getDeclaredTypeOfEnum(getParentOfSymbol(symbol)!);
                if (!links.declaredType) {
                    links.declaredType = enumType;
                }
            }
            return links.declaredType;
        }

        function getDeclaredTypeOfTypeParameter(symbol: Symbol): TypeParameter {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const type = <TypeParameter>createType(TypeFlags.TypeParameter);
                type.symbol = symbol;
                links.declaredType = type;
            }
            return <TypeParameter>links.declaredType;
        }

        function getDeclaredTypeOfAlias(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                links.declaredType = getDeclaredTypeOfSymbol(resolveAlias(symbol));
            }
            return links.declaredType;
        }

        function getDeclaredTypeOfSymbol(symbol: Symbol): Type {
            return tryGetDeclaredTypeOfSymbol(symbol) || errorType;
        }

        function tryGetDeclaredTypeOfSymbol(symbol: Symbol): Type | undefined {
            if (symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                return getDeclaredTypeOfClassOrInterface(symbol);
            }
            if (symbol.flags & SymbolFlags.TypeAlias) {
                return getDeclaredTypeOfTypeAlias(symbol);
            }
            if (symbol.flags & SymbolFlags.TypeParameter) {
                return getDeclaredTypeOfTypeParameter(symbol);
            }
            if (symbol.flags & SymbolFlags.Enum) {
                return getDeclaredTypeOfEnum(symbol);
            }
            if (symbol.flags & SymbolFlags.EnumMember) {
                return getDeclaredTypeOfEnumMember(symbol);
            }
            if (symbol.flags & SymbolFlags.Alias) {
                return getDeclaredTypeOfAlias(symbol);
            }
            return undefined;
        }

        /**
         * A type is free of this references if it's the any, string, number, boolean, symbol, or void keyword, a string
         * literal type, an array with an element type that is free of this references, or a type reference that is
         * free of this references.
         */
        function isThislessType(node: TypeNode): boolean {
            switch (node.kind) {
                case SyntaxKind.AnyKeyword:
                case SyntaxKind.UnknownKeyword:
                case SyntaxKind.StringKeyword:
                case SyntaxKind.NumberKeyword:
                case SyntaxKind.BooleanKeyword:
                case SyntaxKind.SymbolKeyword:
                case SyntaxKind.ObjectKeyword:
                case SyntaxKind.VoidKeyword:
                case SyntaxKind.UndefinedKeyword:
                case SyntaxKind.NullKeyword:
                case SyntaxKind.NeverKeyword:
                case SyntaxKind.LiteralType:
                    return true;
                case SyntaxKind.ArrayType:
                    return isThislessType((<ArrayTypeNode>node).elementType);
                case SyntaxKind.TypeReference:
                    return !(node as TypeReferenceNode).typeArguments || (node as TypeReferenceNode).typeArguments!.every(isThislessType);
            }
            return false;
        }

        /** A type parameter is thisless if its contraint is thisless, or if it has no constraint. */
        function isThislessTypeParameter(node: TypeParameterDeclaration) {
            const constraint = getEffectiveConstraintOfTypeParameter(node);
            return !constraint || isThislessType(constraint);
        }

        /**
         * A variable-like declaration is free of this references if it has a type annotation
         * that is thisless, or if it has no type annotation and no initializer (and is thus of type any).
         */
        function isThislessVariableLikeDeclaration(node: VariableLikeDeclaration): boolean {
            const typeNode = getEffectiveTypeAnnotationNode(node);
            return typeNode ? isThislessType(typeNode) : !hasInitializer(node);
        }

        /**
         * A function-like declaration is considered free of `this` references if it has a return type
         * annotation that is free of this references and if each parameter is thisless and if
         * each type parameter (if present) is thisless.
         */
        function isThislessFunctionLikeDeclaration(node: FunctionLikeDeclaration): boolean {
            const returnType = getEffectiveReturnTypeNode(node);
            const typeParameters = getEffectiveTypeParameterDeclarations(node);
            return (node.kind === SyntaxKind.Constructor || (!!returnType && isThislessType(returnType))) &&
                node.parameters.every(isThislessVariableLikeDeclaration) &&
                typeParameters.every(isThislessTypeParameter);
        }

        /**
         * Returns true if the class or interface member given by the symbol is free of "this" references. The
         * function may return false for symbols that are actually free of "this" references because it is not
         * feasible to perform a complete analysis in all cases. In particular, property members with types
         * inferred from their initializers and function members with inferred return types are conservatively
         * assumed not to be free of "this" references.
         */
        function isThisless(symbol: Symbol): boolean {
            if (symbol.declarations && symbol.declarations.length === 1) {
                const declaration = symbol.declarations[0];
                if (declaration) {
                    switch (declaration.kind) {
                        case SyntaxKind.PropertyDeclaration:
                        case SyntaxKind.PropertySignature:
                            return isThislessVariableLikeDeclaration(<VariableLikeDeclaration>declaration);
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.MethodSignature:
                        case SyntaxKind.Constructor:
                            return isThislessFunctionLikeDeclaration(<FunctionLikeDeclaration>declaration);
                    }
                }
            }
            return false;
        }

        // The mappingThisOnly flag indicates that the only type parameter being mapped is "this". When the flag is true,
        // we check symbols to see if we can quickly conclude they are free of "this" references, thus needing no instantiation.
        function createInstantiatedSymbolTable(symbols: Symbol[], mapper: TypeMapper, mappingThisOnly: boolean): SymbolTable {
            const result = createSymbolTable();
            for (const symbol of symbols) {
                result.set(symbol.escapedName, mappingThisOnly && isThisless(symbol) ? symbol : instantiateSymbol(symbol, mapper));
            }
            return result;
        }

        function addInheritedMembers(symbols: SymbolTable, baseSymbols: Symbol[]) {
            for (const s of baseSymbols) {
                if (!symbols.has(s.escapedName)) {
                    symbols.set(s.escapedName, s);
                }
            }
        }

        function resolveDeclaredMembers(type: InterfaceType): InterfaceTypeWithDeclaredMembers {
            if (!(<InterfaceTypeWithDeclaredMembers>type).declaredProperties) {
                const symbol = type.symbol;
                const members = getMembersOfSymbol(symbol);
                (<InterfaceTypeWithDeclaredMembers>type).declaredProperties = getNamedMembers(members);
                // Start with signatures at empty array in case of recursive types
                (<InterfaceTypeWithDeclaredMembers>type).declaredCallSignatures = emptyArray;
                (<InterfaceTypeWithDeclaredMembers>type).declaredConstructSignatures = emptyArray;

                (<InterfaceTypeWithDeclaredMembers>type).declaredCallSignatures = getSignaturesOfSymbol(members.get(InternalSymbolName.Call));
                (<InterfaceTypeWithDeclaredMembers>type).declaredConstructSignatures = getSignaturesOfSymbol(members.get(InternalSymbolName.New));
                (<InterfaceTypeWithDeclaredMembers>type).declaredStringIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.String);
                (<InterfaceTypeWithDeclaredMembers>type).declaredNumberIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.Number);
            }
            return <InterfaceTypeWithDeclaredMembers>type;
        }

        /**
         * Indicates whether a type can be used as a late-bound name.
         */
        function isTypeUsableAsLateBoundName(type: Type): type is LiteralType | UniqueESSymbolType {
            return !!(type.flags & TypeFlags.StringOrNumberLiteralOrUnique);
        }

        /**
         * Indicates whether a declaration name is definitely late-bindable.
         * A declaration name is only late-bindable if:
         * - It is a `ComputedPropertyName`.
         * - Its expression is an `Identifier` or either a `PropertyAccessExpression` an
         * `ElementAccessExpression` consisting only of these same three types of nodes.
         * - The type of its expression is a string or numeric literal type, or is a `unique symbol` type.
         */
        function isLateBindableName(node: DeclarationName): node is LateBoundName {
            return isComputedPropertyName(node)
                && isEntityNameExpression(node.expression)
                && isTypeUsableAsLateBoundName(checkComputedPropertyName(node));
        }

        function isLateBoundName(name: __String): boolean {
            return (name as string).charCodeAt(0) === CharacterCodes._ &&
                (name as string).charCodeAt(1) === CharacterCodes._ &&
                (name as string).charCodeAt(2) === CharacterCodes.at;
        }

        /**
         * Indicates whether a declaration has a late-bindable dynamic name.
         */
        function hasLateBindableName(node: Declaration): node is LateBoundDeclaration {
            const name = getNameOfDeclaration(node);
            return !!name && isLateBindableName(name);
        }

        /**
         * Indicates whether a declaration has a dynamic name that cannot be late-bound.
         */
        function hasNonBindableDynamicName(node: Declaration) {
            return hasDynamicName(node) && !hasLateBindableName(node);
        }

        /**
         * Indicates whether a declaration name is a dynamic name that cannot be late-bound.
         */
        function isNonBindableDynamicName(node: DeclarationName) {
            return isDynamicName(node) && !isLateBindableName(node);
        }

        /**
         * Gets the symbolic name for a late-bound member from its type.
         */
        function getLateBoundNameFromType(type: LiteralType | UniqueESSymbolType): __String {
            if (type.flags & TypeFlags.UniqueESSymbol) {
                return `__@${type.symbol.escapedName}@${getSymbolId(type.symbol)}` as __String;
            }
            if (type.flags & TypeFlags.StringOrNumberLiteral) {
                return escapeLeadingUnderscores("" + (<LiteralType>type).value);
            }
            return Debug.fail();
        }

        /**
         * Adds a declaration to a late-bound dynamic member. This performs the same function for
         * late-bound members that `addDeclarationToSymbol` in binder.ts performs for early-bound
         * members.
         */
        function addDeclarationToLateBoundSymbol(symbol: Symbol, member: LateBoundDeclaration, symbolFlags: SymbolFlags) {
            Debug.assert(!!(getCheckFlags(symbol) & CheckFlags.Late), "Expected a late-bound symbol.");
            symbol.flags |= symbolFlags;
            getSymbolLinks(member.symbol).lateSymbol = symbol;
            if (!symbol.declarations) {
                symbol.declarations = [member];
            }
            else {
                symbol.declarations.push(member);
            }
            if (symbolFlags & SymbolFlags.Value) {
                if (!symbol.valueDeclaration || symbol.valueDeclaration.kind !== member.kind) {
                    symbol.valueDeclaration = member;
                }
            }
        }

        /**
         * Performs late-binding of a dynamic member. This performs the same function for
         * late-bound members that `declareSymbol` in binder.ts performs for early-bound
         * members.
         *
         * If a symbol is a dynamic name from a computed property, we perform an additional "late"
         * binding phase to attempt to resolve the name for the symbol from the type of the computed
         * property's expression. If the type of the expression is a string-literal, numeric-literal,
         * or unique symbol type, we can use that type as the name of the symbol.
         *
         * For example, given:
         *
         *   const x = Symbol();
         *
         *   interface I {
         *     [x]: number;
         *   }
         *
         * The binder gives the property `[x]: number` a special symbol with the name "__computed".
         * In the late-binding phase we can type-check the expression `x` and see that it has a
         * unique symbol type which we can then use as the name of the member. This allows users
         * to define custom symbols that can be used in the members of an object type.
         *
         * @param parent The containing symbol for the member.
         * @param earlySymbols The early-bound symbols of the parent.
         * @param lateSymbols The late-bound symbols of the parent.
         * @param decl The member to bind.
         */
        function lateBindMember(parent: Symbol, earlySymbols: SymbolTable | undefined, lateSymbols: SymbolTable, decl: LateBoundDeclaration) {
            Debug.assert(!!decl.symbol, "The member is expected to have a symbol.");
            const links = getNodeLinks(decl);
            if (!links.resolvedSymbol) {
                // In the event we attempt to resolve the late-bound name of this member recursively,
                // fall back to the early-bound name of this member.
                links.resolvedSymbol = decl.symbol;
                const type = checkComputedPropertyName(decl.name);
                if (isTypeUsableAsLateBoundName(type)) {
                    const memberName = getLateBoundNameFromType(type);
                    const symbolFlags = decl.symbol.flags;

                    // Get or add a late-bound symbol for the member. This allows us to merge late-bound accessor declarations.
                    let lateSymbol = lateSymbols.get(memberName);
                    if (!lateSymbol) lateSymbols.set(memberName, lateSymbol = createSymbol(SymbolFlags.None, memberName, CheckFlags.Late));

                    // Report an error if a late-bound member has the same name as an early-bound member,
                    // or if we have another early-bound symbol declaration with the same name and
                    // conflicting flags.
                    const earlySymbol = earlySymbols && earlySymbols.get(memberName);
                    if (lateSymbol.flags & getExcludedSymbolFlags(symbolFlags) || earlySymbol) {
                        // If we have an existing early-bound member, combine its declarations so that we can
                        // report an error at each declaration.
                        const declarations = earlySymbol ? concatenate(earlySymbol.declarations, lateSymbol.declarations) : lateSymbol.declarations;
                        const name = declarationNameToString(decl.name);
                        forEach(declarations, declaration => error(getNameOfDeclaration(declaration) || declaration, Diagnostics.Duplicate_declaration_0, name));
                        error(decl.name || decl, Diagnostics.Duplicate_declaration_0, name);
                        lateSymbol = createSymbol(SymbolFlags.None, memberName, CheckFlags.Late);
                    }
                    lateSymbol.nameType = type;
                    addDeclarationToLateBoundSymbol(lateSymbol, decl, symbolFlags);
                    if (lateSymbol.parent) {
                        Debug.assert(lateSymbol.parent === parent, "Existing symbol parent should match new one");
                    }
                    else {
                        lateSymbol.parent = parent;
                    }
                    return links.resolvedSymbol = lateSymbol;
                }
            }
            return links.resolvedSymbol;
        }

        function getResolvedMembersOrExportsOfSymbol(symbol: Symbol, resolutionKind: MembersOrExportsResolutionKind): UnderscoreEscapedMap<Symbol> {
            const links = getSymbolLinks(symbol);
            if (!links[resolutionKind]) {
                const isStatic = resolutionKind === MembersOrExportsResolutionKind.resolvedExports;
                const earlySymbols = !isStatic ? symbol.members :
                    symbol.flags & SymbolFlags.Module ? getExportsOfModuleWorker(symbol) :
                    symbol.exports;

                // In the event we recursively resolve the members/exports of the symbol, we
                // set the initial value of resolvedMembers/resolvedExports to the early-bound
                // members/exports of the symbol.
                links[resolutionKind] = earlySymbols || emptySymbols;

                // fill in any as-yet-unresolved late-bound members.
                const lateSymbols = createSymbolTable();
                for (const decl of symbol.declarations) {
                    const members = getMembersOfDeclaration(decl);
                    if (members) {
                        for (const member of members) {
                            if (isStatic === hasStaticModifier(member) && hasLateBindableName(member)) {
                                lateBindMember(symbol, earlySymbols, lateSymbols, member);
                            }
                        }
                    }
                }

                links[resolutionKind] = combineSymbolTables(earlySymbols, lateSymbols) || emptySymbols;
            }

            return links[resolutionKind]!;
        }

        /**
         * Gets a SymbolTable containing both the early- and late-bound members of a symbol.
         *
         * For a description of late-binding, see `lateBindMember`.
         */
        function getMembersOfSymbol(symbol: Symbol) {
            return symbol.flags & SymbolFlags.LateBindingContainer
                ? getResolvedMembersOrExportsOfSymbol(symbol, MembersOrExportsResolutionKind.resolvedMembers)
                : symbol.members || emptySymbols;
        }

        /**
         * If a symbol is the dynamic name of the member of an object type, get the late-bound
         * symbol of the member.
         *
         * For a description of late-binding, see `lateBindMember`.
         */
        function getLateBoundSymbol(symbol: Symbol): Symbol {
            if (symbol.flags & SymbolFlags.ClassMember && symbol.escapedName === InternalSymbolName.Computed) {
                const links = getSymbolLinks(symbol);
                if (!links.lateSymbol && some(symbol.declarations, hasLateBindableName)) {
                    // force late binding of members/exports. This will set the late-bound symbol
                    if (some(symbol.declarations, hasStaticModifier)) {
                        getExportsOfSymbol(symbol.parent!);
                    }
                    else {
                        getMembersOfSymbol(symbol.parent!);
                    }
                }
                return links.lateSymbol || (links.lateSymbol = symbol);
            }
            return symbol;
        }

        function getTypeWithThisArgument(type: Type, thisArgument?: Type, needApparentType?: boolean): Type {
            if (getObjectFlags(type) & ObjectFlags.Reference) {
                const target = (<TypeReference>type).target;
                const typeArguments = (<TypeReference>type).typeArguments;
                if (length(target.typeParameters) === length(typeArguments)) {
                    const ref = createTypeReference(target, concatenate(typeArguments, [thisArgument || target.thisType!]));
                    return needApparentType ? getApparentType(ref) : ref;
                }
            }
            else if (type.flags & TypeFlags.Intersection) {
                return getIntersectionType(map((<IntersectionType>type).types, t => getTypeWithThisArgument(t, thisArgument, needApparentType)));
            }
            return needApparentType ? getApparentType(type) : type;
        }

        function resolveObjectTypeMembers(type: ObjectType, source: InterfaceTypeWithDeclaredMembers, typeParameters: ReadonlyArray<TypeParameter>, typeArguments: ReadonlyArray<Type>) {
            let mapper: TypeMapper;
            let members: SymbolTable;
            let callSignatures: ReadonlyArray<Signature>;
            let constructSignatures: ReadonlyArray<Signature> | undefined;
            let stringIndexInfo: IndexInfo | undefined;
            let numberIndexInfo: IndexInfo | undefined;
            if (rangeEquals(typeParameters, typeArguments, 0, typeParameters.length)) {
                mapper = identityMapper;
                members = source.symbol ? getMembersOfSymbol(source.symbol) : createSymbolTable(source.declaredProperties);
                callSignatures = source.declaredCallSignatures;
                constructSignatures = source.declaredConstructSignatures;
                stringIndexInfo = source.declaredStringIndexInfo;
                numberIndexInfo = source.declaredNumberIndexInfo;
            }
            else {
                mapper = createTypeMapper(typeParameters, typeArguments);
                members = createInstantiatedSymbolTable(source.declaredProperties, mapper, /*mappingThisOnly*/ typeParameters.length === 1);
                callSignatures = instantiateSignatures(source.declaredCallSignatures, mapper);
                constructSignatures = instantiateSignatures(source.declaredConstructSignatures, mapper);
                stringIndexInfo = instantiateIndexInfo(source.declaredStringIndexInfo, mapper);
                numberIndexInfo = instantiateIndexInfo(source.declaredNumberIndexInfo, mapper);
            }
            const baseTypes = getBaseTypes(source);
            if (baseTypes.length) {
                if (source.symbol && members === getMembersOfSymbol(source.symbol)) {
                    members = createSymbolTable(source.declaredProperties);
                }
                setStructuredTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
                const thisArgument = lastOrUndefined(typeArguments);
                for (const baseType of baseTypes) {
                    const instantiatedBaseType = thisArgument ? getTypeWithThisArgument(instantiateType(baseType, mapper), thisArgument) : baseType;
                    addInheritedMembers(members, getPropertiesOfType(instantiatedBaseType));
                    callSignatures = concatenate(callSignatures, getSignaturesOfType(instantiatedBaseType, SignatureKind.Call));
                    constructSignatures = concatenate(constructSignatures, getSignaturesOfType(instantiatedBaseType, SignatureKind.Construct));
                    if (!stringIndexInfo) {
                        stringIndexInfo = instantiatedBaseType === anyType ?
                            createIndexInfo(anyType, /*isReadonly*/ false) :
                            getIndexInfoOfType(instantiatedBaseType, IndexKind.String);
                    }
                    numberIndexInfo = numberIndexInfo || getIndexInfoOfType(instantiatedBaseType, IndexKind.Number);
                }
            }
            setStructuredTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function resolveClassOrInterfaceMembers(type: InterfaceType): void {
            resolveObjectTypeMembers(type, resolveDeclaredMembers(type), emptyArray, emptyArray);
        }

        function resolveTypeReferenceMembers(type: TypeReference): void {
            const source = resolveDeclaredMembers(type.target);
            const typeParameters = concatenate(source.typeParameters!, [source.thisType!]);
            const typeArguments = type.typeArguments && type.typeArguments.length === typeParameters.length ?
                type.typeArguments : concatenate(type.typeArguments, [type]);
            resolveObjectTypeMembers(type, source, typeParameters, typeArguments);
        }

        function createSignature(
            declaration: SignatureDeclaration | JSDocSignature | undefined,
            typeParameters: ReadonlyArray<TypeParameter> | undefined,
            thisParameter: Symbol | undefined,
            parameters: ReadonlyArray<Symbol>,
            resolvedReturnType: Type | undefined,
            resolvedTypePredicate: TypePredicate | undefined,
            minArgumentCount: number,
            hasRestParameter: boolean,
            hasLiteralTypes: boolean,
        ): Signature {
            const sig = new Signature(checker);
            sig.declaration = declaration;
            sig.typeParameters = typeParameters;
            sig.parameters = parameters;
            sig.thisParameter = thisParameter;
            sig.resolvedReturnType = resolvedReturnType;
            sig.resolvedTypePredicate = resolvedTypePredicate;
            sig.minArgumentCount = minArgumentCount;
            sig.hasRestParameter = hasRestParameter;
            sig.hasLiteralTypes = hasLiteralTypes;
            sig.target = undefined;
            sig.mapper = undefined;
            return sig;
        }

        function cloneSignature(sig: Signature): Signature {
            return createSignature(sig.declaration, sig.typeParameters, sig.thisParameter, sig.parameters, /*resolvedReturnType*/ undefined,
                /*resolvedTypePredicate*/ undefined, sig.minArgumentCount, sig.hasRestParameter, sig.hasLiteralTypes);
        }

        function getExpandedParameters(sig: Signature): ReadonlyArray<Symbol> {
            if (sig.hasRestParameter) {
                const restIndex = sig.parameters.length - 1;
                const restParameter = sig.parameters[restIndex];
                const restType = getTypeOfSymbol(restParameter);
                if (isTupleType(restType)) {
                    const elementTypes = restType.typeArguments || emptyArray;
                    const minLength = restType.target.minLength;
                    const tupleRestIndex = restType.target.hasRestElement ? elementTypes.length - 1 : -1;
                    const restParams = map(elementTypes, (t, i) => {
                        const name = getParameterNameAtPosition(sig, restIndex + i);
                        const checkFlags = i === tupleRestIndex ? CheckFlags.RestParameter :
                            i >= minLength ? CheckFlags.OptionalParameter : 0;
                        const symbol = createSymbol(SymbolFlags.FunctionScopedVariable, name, checkFlags);
                        symbol.type = i === tupleRestIndex ? createArrayType(t) : t;
                        return symbol;
                    });
                    return concatenate(sig.parameters.slice(0, restIndex), restParams);
                }
            }
            return sig.parameters;
        }

        function getDefaultConstructSignatures(classType: InterfaceType): Signature[] {
            const baseConstructorType = getBaseConstructorTypeOfClass(classType);
            const baseSignatures = getSignaturesOfType(baseConstructorType, SignatureKind.Construct);
            if (baseSignatures.length === 0) {
                return [createSignature(undefined, classType.localTypeParameters, undefined, emptyArray, classType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false)]; // TODO: GH#18217
            }
            const baseTypeNode = getBaseTypeNodeOfClass(classType)!;
            const isJavaScript = isInJSFile(baseTypeNode);
            const typeArguments = typeArgumentsFromTypeReferenceNode(baseTypeNode);
            const typeArgCount = length(typeArguments);
            const result: Signature[] = [];
            for (const baseSig of baseSignatures) {
                const minTypeArgumentCount = getMinTypeArgumentCount(baseSig.typeParameters);
                const typeParamCount = length(baseSig.typeParameters);
                if (isJavaScript || typeArgCount >= minTypeArgumentCount && typeArgCount <= typeParamCount) {
                    const sig = typeParamCount ? createSignatureInstantiation(baseSig, fillMissingTypeArguments(typeArguments, baseSig.typeParameters, minTypeArgumentCount, isJavaScript)!) : cloneSignature(baseSig);
                    sig.typeParameters = classType.localTypeParameters;
                    sig.resolvedReturnType = classType;
                    result.push(sig);
                }
            }
            return result;
        }

        function findMatchingSignature(signatureList: ReadonlyArray<Signature>, signature: Signature, partialMatch: boolean, ignoreThisTypes: boolean, ignoreReturnTypes: boolean): Signature | undefined {
            for (const s of signatureList) {
                if (compareSignaturesIdentical(s, signature, partialMatch, ignoreThisTypes, ignoreReturnTypes, compareTypesIdentical)) {
                    return s;
                }
            }
        }

        function findMatchingSignatures(signatureLists: ReadonlyArray<ReadonlyArray<Signature>>, signature: Signature, listIndex: number): Signature[] | undefined {
            if (signature.typeParameters) {
                // We require an exact match for generic signatures, so we only return signatures from the first
                // signature list and only if they have exact matches in the other signature lists.
                if (listIndex > 0) {
                    return undefined;
                }
                for (let i = 1; i < signatureLists.length; i++) {
                    if (!findMatchingSignature(signatureLists[i], signature, /*partialMatch*/ false, /*ignoreThisTypes*/ false, /*ignoreReturnTypes*/ false)) {
                        return undefined;
                    }
                }
                return [signature];
            }
            let result: Signature[] | undefined;
            for (let i = 0; i < signatureLists.length; i++) {
                // Allow matching non-generic signatures to have excess parameters and different return types
                const match = i === listIndex ? signature : findMatchingSignature(signatureLists[i], signature, /*partialMatch*/ true, /*ignoreThisTypes*/ true, /*ignoreReturnTypes*/ true);
                if (!match) {
                    return undefined;
                }
                result = appendIfUnique(result, match);
            }
            return result;
        }

        // The signatures of a union type are those signatures that are present in each of the constituent types.
        // Generic signatures must match exactly, but non-generic signatures are allowed to have extra optional
        // parameters and may differ in return types. When signatures differ in return types, the resulting return
        // type is the union of the constituent return types.
        function getUnionSignatures(types: ReadonlyArray<Type>, kind: SignatureKind): Signature[] {
            const signatureLists = map(types, t => getSignaturesOfType(t, kind));
            let result: Signature[] | undefined;
            for (let i = 0; i < signatureLists.length; i++) {
                for (const signature of signatureLists[i]) {
                    // Only process signatures with parameter lists that aren't already in the result list
                    if (!result || !findMatchingSignature(result, signature, /*partialMatch*/ false, /*ignoreThisTypes*/ true, /*ignoreReturnTypes*/ true)) {
                        const unionSignatures = findMatchingSignatures(signatureLists, signature, i);
                        if (unionSignatures) {
                            let s = signature;
                            // Union the result types when more than one signature matches
                            if (unionSignatures.length > 1) {
                                let thisParameter = signature.thisParameter;
                                if (forEach(unionSignatures, sig => sig.thisParameter)) {
                                    // TODO: GH#18217 We tested that *some* has thisParameter and now act as if *all* do
                                    const thisType = getUnionType(map(unionSignatures, sig => sig.thisParameter ? getTypeOfSymbol(sig.thisParameter) : anyType), UnionReduction.Subtype);
                                    thisParameter = createSymbolWithType(signature.thisParameter!, thisType);
                                }
                                s = cloneSignature(signature);
                                s.thisParameter = thisParameter;
                                s.unionSignatures = unionSignatures;
                            }
                            (result || (result = [])).push(s);
                        }
                    }
                }
            }
            return result || emptyArray;
        }

        function getUnionIndexInfo(types: ReadonlyArray<Type>, kind: IndexKind): IndexInfo | undefined {
            const indexTypes: Type[] = [];
            let isAnyReadonly = false;
            for (const type of types) {
                const indexInfo = getIndexInfoOfType(type, kind);
                if (!indexInfo) {
                    return undefined;
                }
                indexTypes.push(indexInfo.type);
                isAnyReadonly = isAnyReadonly || indexInfo.isReadonly;
            }
            return createIndexInfo(getUnionType(indexTypes, UnionReduction.Subtype), isAnyReadonly);
        }

        function resolveUnionTypeMembers(type: UnionType) {
            // The members and properties collections are empty for union types. To get all properties of a union
            // type use getPropertiesOfType (only the language service uses this).
            const callSignatures = getUnionSignatures(type.types, SignatureKind.Call);
            const constructSignatures = getUnionSignatures(type.types, SignatureKind.Construct);
            const stringIndexInfo = getUnionIndexInfo(type.types, IndexKind.String);
            const numberIndexInfo = getUnionIndexInfo(type.types, IndexKind.Number);
            setStructuredTypeMembers(type, emptySymbols, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function intersectTypes(type1: Type, type2: Type): Type;
        function intersectTypes(type1: Type | undefined, type2: Type | undefined): Type | undefined;
        function intersectTypes(type1: Type | undefined, type2: Type | undefined): Type | undefined {
            return !type1 ? type2 : !type2 ? type1 : getIntersectionType([type1, type2]);
        }

        function intersectIndexInfos(info1: IndexInfo | undefined, info2: IndexInfo | undefined): IndexInfo | undefined {
            return !info1 ? info2 : !info2 ? info1 : createIndexInfo(
                getIntersectionType([info1.type, info2.type]), info1.isReadonly && info2.isReadonly);
        }

        function unionSpreadIndexInfos(info1: IndexInfo | undefined, info2: IndexInfo | undefined): IndexInfo | undefined {
            return info1 && info2 && createIndexInfo(
                getUnionType([info1.type, info2.type]), info1.isReadonly || info2.isReadonly);
        }

        function includeMixinType(type: Type, types: ReadonlyArray<Type>, index: number): Type {
            const mixedTypes: Type[] = [];
            for (let i = 0; i < types.length; i++) {
                if (i === index) {
                    mixedTypes.push(type);
                }
                else if (isMixinConstructorType(types[i])) {
                    mixedTypes.push(getReturnTypeOfSignature(getSignaturesOfType(types[i], SignatureKind.Construct)[0]));
                }
            }
            return getIntersectionType(mixedTypes);
        }

        function resolveIntersectionTypeMembers(type: IntersectionType) {
            // The members and properties collections are empty for intersection types. To get all properties of an
            // intersection type use getPropertiesOfType (only the language service uses this).
            let callSignatures: ReadonlyArray<Signature> = emptyArray;
            let constructSignatures: ReadonlyArray<Signature> = emptyArray;
            let stringIndexInfo: IndexInfo | undefined;
            let numberIndexInfo: IndexInfo | undefined;
            const types = type.types;
            const mixinCount = countWhere(types, isMixinConstructorType);
            for (let i = 0; i < types.length; i++) {
                const t = type.types[i];
                // When an intersection type contains mixin constructor types, the construct signatures from
                // those types are discarded and their return types are mixed into the return types of all
                // other construct signatures in the intersection type. For example, the intersection type
                // '{ new(...args: any[]) => A } & { new(s: string) => B }' has a single construct signature
                // 'new(s: string) => A & B'.
                if (mixinCount === 0 || mixinCount === types.length && i === 0 || !isMixinConstructorType(t)) {
                    let signatures = getSignaturesOfType(t, SignatureKind.Construct);
                    if (signatures.length && mixinCount > 0) {
                        signatures = map(signatures, s => {
                            const clone = cloneSignature(s);
                            clone.resolvedReturnType = includeMixinType(getReturnTypeOfSignature(s), types, i);
                            return clone;
                        });
                    }
                    constructSignatures = concatenate(constructSignatures, signatures);
                }
                callSignatures = concatenate(callSignatures, getSignaturesOfType(t, SignatureKind.Call));
                stringIndexInfo = intersectIndexInfos(stringIndexInfo, getIndexInfoOfType(t, IndexKind.String));
                numberIndexInfo = intersectIndexInfos(numberIndexInfo, getIndexInfoOfType(t, IndexKind.Number));
            }
            setStructuredTypeMembers(type, emptySymbols, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        /**
         * Converts an AnonymousType to a ResolvedType.
         */
        function resolveAnonymousTypeMembers(type: AnonymousType) {
            const symbol = type.symbol;
            if (type.target) {
                setStructuredTypeMembers(type, emptySymbols, emptyArray, emptyArray, undefined, undefined);
                const members = createInstantiatedSymbolTable(getPropertiesOfObjectType(type.target), type.mapper!, /*mappingThisOnly*/ false);
                const callSignatures = instantiateSignatures(getSignaturesOfType(type.target, SignatureKind.Call), type.mapper!);
                const constructSignatures = instantiateSignatures(getSignaturesOfType(type.target, SignatureKind.Construct), type.mapper!);
                const stringIndexInfo = instantiateIndexInfo(getIndexInfoOfType(type.target, IndexKind.String), type.mapper!);
                const numberIndexInfo = instantiateIndexInfo(getIndexInfoOfType(type.target, IndexKind.Number), type.mapper!);
                setStructuredTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
            }
            else if (symbol.flags & SymbolFlags.TypeLiteral) {
                setStructuredTypeMembers(type, emptySymbols, emptyArray, emptyArray, undefined, undefined);
                const members = getMembersOfSymbol(symbol);
                const callSignatures = getSignaturesOfSymbol(members.get(InternalSymbolName.Call));
                const constructSignatures = getSignaturesOfSymbol(members.get(InternalSymbolName.New));
                const stringIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.String);
                const numberIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.Number);
                setStructuredTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
            }
            else {
                // Combinations of function, class, enum and module
                let members = emptySymbols;
                let stringIndexInfo: IndexInfo | undefined;
                if (symbol.exports) {
                    members = getExportsOfSymbol(symbol);
                }
                setStructuredTypeMembers(type, members, emptyArray, emptyArray, undefined, undefined);
                if (symbol.flags & SymbolFlags.Class) {
                    const classType = getDeclaredTypeOfClassOrInterface(symbol);
                    const baseConstructorType = getBaseConstructorTypeOfClass(classType);
                    if (baseConstructorType.flags & (TypeFlags.Object | TypeFlags.Intersection | TypeFlags.TypeVariable)) {
                        members = createSymbolTable(getNamedMembers(members));
                        addInheritedMembers(members, getPropertiesOfType(baseConstructorType));
                    }
                    else if (baseConstructorType === anyType) {
                        stringIndexInfo = createIndexInfo(anyType, /*isReadonly*/ false);
                    }
                }
                const numberIndexInfo = symbol.flags & SymbolFlags.Enum ? enumNumberIndexInfo : undefined;
                setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
                // We resolve the members before computing the signatures because a signature may use
                // typeof with a qualified name expression that circularly references the type we are
                // in the process of resolving (see issue #6072). The temporarily empty signature list
                // will never be observed because a qualified name can't reference signatures.
                if (symbol.flags & (SymbolFlags.Function | SymbolFlags.Method)) {
                    type.callSignatures = getSignaturesOfSymbol(symbol);
                }
                // And likewise for construct signatures for classes
                if (symbol.flags & SymbolFlags.Class) {
                    const classType = getDeclaredTypeOfClassOrInterface(symbol);
                    let constructSignatures = getSignaturesOfSymbol(symbol.members!.get(InternalSymbolName.Constructor));
                    if (!constructSignatures.length) {
                        constructSignatures = getDefaultConstructSignatures(classType);
                    }
                    type.constructSignatures = constructSignatures;
                }
            }
        }

        function resolveReverseMappedTypeMembers(type: ReverseMappedType) {
            const indexInfo = getIndexInfoOfType(type.source, IndexKind.String);
            const modifiers = getMappedTypeModifiers(type.mappedType);
            const readonlyMask = modifiers & MappedTypeModifiers.IncludeReadonly ? false : true;
            const optionalMask = modifiers & MappedTypeModifiers.IncludeOptional ? 0 : SymbolFlags.Optional;
            const stringIndexInfo = indexInfo && createIndexInfo(inferReverseMappedType(indexInfo.type, type.mappedType), readonlyMask && indexInfo.isReadonly);
            const members = createSymbolTable();
            for (const prop of getPropertiesOfType(type.source)) {
                const checkFlags = CheckFlags.ReverseMapped | (readonlyMask && isReadonlySymbol(prop) ? CheckFlags.Readonly : 0);
                const inferredProp = createSymbol(SymbolFlags.Property | prop.flags & optionalMask, prop.escapedName, checkFlags) as ReverseMappedSymbol;
                inferredProp.declarations = prop.declarations;
                inferredProp.nameType = prop.nameType;
                inferredProp.propertyType = getTypeOfSymbol(prop);
                inferredProp.mappedType = type.mappedType;
                members.set(prop.escapedName, inferredProp);
            }
            setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, undefined);
        }

        /** Resolve the members of a mapped type { [P in K]: T } */
        function resolveMappedTypeMembers(type: MappedType) {
            const members: SymbolTable = createSymbolTable();
            let stringIndexInfo: IndexInfo | undefined;
            let numberIndexInfo: IndexInfo | undefined;
            // Resolve upfront such that recursive references see an empty object type.
            setStructuredTypeMembers(type, emptySymbols, emptyArray, emptyArray, undefined, undefined);
            // In { [P in K]: T }, we refer to P as the type parameter type, K as the constraint type,
            // and T as the template type.
            const typeParameter = getTypeParameterFromMappedType(type);
            const constraintType = getConstraintTypeFromMappedType(type);
            const templateType = getTemplateTypeFromMappedType(<MappedType>type.target || type);
            const modifiersType = getApparentType(getModifiersTypeFromMappedType(type)); // The 'T' in 'keyof T'
            const templateModifiers = getMappedTypeModifiers(type);
            const include = keyofStringsOnly ? TypeFlags.StringLiteral : TypeFlags.StringOrNumberLiteralOrUnique;
            if (isMappedTypeWithKeyofConstraintDeclaration(type)) {
                // We have a { [P in keyof T]: X }
                for (const prop of getPropertiesOfType(modifiersType)) {
                    addMemberForKeyType(getLiteralTypeFromPropertyName(prop, include));
                }
                if (modifiersType.flags & TypeFlags.Any || getIndexInfoOfType(modifiersType, IndexKind.String)) {
                    addMemberForKeyType(stringType);
                }
                if (!keyofStringsOnly && getIndexInfoOfType(modifiersType, IndexKind.Number)) {
                    addMemberForKeyType(numberType);
                }
            }
            else {
                // First, if the constraint type is a type parameter, obtain the base constraint. Then,
                // if the key type is a 'keyof X', obtain 'keyof C' where C is the base constraint of X.
                // Finally, iterate over the constituents of the resulting iteration type.
                const keyType = constraintType.flags & TypeFlags.InstantiableNonPrimitive ? getApparentType(constraintType) : constraintType;
                const iterationType = keyType.flags & TypeFlags.Index ? getIndexType(getApparentType((<IndexType>keyType).type)) : keyType;
                forEachType(iterationType, addMemberForKeyType);
            }
            setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);

            function addMemberForKeyType(t: Type) {
                // Create a mapper from T to the current iteration type constituent. Then, if the
                // mapped type is itself an instantiated type, combine the iteration mapper with the
                // instantiation mapper.
                const templateMapper = combineTypeMappers(type.mapper, createTypeMapper([typeParameter], [t]));
                const propType = instantiateType(templateType, templateMapper);
                // If the current iteration type constituent is a string literal type, create a property.
                // Otherwise, for type string create a string index signature.
                if (t.flags & TypeFlags.StringOrNumberLiteralOrUnique) {
                    const propName = getLateBoundNameFromType(t as LiteralType);
                    const modifiersProp = getPropertyOfType(modifiersType, propName);
                    const isOptional = !!(templateModifiers & MappedTypeModifiers.IncludeOptional ||
                        !(templateModifiers & MappedTypeModifiers.ExcludeOptional) && modifiersProp && modifiersProp.flags & SymbolFlags.Optional);
                    const isReadonly = !!(templateModifiers & MappedTypeModifiers.IncludeReadonly ||
                        !(templateModifiers & MappedTypeModifiers.ExcludeReadonly) && modifiersProp && isReadonlySymbol(modifiersProp));
                    const prop = createSymbol(SymbolFlags.Property | (isOptional ? SymbolFlags.Optional : 0), propName, isReadonly ? CheckFlags.Readonly : 0);
                    // When creating an optional property in strictNullChecks mode, if 'undefined' isn't assignable to the
                    // type, we include 'undefined' in the type. Similarly, when creating a non-optional property in strictNullChecks
                    // mode, if the underlying property is optional we remove 'undefined' from the type.
                    prop.type = strictNullChecks && isOptional && !isTypeAssignableTo(undefinedType, propType) ? getOptionalType(propType) :
                        strictNullChecks && !isOptional && modifiersProp && modifiersProp.flags & SymbolFlags.Optional ? getTypeWithFacts(propType, TypeFacts.NEUndefined) :
                        propType;
                    if (modifiersProp) {
                        prop.syntheticOrigin = modifiersProp;
                        prop.declarations = modifiersProp.declarations;
                    }
                    prop.nameType = t;
                    members.set(propName, prop);
                }
                else if (t.flags & (TypeFlags.Any | TypeFlags.String)) {
                    stringIndexInfo = createIndexInfo(propType, !!(templateModifiers & MappedTypeModifiers.IncludeReadonly));
                }
                else if (t.flags & TypeFlags.Number) {
                    numberIndexInfo = createIndexInfo(propType, !!(templateModifiers & MappedTypeModifiers.IncludeReadonly));
                }
            }
        }

        function getTypeParameterFromMappedType(type: MappedType) {
            return type.typeParameter ||
                (type.typeParameter = getDeclaredTypeOfTypeParameter(getSymbolOfNode(type.declaration.typeParameter)));
        }

        function getConstraintTypeFromMappedType(type: MappedType) {
            return type.constraintType ||
                (type.constraintType = instantiateType(getConstraintOfTypeParameter(getTypeParameterFromMappedType(type)), type.mapper || identityMapper) || errorType);
        }

        function getTemplateTypeFromMappedType(type: MappedType) {
            return type.templateType ||
                (type.templateType = type.declaration.type ?
                    instantiateType(addOptionality(getTypeFromTypeNode(type.declaration.type), !!(getMappedTypeModifiers(type) & MappedTypeModifiers.IncludeOptional)), type.mapper || identityMapper) :
                    errorType);
        }

        function getConstraintDeclarationForMappedType(type: MappedType) {
            return getEffectiveConstraintOfTypeParameter(type.declaration.typeParameter);
        }

        function isMappedTypeWithKeyofConstraintDeclaration(type: MappedType) {
            const constraintDeclaration = getConstraintDeclarationForMappedType(type)!; // TODO: GH#18217
            return constraintDeclaration.kind === SyntaxKind.TypeOperator &&
                (<TypeOperatorNode>constraintDeclaration).operator === SyntaxKind.KeyOfKeyword;
        }

        function getModifiersTypeFromMappedType(type: MappedType) {
            if (!type.modifiersType) {
                if (isMappedTypeWithKeyofConstraintDeclaration(type)) {
                    // If the constraint declaration is a 'keyof T' node, the modifiers type is T. We check
                    // AST nodes here because, when T is a non-generic type, the logic below eagerly resolves
                    // 'keyof T' to a literal union type and we can't recover T from that type.
                    type.modifiersType = instantiateType(getTypeFromTypeNode((<TypeOperatorNode>getConstraintDeclarationForMappedType(type)).type), type.mapper || identityMapper);
                }
                else {
                    // Otherwise, get the declared constraint type, and if the constraint type is a type parameter,
                    // get the constraint of that type parameter. If the resulting type is an indexed type 'keyof T',
                    // the modifiers type is T. Otherwise, the modifiers type is {}.
                    const declaredType = <MappedType>getTypeFromMappedTypeNode(type.declaration);
                    const constraint = getConstraintTypeFromMappedType(declaredType);
                    const extendedConstraint = constraint && constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
                    type.modifiersType = extendedConstraint && extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
                }
            }
            return type.modifiersType;
        }

        function getMappedTypeModifiers(type: MappedType): MappedTypeModifiers {
            const declaration = type.declaration;
            return (declaration.readonlyToken ? declaration.readonlyToken.kind === SyntaxKind.MinusToken ? MappedTypeModifiers.ExcludeReadonly : MappedTypeModifiers.IncludeReadonly : 0) |
                (declaration.questionToken ? declaration.questionToken.kind === SyntaxKind.MinusToken ? MappedTypeModifiers.ExcludeOptional : MappedTypeModifiers.IncludeOptional : 0);
        }

        function getMappedTypeOptionality(type: MappedType): number {
            const modifiers = getMappedTypeModifiers(type);
            return modifiers & MappedTypeModifiers.ExcludeOptional ? -1 : modifiers & MappedTypeModifiers.IncludeOptional ? 1 : 0;
        }

        function getCombinedMappedTypeOptionality(type: MappedType): number {
            const optionality = getMappedTypeOptionality(type);
            const modifiersType = getModifiersTypeFromMappedType(type);
            return optionality || (isGenericMappedType(modifiersType) ? getMappedTypeOptionality(modifiersType) : 0);
        }

        function isPartialMappedType(type: Type) {
            return !!(getObjectFlags(type) & ObjectFlags.Mapped && getMappedTypeModifiers(<MappedType>type) & MappedTypeModifiers.IncludeOptional);
        }

        function isGenericMappedType(type: Type): type is MappedType {
            return !!(getObjectFlags(type) & ObjectFlags.Mapped) && isGenericIndexType(getConstraintTypeFromMappedType(<MappedType>type));
        }

        function resolveStructuredTypeMembers(type: StructuredType): ResolvedType {
            if (!(<ResolvedType>type).members) {
                if (type.flags & TypeFlags.Object) {
                    if ((<ObjectType>type).objectFlags & ObjectFlags.Reference) {
                        resolveTypeReferenceMembers(<TypeReference>type);
                    }
                    else if ((<ObjectType>type).objectFlags & ObjectFlags.ClassOrInterface) {
                        resolveClassOrInterfaceMembers(<InterfaceType>type);
                    }
                    else if ((<ReverseMappedType>type).objectFlags & ObjectFlags.ReverseMapped) {
                        resolveReverseMappedTypeMembers(type as ReverseMappedType);
                    }
                    else if ((<ObjectType>type).objectFlags & ObjectFlags.Anonymous) {
                        resolveAnonymousTypeMembers(<AnonymousType>type);
                    }
                    else if ((<MappedType>type).objectFlags & ObjectFlags.Mapped) {
                        resolveMappedTypeMembers(<MappedType>type);
                    }
                }
                else if (type.flags & TypeFlags.Union) {
                    resolveUnionTypeMembers(<UnionType>type);
                }
                else if (type.flags & TypeFlags.Intersection) {
                    resolveIntersectionTypeMembers(<IntersectionType>type);
                }
            }
            return <ResolvedType>type;
        }

        /** Return properties of an object type or an empty array for other types */
        function getPropertiesOfObjectType(type: Type): Symbol[] {
            if (type.flags & TypeFlags.Object) {
                return resolveStructuredTypeMembers(<ObjectType>type).properties;
            }
            return emptyArray;
        }

        /** If the given type is an object type and that type has a property by the given name,
         * return the symbol for that property. Otherwise return undefined.
         */
        function getPropertyOfObjectType(type: Type, name: __String): Symbol | undefined {
            if (type.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                const symbol = resolved.members.get(name);
                if (symbol && symbolIsValue(symbol)) {
                    return symbol;
                }
            }
        }

        function getPropertiesOfUnionOrIntersectionType(type: UnionOrIntersectionType): Symbol[] {
            if (!type.resolvedProperties) {
                const members = createSymbolTable();
                for (const current of type.types) {
                    for (const prop of getPropertiesOfType(current)) {
                        if (!members.has(prop.escapedName)) {
                            const combinedProp = getPropertyOfUnionOrIntersectionType(type, prop.escapedName);
                            if (combinedProp) {
                                members.set(prop.escapedName, combinedProp);
                            }
                        }
                    }
                    // The properties of a union type are those that are present in all constituent types, so
                    // we only need to check the properties of the first type
                    if (type.flags & TypeFlags.Union) {
                        break;
                    }
                }
                type.resolvedProperties = getNamedMembers(members);
            }
            return type.resolvedProperties;
        }

        function getPropertiesOfType(type: Type): Symbol[] {
            type = getApparentType(type);
            return type.flags & TypeFlags.UnionOrIntersection ?
                getPropertiesOfUnionOrIntersectionType(<UnionType>type) :
                getPropertiesOfObjectType(type);
        }

        function isTypeInvalidDueToUnionDiscriminant(contextualType: Type, obj: ObjectLiteralExpression): boolean {
            return obj.properties.some(property => {
                const name = property.name && getTextOfPropertyName(property.name);
                const expected = name === undefined ? undefined : getTypeOfPropertyOfType(contextualType, name);
                return !!expected && typeIsLiteralType(expected) && !isTypeIdenticalTo(getTypeOfNode(property), expected);
            });
        }

        function getAllPossiblePropertiesOfTypes(types: ReadonlyArray<Type>): Symbol[] {
            const unionType = getUnionType(types);
            if (!(unionType.flags & TypeFlags.Union)) {
                return getAugmentedPropertiesOfType(unionType);
            }

            const props = createSymbolTable();
            for (const memberType of types) {
                for (const { escapedName } of getAugmentedPropertiesOfType(memberType)) {
                    if (!props.has(escapedName)) {
                        const prop = createUnionOrIntersectionProperty(unionType as UnionType, escapedName);
                        // May be undefined if the property is private
                        if (prop) props.set(escapedName, prop);
                    }
                }
            }
            return arrayFrom(props.values());
        }

        function getConstraintOfType(type: InstantiableType | UnionOrIntersectionType): Type | undefined {
            return type.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>type) :
                type.flags & TypeFlags.IndexedAccess ? getConstraintOfIndexedAccess(<IndexedAccessType>type) :
                type.flags & TypeFlags.Conditional ? getConstraintOfConditionalType(<ConditionalType>type) :
                getBaseConstraintOfType(type);
        }

        function getConstraintOfTypeParameter(typeParameter: TypeParameter): Type | undefined {
            return hasNonCircularBaseConstraint(typeParameter) ? getConstraintFromTypeParameter(typeParameter) : undefined;
        }

        function getConstraintOfIndexedAccess(type: IndexedAccessType) {
            const objectType = getConstraintOfType(type.objectType) || type.objectType;
            if (objectType !== type.objectType) {
                const constraint = getIndexedAccessType(objectType, type.indexType, /*accessNode*/ undefined, errorType);
                if (constraint && constraint !== errorType) {
                    return constraint;
                }
            }
            const baseConstraint = getBaseConstraintOfType(type);
            return baseConstraint && baseConstraint !== type ? baseConstraint : undefined;
        }

        function getDefaultConstraintOfConditionalType(type: ConditionalType) {
            if (!type.resolvedDefaultConstraint) {
                const rootTrueType = type.root.trueType;
                const rootTrueConstraint = rootTrueType.flags & TypeFlags.Substitution ? (<SubstitutionType>rootTrueType).substitute : rootTrueType;
                type.resolvedDefaultConstraint = getUnionType([instantiateType(rootTrueConstraint, type.combinedMapper || type.mapper), getFalseTypeFromConditionalType(type)]);
            }
            return type.resolvedDefaultConstraint;
        }

        function getConstraintOfDistributiveConditionalType(type: ConditionalType): Type | undefined {
            // Check if we have a conditional type of the form 'T extends U ? X : Y', where T is a constrained
            // type parameter. If so, create an instantiation of the conditional type where T is replaced
            // with its constraint. We do this because if the constraint is a union type it will be distributed
            // over the conditional type and possibly reduced. For example, 'T extends undefined ? never : T'
            // removes 'undefined' from T.
            if (type.root.isDistributive) {
                const simplified = getSimplifiedType(type.checkType);
                const constraint = simplified === type.checkType ? getConstraintOfType(simplified) : simplified;
                if (constraint) {
                    const mapper = makeUnaryTypeMapper(type.root.checkType, constraint);
                    const instantiated = getConditionalTypeInstantiation(type, combineTypeMappers(mapper, type.mapper));
                    if (!(instantiated.flags & TypeFlags.Never)) {
                        return instantiated;
                    }
                }
            }
            return undefined;
        }

        function getConstraintOfConditionalType(type: ConditionalType) {
            return getConstraintOfDistributiveConditionalType(type) || getDefaultConstraintOfConditionalType(type);
        }

        function getUnionConstraintOfIntersection(type: IntersectionType, targetIsUnion: boolean) {
            let constraints: Type[] | undefined;
            let hasDisjointDomainType = false;
            for (const t of type.types) {
                if (t.flags & TypeFlags.Instantiable) {
                    // We keep following constraints as long as we have an instantiable type that is known
                    // not to be circular or infinite (hence we stop on index access types).
                    let constraint = getConstraintOfType(t);
                    while (constraint && constraint.flags & (TypeFlags.TypeParameter | TypeFlags.Index | TypeFlags.Conditional)) {
                        constraint = getConstraintOfType(constraint);
                    }
                    if (constraint) {
                        // A constraint that isn't a union type implies that the final type would be a non-union
                        // type as well. Since non-union constraints are of no interest, we can exit here.
                        if (!(constraint.flags & TypeFlags.Union)) {
                            return undefined;
                        }
                        constraints = append(constraints, constraint);
                    }
                }
                else if (t.flags & TypeFlags.DisjointDomains) {
                    hasDisjointDomainType = true;
                }
            }
            // If the target is a union type or if we are intersecting with types belonging to one of the
            // disjoint domans, we may end up producing a constraint that hasn't been examined before.
            if (constraints && (targetIsUnion || hasDisjointDomainType)) {
                if (hasDisjointDomainType) {
                    // We add any types belong to one of the disjoint domans because they might cause the final
                    // intersection operation to reduce the union constraints.
                    for (const t of type.types) {
                        if (t.flags & TypeFlags.DisjointDomains) {
                            constraints = append(constraints, t);
                        }
                    }
                }
                return getIntersectionType(constraints);
            }
            return undefined;
        }

        function getBaseConstraintOfType(type: Type): Type | undefined {
            if (type.flags & (TypeFlags.InstantiableNonPrimitive | TypeFlags.UnionOrIntersection)) {
                const constraint = getResolvedBaseConstraint(<InstantiableType | UnionOrIntersectionType>type);
                return constraint !== noConstraintType && constraint !== circularConstraintType ? constraint : undefined;
            }
            return type.flags & TypeFlags.Index ? keyofConstraintType : undefined;
        }

        /**
         * This is similar to `getBaseConstraintOfType` except it returns the input type if there's no base constraint, instead of `undefined`
         * It also doesn't map indexes to `string`, as where this is used this would be unneeded (and likely undesirable)
         */
        function getBaseConstraintOrType(type: Type) {
            return getBaseConstraintOfType(type) || type;
        }

        function hasNonCircularBaseConstraint(type: InstantiableType): boolean {
            return getResolvedBaseConstraint(type) !== circularConstraintType;
        }

        /**
         * Return the resolved base constraint of a type variable. The noConstraintType singleton is returned if the
         * type variable has no constraint, and the circularConstraintType singleton is returned if the constraint
         * circularly references the type variable.
         */
        function getResolvedBaseConstraint(type: InstantiableType | UnionOrIntersectionType): Type {
            let nonTerminating = false;
            return type.resolvedBaseConstraint ||
                (type.resolvedBaseConstraint = getTypeWithThisArgument(getImmediateBaseConstraint(type), type));

            function getImmediateBaseConstraint(t: Type): Type {
                if (!t.immediateBaseConstraint) {
                    if (!pushTypeResolution(t, TypeSystemPropertyName.ImmediateBaseConstraint)) {
                        return circularConstraintType;
                    }
                    if (constraintDepth === 50) {
                        // We have reached 50 recursive invocations of getImmediateBaseConstraint and there is a
                        // very high likelyhood we're dealing with an infinite generic type that perpetually generates
                        // new type identities as we descend into it. We stop the recursion here and mark this type
                        // and the outer types as having circular constraints.
                        nonTerminating = true;
                        return t.immediateBaseConstraint = noConstraintType;
                    }
                    constraintDepth++;
                    let result = computeBaseConstraint(getSimplifiedType(t));
                    constraintDepth--;
                    if (!popTypeResolution() || nonTerminating) {
                        result = circularConstraintType;
                    }
                    t.immediateBaseConstraint = result || noConstraintType;
                }
                return t.immediateBaseConstraint;
            }

            function getBaseConstraint(t: Type): Type | undefined {
                const c = getImmediateBaseConstraint(t);
                return c !== noConstraintType && c !== circularConstraintType ? c : undefined;
            }

            function computeBaseConstraint(t: Type): Type | undefined {
                if (t.flags & TypeFlags.TypeParameter) {
                    const constraint = getConstraintFromTypeParameter(<TypeParameter>t);
                    return (t as TypeParameter).isThisType || !constraint ?
                        constraint :
                        getBaseConstraint(constraint);
                }
                if (t.flags & TypeFlags.UnionOrIntersection) {
                    const types = (<UnionOrIntersectionType>t).types;
                    const baseTypes: Type[] = [];
                    for (const type of types) {
                        const baseType = getBaseConstraint(type);
                        if (baseType) {
                            baseTypes.push(baseType);
                        }
                    }
                    return t.flags & TypeFlags.Union && baseTypes.length === types.length ? getUnionType(baseTypes) :
                        t.flags & TypeFlags.Intersection && baseTypes.length ? getIntersectionType(baseTypes) :
                            undefined;
                }
                if (t.flags & TypeFlags.Index) {
                    return keyofConstraintType;
                }
                if (t.flags & TypeFlags.IndexedAccess) {
                    const baseObjectType = getBaseConstraint((<IndexedAccessType>t).objectType);
                    const baseIndexType = getBaseConstraint((<IndexedAccessType>t).indexType);
                    const baseIndexedAccess = baseObjectType && baseIndexType ? getIndexedAccessType(baseObjectType, baseIndexType, /*accessNode*/ undefined, errorType) : undefined;
                    return baseIndexedAccess && baseIndexedAccess !== errorType ? getBaseConstraint(baseIndexedAccess) : undefined;
                }
                if (t.flags & TypeFlags.Conditional) {
                    const constraint = getConstraintOfConditionalType(<ConditionalType>t);
                    return constraint && getBaseConstraint(constraint);
                }
                if (t.flags & TypeFlags.Substitution) {
                    return getBaseConstraint((<SubstitutionType>t).substitute);
                }
                return t;
            }
        }

        function getApparentTypeOfIntersectionType(type: IntersectionType) {
            return type.resolvedApparentType || (type.resolvedApparentType = getTypeWithThisArgument(type, type, /*apparentType*/ true));
        }

        function getResolvedTypeParameterDefault(typeParameter: TypeParameter): Type | undefined {
            if (!typeParameter.default) {
                if (typeParameter.target) {
                    const targetDefault = getResolvedTypeParameterDefault(typeParameter.target);
                    typeParameter.default = targetDefault ? instantiateType(targetDefault, typeParameter.mapper!) : noConstraintType;
                }
                else {
                    // To block recursion, set the initial value to the resolvingDefaultType.
                    typeParameter.default = resolvingDefaultType;
                    const defaultDeclaration = typeParameter.symbol && forEach(typeParameter.symbol.declarations, decl => isTypeParameterDeclaration(decl) && decl.default);
                    const defaultType = defaultDeclaration ? getTypeFromTypeNode(defaultDeclaration) : noConstraintType;
                    if (typeParameter.default === resolvingDefaultType) {
                        // If we have not been called recursively, set the correct default type.
                        typeParameter.default = defaultType;
                    }
                }
            }
            else if (typeParameter.default === resolvingDefaultType) {
                // If we are called recursively for this type parameter, mark the default as circular.
                typeParameter.default = circularConstraintType;
            }
            return typeParameter.default;
        }

        /**
         * Gets the default type for a type parameter.
         *
         * If the type parameter is the result of an instantiation, this gets the instantiated
         * default type of its target. If the type parameter has no default type or the default is
         * circular, `undefined` is returned.
         */
        function getDefaultFromTypeParameter(typeParameter: TypeParameter): Type | undefined {
            const defaultType = getResolvedTypeParameterDefault(typeParameter);
            return defaultType !== noConstraintType && defaultType !== circularConstraintType ? defaultType : undefined;
        }

        function hasNonCircularTypeParameterDefault(typeParameter: TypeParameter) {
            return getResolvedTypeParameterDefault(typeParameter) !== circularConstraintType;
        }

        /**
         * Indicates whether the declaration of a typeParameter has a default type.
         */
        function hasTypeParameterDefault(typeParameter: TypeParameter): boolean {
            return !!(typeParameter.symbol && forEach(typeParameter.symbol.declarations, decl => isTypeParameterDeclaration(decl) && decl.default));
        }

        function getApparentTypeOfMappedType(type: MappedType) {
            return type.resolvedApparentType || (type.resolvedApparentType = getResolvedApparentTypeOfMappedType(type));
        }

        function getResolvedApparentTypeOfMappedType(type: MappedType) {
            const typeVariable = getHomomorphicTypeVariable(type);
            if (typeVariable) {
                const constraint = getConstraintOfTypeParameter(typeVariable);
                if (constraint && (isArrayType(constraint) || isReadonlyArrayType(constraint) || isTupleType(constraint))) {
                    const mapper = makeUnaryTypeMapper(typeVariable, constraint);
                    return instantiateType(type, combineTypeMappers(mapper, type.mapper));
                }
            }
            return type;
        }

        /**
         * For a type parameter, return the base constraint of the type parameter. For the string, number,
         * boolean, and symbol primitive types, return the corresponding object types. Otherwise return the
         * type itself. Note that the apparent type of a union type is the union type itself.
         */
        function getApparentType(type: Type): Type {
            const t = type.flags & TypeFlags.Instantiable ? getBaseConstraintOfType(type) || emptyObjectType : type;
            return getObjectFlags(t) & ObjectFlags.Mapped ? getApparentTypeOfMappedType(<MappedType>t) :
                t.flags & TypeFlags.Intersection ? getApparentTypeOfIntersectionType(<IntersectionType>t) :
                t.flags & TypeFlags.StringLike ? globalStringType :
                t.flags & TypeFlags.NumberLike ? globalNumberType :
                t.flags & TypeFlags.BooleanLike ? globalBooleanType :
                t.flags & TypeFlags.ESSymbolLike ? getGlobalESSymbolType(/*reportErrors*/ languageVersion >= ScriptTarget.ES2015) :
                t.flags & TypeFlags.NonPrimitive ? emptyObjectType :
                t.flags & TypeFlags.Index ? keyofConstraintType :
                t;
        }

        function createUnionOrIntersectionProperty(containingType: UnionOrIntersectionType, name: __String): Symbol | undefined {
            let props: Symbol[] | undefined;
            let indexTypes: Type[] | undefined;
            const isUnion = containingType.flags & TypeFlags.Union;
            const excludeModifiers = isUnion ? ModifierFlags.NonPublicAccessibilityModifier : 0;
            // Flags we want to propagate to the result if they exist in all source symbols
            let commonFlags = isUnion ? SymbolFlags.None : SymbolFlags.Optional;
            let syntheticFlag = CheckFlags.SyntheticMethod;
            let checkFlags = 0;
            for (const current of containingType.types) {
                const type = getApparentType(current);
                if (type !== errorType) {
                    const prop = getPropertyOfType(type, name);
                    const modifiers = prop ? getDeclarationModifierFlagsFromSymbol(prop) : 0;
                    if (prop && !(modifiers & excludeModifiers)) {
                        commonFlags &= prop.flags;
                        props = appendIfUnique(props, prop);
                        checkFlags |= (isReadonlySymbol(prop) ? CheckFlags.Readonly : 0) |
                            (!(modifiers & ModifierFlags.NonPublicAccessibilityModifier) ? CheckFlags.ContainsPublic : 0) |
                            (modifiers & ModifierFlags.Protected ? CheckFlags.ContainsProtected : 0) |
                            (modifiers & ModifierFlags.Private ? CheckFlags.ContainsPrivate : 0) |
                            (modifiers & ModifierFlags.Static ? CheckFlags.ContainsStatic : 0);
                        if (!isPrototypeProperty(prop)) {
                            syntheticFlag = CheckFlags.SyntheticProperty;
                        }
                    }
                    else if (isUnion) {
                        const index = !isLateBoundName(name) && ((isNumericLiteralName(name) && getIndexInfoOfType(type, IndexKind.Number)) || getIndexInfoOfType(type, IndexKind.String));
                        if (index) {
                            checkFlags |= index.isReadonly ? CheckFlags.Readonly : 0;
                            indexTypes = append(indexTypes, index.type);
                        }
                        else {
                            checkFlags |= CheckFlags.Partial;
                        }
                    }
                }
            }
            if (!props) {
                return undefined;
            }
            if (props.length === 1 && !(checkFlags & CheckFlags.Partial) && !indexTypes) {
                return props[0];
            }
            let declarations: Declaration[] | undefined;
            let commonType: Type | undefined;
            let nameType: Type | undefined;
            const propTypes: Type[] = [];
            let first = true;
            let commonValueDeclaration: Declaration | undefined;
            let hasNonUniformValueDeclaration = false;
            for (const prop of props) {
                if (!commonValueDeclaration) {
                    commonValueDeclaration = prop.valueDeclaration;
                }
                else if (prop.valueDeclaration !== commonValueDeclaration) {
                    hasNonUniformValueDeclaration = true;
                }
                declarations = addRange(declarations, prop.declarations);
                const type = getTypeOfSymbol(prop);
                if (first) {
                    commonType = type;
                    nameType = prop.nameType;
                    first = false;
                }
                else {
                    if (type !== commonType) {
                        checkFlags |= CheckFlags.HasNonUniformType;
                    }
                }
                propTypes.push(type);
            }
            addRange(propTypes, indexTypes);
            const result = createSymbol(SymbolFlags.Property | commonFlags, name, syntheticFlag | checkFlags);
            result.containingType = containingType;
            if (!hasNonUniformValueDeclaration && commonValueDeclaration) {
                result.valueDeclaration = commonValueDeclaration;
            }
            result.declarations = declarations!;
            result.nameType = nameType;
            result.type = isUnion ? getUnionType(propTypes) : getIntersectionType(propTypes);
            return result;
        }

        // Return the symbol for a given property in a union or intersection type, or undefined if the property
        // does not exist in any constituent type. Note that the returned property may only be present in some
        // constituents, in which case the isPartial flag is set when the containing type is union type. We need
        // these partial properties when identifying discriminant properties, but otherwise they are filtered out
        // and do not appear to be present in the union type.
        function getUnionOrIntersectionProperty(type: UnionOrIntersectionType, name: __String): Symbol | undefined {
            const properties = type.propertyCache || (type.propertyCache = createSymbolTable());
            let property = properties.get(name);
            if (!property) {
                property = createUnionOrIntersectionProperty(type, name);
                if (property) {
                    properties.set(name, property);
                }
            }
            return property;
        }

        function getPropertyOfUnionOrIntersectionType(type: UnionOrIntersectionType, name: __String): Symbol | undefined {
            const property = getUnionOrIntersectionProperty(type, name);
            // We need to filter out partial properties in union types
            return property && !(getCheckFlags(property) & CheckFlags.Partial) ? property : undefined;
        }

        /**
         * Return the symbol for the property with the given name in the given type. Creates synthetic union properties when
         * necessary, maps primitive types and type parameters are to their apparent types, and augments with properties from
         * Object and Function as appropriate.
         *
         * @param type a type to look up property from
         * @param name a name of property to look up in a given type
         */
        function getPropertyOfType(type: Type, name: __String): Symbol | undefined {
            type = getApparentType(type);
            if (type.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                const symbol = resolved.members.get(name);
                if (symbol && symbolIsValue(symbol)) {
                    return symbol;
                }
                if (resolved === anyFunctionType || resolved.callSignatures.length || resolved.constructSignatures.length) {
                    const symbol = getPropertyOfObjectType(globalFunctionType, name);
                    if (symbol) {
                        return symbol;
                    }
                }
                return getPropertyOfObjectType(globalObjectType, name);
            }
            if (type.flags & TypeFlags.UnionOrIntersection) {
                return getPropertyOfUnionOrIntersectionType(<UnionOrIntersectionType>type, name);
            }
            return undefined;
        }

        function getSignaturesOfStructuredType(type: Type, kind: SignatureKind): ReadonlyArray<Signature> {
            if (type.flags & TypeFlags.StructuredType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                return kind === SignatureKind.Call ? resolved.callSignatures : resolved.constructSignatures;
            }
            return emptyArray;
        }

        /**
         * Return the signatures of the given kind in the given type. Creates synthetic union signatures when necessary and
         * maps primitive types and type parameters are to their apparent types.
         */
        function getSignaturesOfType(type: Type, kind: SignatureKind): ReadonlyArray<Signature> {
            return getSignaturesOfStructuredType(getApparentType(type), kind);
        }

        function getIndexInfoOfStructuredType(type: Type, kind: IndexKind): IndexInfo | undefined {
            if (type.flags & TypeFlags.StructuredType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                return kind === IndexKind.String ? resolved.stringIndexInfo : resolved.numberIndexInfo;
            }
        }

        function getIndexTypeOfStructuredType(type: Type, kind: IndexKind): Type | undefined {
            const info = getIndexInfoOfStructuredType(type, kind);
            return info && info.type;
        }

        // Return the indexing info of the given kind in the given type. Creates synthetic union index types when necessary and
        // maps primitive types and type parameters are to their apparent types.
        function getIndexInfoOfType(type: Type, kind: IndexKind): IndexInfo | undefined {
            return getIndexInfoOfStructuredType(getApparentType(type), kind);
        }

        // Return the index type of the given kind in the given type. Creates synthetic union index types when necessary and
        // maps primitive types and type parameters are to their apparent types.
        function getIndexTypeOfType(type: Type, kind: IndexKind): Type | undefined {
            return getIndexTypeOfStructuredType(getApparentType(type), kind);
        }

        function getImplicitIndexTypeOfType(type: Type, kind: IndexKind): Type | undefined {
            if (isObjectTypeWithInferableIndex(type)) {
                const propTypes: Type[] = [];
                for (const prop of getPropertiesOfType(type)) {
                    if (kind === IndexKind.String || isNumericLiteralName(prop.escapedName)) {
                        propTypes.push(getTypeOfSymbol(prop));
                    }
                }
                if (propTypes.length) {
                    return getUnionType(propTypes, UnionReduction.Subtype);
                }
            }
            return undefined;
        }

        // Return list of type parameters with duplicates removed (duplicate identifier errors are generated in the actual
        // type checking functions).
        function getTypeParametersFromDeclaration(declaration: DeclarationWithTypeParameters): TypeParameter[] | undefined {
            let result: TypeParameter[] | undefined;
            for (const node of getEffectiveTypeParameterDeclarations(declaration)) {
                result = appendIfUnique(result, getDeclaredTypeOfTypeParameter(node.symbol));
            }
            return result;
        }

        function symbolsToArray(symbols: SymbolTable): Symbol[] {
            const result: Symbol[] = [];
            symbols.forEach((symbol, id) => {
                if (!isReservedMemberName(id)) {
                    result.push(symbol);
                }
            });
            return result;
        }

        function isJSDocOptionalParameter(node: ParameterDeclaration) {
            return isInJSFile(node) && (
                // node.type should only be a JSDocOptionalType when node is a parameter of a JSDocFunctionType
                node.type && node.type.kind === SyntaxKind.JSDocOptionalType
                || getJSDocParameterTags(node).some(({ isBracketed, typeExpression }) =>
                    isBracketed || !!typeExpression && typeExpression.type.kind === SyntaxKind.JSDocOptionalType));
        }

        function tryFindAmbientModule(moduleName: string, withAugmentations: boolean) {
            if (isExternalModuleNameRelative(moduleName)) {
                return undefined;
            }
            const symbol = getSymbol(globals, '"' + moduleName + '"' as __String, SymbolFlags.ValueModule);
            // merged symbol is module declaration symbol combined with all augmentations
            return symbol && withAugmentations ? getMergedSymbol(symbol) : symbol;
        }

        function isOptionalParameter(node: ParameterDeclaration | JSDocParameterTag) {
            if (hasQuestionToken(node) || isOptionalJSDocParameterTag(node) || isJSDocOptionalParameter(node)) {
                return true;
            }

            if (node.initializer) {
                const signature = getSignatureFromDeclaration(node.parent);
                const parameterIndex = node.parent.parameters.indexOf(node);
                Debug.assert(parameterIndex >= 0);
                return parameterIndex >= getMinArgumentCount(signature);
            }
            const iife = getImmediatelyInvokedFunctionExpression(node.parent);
            if (iife) {
                return !node.type &&
                    !node.dotDotDotToken &&
                    node.parent.parameters.indexOf(node) >= iife.arguments.length;
            }

            return false;
        }

        function isOptionalJSDocParameterTag(node: Node): node is JSDocParameterTag {
            if (!isJSDocParameterTag(node)) {
                return false;
            }
            const { isBracketed, typeExpression } = node;
            return isBracketed || !!typeExpression && typeExpression.type.kind === SyntaxKind.JSDocOptionalType;
        }

        function createIdentifierTypePredicate(parameterName: string, parameterIndex: number, type: Type): IdentifierTypePredicate {
            return { kind: TypePredicateKind.Identifier, parameterName, parameterIndex, type };
        }

        function createThisTypePredicate(type: Type): ThisTypePredicate {
            return { kind: TypePredicateKind.This, type };
        }

        /**
         * Gets the minimum number of type arguments needed to satisfy all non-optional type
         * parameters.
         */
        function getMinTypeArgumentCount(typeParameters: ReadonlyArray<TypeParameter> | undefined): number {
            let minTypeArgumentCount = 0;
            if (typeParameters) {
                for (let i = 0; i < typeParameters.length; i++) {
                    if (!hasTypeParameterDefault(typeParameters[i])) {
                        minTypeArgumentCount = i + 1;
                    }
                }
            }
            return minTypeArgumentCount;
        }

        /**
         * Fill in default types for unsupplied type arguments. If `typeArguments` is undefined
         * when a default type is supplied, a new array will be created and returned.
         *
         * @param typeArguments The supplied type arguments.
         * @param typeParameters The requested type parameters.
         * @param minTypeArgumentCount The minimum number of required type arguments.
         */
        function fillMissingTypeArguments(typeArguments: Type[], typeParameters: ReadonlyArray<TypeParameter> | undefined, minTypeArgumentCount: number, isJavaScriptImplicitAny: boolean): Type[];
        function fillMissingTypeArguments(typeArguments: Type[] | undefined, typeParameters: ReadonlyArray<TypeParameter> | undefined, minTypeArgumentCount: number, isJavaScriptImplicitAny: boolean): Type[] | undefined;
        function fillMissingTypeArguments(typeArguments: Type[] | undefined, typeParameters: ReadonlyArray<TypeParameter> | undefined, minTypeArgumentCount: number, isJavaScriptImplicitAny: boolean) {
            const numTypeParameters = length(typeParameters);
            if (numTypeParameters) {
                const numTypeArguments = length(typeArguments);
                if (isJavaScriptImplicitAny || (numTypeArguments >= minTypeArgumentCount && numTypeArguments <= numTypeParameters)) {
                    if (!typeArguments) {
                        typeArguments = [];
                    }

                    // Map an unsatisfied type parameter with a default type.
                    // If a type parameter does not have a default type, or if the default type
                    // is a forward reference, the empty object type is used.
                    for (let i = numTypeArguments; i < numTypeParameters; i++) {
                        typeArguments[i] = getDefaultTypeArgumentType(isJavaScriptImplicitAny);
                    }
                    for (let i = numTypeArguments; i < numTypeParameters; i++) {
                        const mapper = createTypeMapper(typeParameters!, typeArguments);
                        let defaultType = getDefaultFromTypeParameter(typeParameters![i]);
                        if (isJavaScriptImplicitAny && defaultType && isTypeIdenticalTo(defaultType, emptyObjectType)) {
                            defaultType = anyType;
                        }
                        typeArguments[i] = defaultType ? instantiateType(defaultType, mapper) : getDefaultTypeArgumentType(isJavaScriptImplicitAny);
                    }
                    typeArguments.length = typeParameters!.length;
                }
            }
            return typeArguments;
        }

        function getSignatureFromDeclaration(declaration: SignatureDeclaration | JSDocSignature): Signature {
            const links = getNodeLinks(declaration);
            if (!links.resolvedSignature) {
                const parameters: Symbol[] = [];
                let hasLiteralTypes = false;
                let minArgumentCount = 0;
                let thisParameter: Symbol | undefined;
                let hasThisParameter = false;
                const iife = getImmediatelyInvokedFunctionExpression(declaration);
                const isJSConstructSignature = isJSDocConstructSignature(declaration);
                const isUntypedSignatureInJSFile = !iife &&
                    isInJSFile(declaration) &&
                    isValueSignatureDeclaration(declaration) &&
                    !hasJSDocParameterTags(declaration) &&
                    !getJSDocType(declaration);

                // If this is a JSDoc construct signature, then skip the first parameter in the
                // parameter list.  The first parameter represents the return type of the construct
                // signature.
                for (let i = isJSConstructSignature ? 1 : 0; i < declaration.parameters.length; i++) {
                    const param = declaration.parameters[i];

                    let paramSymbol = param.symbol;
                    const type = isJSDocParameterTag(param) ? (param.typeExpression && param.typeExpression.type) : param.type;
                    // Include parameter symbol instead of property symbol in the signature
                    if (paramSymbol && !!(paramSymbol.flags & SymbolFlags.Property) && !isBindingPattern(param.name)) {
                        const resolvedSymbol = resolveName(param, paramSymbol.escapedName, SymbolFlags.Value, undefined, undefined, /*isUse*/ false);
                        paramSymbol = resolvedSymbol!;
                    }
                    if (i === 0 && paramSymbol.escapedName === InternalSymbolName.This) {
                        hasThisParameter = true;
                        thisParameter = param.symbol;
                    }
                    else {
                        parameters.push(paramSymbol);
                    }

                    if (type && type.kind === SyntaxKind.LiteralType) {
                        hasLiteralTypes = true;
                    }

                    // Record a new minimum argument count if this is not an optional parameter
                    const isOptionalParameter = isOptionalJSDocParameterTag(param) ||
                        param.initializer || param.questionToken || param.dotDotDotToken ||
                        iife && parameters.length > iife.arguments.length && !type ||
                        isUntypedSignatureInJSFile ||
                        isJSDocOptionalParameter(param);
                    if (!isOptionalParameter) {
                        minArgumentCount = parameters.length;
                    }
                }

                // If only one accessor includes a this-type annotation, the other behaves as if it had the same type annotation
                if ((declaration.kind === SyntaxKind.GetAccessor || declaration.kind === SyntaxKind.SetAccessor) &&
                    !hasNonBindableDynamicName(declaration) &&
                    (!hasThisParameter || !thisParameter)) {
                    const otherKind = declaration.kind === SyntaxKind.GetAccessor ? SyntaxKind.SetAccessor : SyntaxKind.GetAccessor;
                    const other = getDeclarationOfKind<AccessorDeclaration>(getSymbolOfNode(declaration), otherKind);
                    if (other) {
                        thisParameter = getAnnotatedAccessorThisParameter(other);
                    }
                }

                const classType = declaration.kind === SyntaxKind.Constructor ?
                    getDeclaredTypeOfClassOrInterface(getMergedSymbol((<ClassDeclaration>declaration.parent).symbol))
                    : undefined;
                const typeParameters = classType ? classType.localTypeParameters : getTypeParametersFromDeclaration(declaration);
                const hasRestLikeParameter = hasRestParameter(declaration) || isInJSFile(declaration) && maybeAddJsSyntheticRestParameter(declaration, parameters);
                links.resolvedSignature = createSignature(declaration, typeParameters, thisParameter, parameters,
                    /*resolvedReturnType*/ undefined, /*resolvedTypePredicate*/ undefined,
                    minArgumentCount, hasRestLikeParameter, hasLiteralTypes);
            }
            return links.resolvedSignature;
        }

        /**
         * A JS function gets a synthetic rest parameter if it references `arguments` AND:
         * 1. It has no parameters but at least one `@param` with a type that starts with `...`
         * OR
         * 2. It has at least one parameter, and the last parameter has a matching `@param` with a type that starts with `...`
         */
        function maybeAddJsSyntheticRestParameter(declaration: SignatureDeclaration | JSDocSignature, parameters: Symbol[]): boolean {
            if (isJSDocSignature(declaration) || !containsArgumentsReference(declaration)) {
                return false;
            }
            const lastParam = lastOrUndefined(declaration.parameters);
            const lastParamTags = lastParam ? getJSDocParameterTags(lastParam) : getJSDocTags(declaration).filter(isJSDocParameterTag);
            const lastParamVariadicType = firstDefined(lastParamTags, p =>
                p.typeExpression && isJSDocVariadicType(p.typeExpression.type) ? p.typeExpression.type : undefined);

            const syntheticArgsSymbol = createSymbol(SymbolFlags.Variable, "args" as __String, CheckFlags.RestParameter);
            syntheticArgsSymbol.type = lastParamVariadicType ? createArrayType(getTypeFromTypeNode(lastParamVariadicType.type)) : anyArrayType;
            if (lastParamVariadicType) {
                // Replace the last parameter with a rest parameter.
                parameters.pop();
            }
            parameters.push(syntheticArgsSymbol);
            return true;
        }

        function getSignatureOfTypeTag(node: SignatureDeclaration | JSDocSignature) {
            const typeTag = isInJSFile(node) ? getJSDocTypeTag(node) : undefined;
            const signature = typeTag && typeTag.typeExpression && getSingleCallSignature(getTypeFromTypeNode(typeTag.typeExpression));
            return signature && getErasedSignature(signature);
        }

        function getReturnTypeOfTypeTag(node: SignatureDeclaration | JSDocSignature) {
            const signature = getSignatureOfTypeTag(node);
            return signature && getReturnTypeOfSignature(signature);
        }

        function containsArgumentsReference(declaration: SignatureDeclaration): boolean {
            const links = getNodeLinks(declaration);
            if (links.containsArgumentsReference === undefined) {
                if (links.flags & NodeCheckFlags.CaptureArguments) {
                    links.containsArgumentsReference = true;
                }
                else {
                    links.containsArgumentsReference = traverse((declaration as FunctionLikeDeclaration).body!);
                }
            }
            return links.containsArgumentsReference;

            function traverse(node: Node): boolean {
                if (!node) return false;
                switch (node.kind) {
                    case SyntaxKind.Identifier:
                        return (<Identifier>node).escapedText === "arguments" && isExpressionNode(node);

                    case SyntaxKind.PropertyDeclaration:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        return (<NamedDeclaration>node).name!.kind === SyntaxKind.ComputedPropertyName
                            && traverse((<NamedDeclaration>node).name!);

                    default:
                        return !nodeStartsNewLexicalEnvironment(node) && !isPartOfTypeNode(node) && !!forEachChild(node, traverse);
                }
            }
        }

        function getSignaturesOfSymbol(symbol: Symbol | undefined): Signature[] {
            if (!symbol) return emptyArray;
            const result: Signature[] = [];
            for (let i = 0; i < symbol.declarations.length; i++) {
                const decl = symbol.declarations[i];
                if (!isFunctionLike(decl)) continue;
                // Don't include signature if node is the implementation of an overloaded function. A node is considered
                // an implementation node if it has a body and the previous node is of the same kind and immediately
                // precedes the implementation node (i.e. has the same parent and ends where the implementation starts).
                if (i > 0 && (decl as FunctionLikeDeclaration).body) {
                    const previous = symbol.declarations[i - 1];
                    if (decl.parent === previous.parent && decl.kind === previous.kind && decl.pos === previous.end) {
                        continue;
                    }
                }
                result.push(getSignatureFromDeclaration(decl));
            }
            return result;
        }

        function resolveExternalModuleTypeByLiteral(name: StringLiteral) {
            const moduleSym = resolveExternalModuleName(name, name);
            if (moduleSym) {
                const resolvedModuleSymbol = resolveExternalModuleSymbol(moduleSym);
                if (resolvedModuleSymbol) {
                    return getTypeOfSymbol(resolvedModuleSymbol);
                }
            }

            return anyType;
        }

        function getThisTypeOfSignature(signature: Signature): Type | undefined {
            if (signature.thisParameter) {
                return getTypeOfSymbol(signature.thisParameter);
            }
        }

        function signatureHasTypePredicate(signature: Signature): boolean {
            return getTypePredicateOfSignature(signature) !== undefined;
        }

        function getTypePredicateOfSignature(signature: Signature): TypePredicate | undefined {
            if (!signature.resolvedTypePredicate) {
                if (signature.target) {
                    const targetTypePredicate = getTypePredicateOfSignature(signature.target);
                    signature.resolvedTypePredicate = targetTypePredicate ? instantiateTypePredicate(targetTypePredicate, signature.mapper!) : noTypePredicate;
                }
                else if (signature.unionSignatures) {
                    signature.resolvedTypePredicate = getUnionTypePredicate(signature.unionSignatures) || noTypePredicate;
                }
                else {
                    const type = signature.declaration && getEffectiveReturnTypeNode(signature.declaration);
                    let jsdocPredicate: TypePredicate | undefined;
                    if (!type && isInJSFile(signature.declaration)) {
                        const jsdocSignature = getSignatureOfTypeTag(signature.declaration!);
                        if (jsdocSignature && signature !== jsdocSignature) {
                            jsdocPredicate = getTypePredicateOfSignature(jsdocSignature);
                        }
                    }
                    signature.resolvedTypePredicate = type && isTypePredicateNode(type) ?
                        createTypePredicateFromTypePredicateNode(type, signature.declaration!) :
                        jsdocPredicate || noTypePredicate;
                }
                Debug.assert(!!signature.resolvedTypePredicate);
            }
            return signature.resolvedTypePredicate === noTypePredicate ? undefined : signature.resolvedTypePredicate;
        }

        function createTypePredicateFromTypePredicateNode(node: TypePredicateNode, func: SignatureDeclaration | JSDocSignature): IdentifierTypePredicate | ThisTypePredicate {
            const { parameterName } = node;
            const type = getTypeFromTypeNode(node.type);
            if (parameterName.kind === SyntaxKind.Identifier) {
                return createIdentifierTypePredicate(
                    parameterName.escapedText as string,
                    getTypePredicateParameterIndex(func.parameters, parameterName),
                    type);
            }
            else {
                return createThisTypePredicate(type);
            }
        }

        function getTypePredicateParameterIndex(parameterList: ReadonlyArray<ParameterDeclaration | JSDocParameterTag>, parameter: Identifier): number {
            for (let i = 0; i < parameterList.length; i++) {
                const param = parameterList[i];
                if (param.name.kind === SyntaxKind.Identifier && param.name.escapedText === parameter.escapedText) {
                    return i;
                }
            }
            return -1;
        }

        function getReturnTypeOfSignature(signature: Signature): Type {
            if (!signature.resolvedReturnType) {
                if (!pushTypeResolution(signature, TypeSystemPropertyName.ResolvedReturnType)) {
                    return errorType;
                }
                let type = signature.target ? instantiateType(getReturnTypeOfSignature(signature.target), signature.mapper!) :
                    signature.unionSignatures ? getUnionType(map(signature.unionSignatures, getReturnTypeOfSignature), UnionReduction.Subtype) :
                    getReturnTypeFromAnnotation(signature.declaration!) ||
                    (nodeIsMissing((<FunctionLikeDeclaration>signature.declaration).body) ? anyType : getReturnTypeFromBody(<FunctionLikeDeclaration>signature.declaration));
                if (!popTypeResolution()) {
                    if (signature.declaration) {
                        const typeNode = getEffectiveReturnTypeNode(signature.declaration);
                        if (typeNode) {
                            error(typeNode, Diagnostics.Return_type_annotation_circularly_references_itself);
                        }
                        else if (noImplicitAny) {
                            const declaration = <Declaration>signature.declaration;
                            const name = getNameOfDeclaration(declaration);
                            if (name) {
                                error(name, Diagnostics._0_implicitly_has_return_type_any_because_it_does_not_have_a_return_type_annotation_and_is_referenced_directly_or_indirectly_in_one_of_its_return_expressions, declarationNameToString(name));
                            }
                            else {
                                error(declaration, Diagnostics.Function_implicitly_has_return_type_any_because_it_does_not_have_a_return_type_annotation_and_is_referenced_directly_or_indirectly_in_one_of_its_return_expressions);
                            }
                        }
                    }
                    type = anyType;
                }
                signature.resolvedReturnType = type;
            }
            return signature.resolvedReturnType;
        }

        function getReturnTypeFromAnnotation(declaration: SignatureDeclaration | JSDocSignature) {
            if (declaration.kind === SyntaxKind.Constructor) {
                return getDeclaredTypeOfClassOrInterface(getMergedSymbol((<ClassDeclaration>declaration.parent).symbol));
            }
            if (isJSDocConstructSignature(declaration)) {
                return getTypeFromTypeNode((declaration.parameters[0] as ParameterDeclaration).type!); // TODO: GH#18217
            }
            const typeNode = getEffectiveReturnTypeNode(declaration);
            if (typeNode) {
                return getTypeFromTypeNode(typeNode);
            }
            if (declaration.kind === SyntaxKind.GetAccessor && !hasNonBindableDynamicName(declaration)) {
                const jsDocType = isInJSFile(declaration) && getTypeForDeclarationFromJSDocComment(declaration);
                if (jsDocType) {
                    return jsDocType;
                }
                const setter = getDeclarationOfKind<AccessorDeclaration>(getSymbolOfNode(declaration), SyntaxKind.SetAccessor);
                const setterType = getAnnotatedAccessorType(setter);
                if (setterType) {
                    return setterType;
                }
            }
            return getReturnTypeOfTypeTag(declaration);
        }

        function isResolvingReturnTypeOfSignature(signature: Signature) {
            return !signature.resolvedReturnType && findResolutionCycleStartIndex(signature, TypeSystemPropertyName.ResolvedReturnType) >= 0;
        }

        function getRestTypeOfSignature(signature: Signature): Type {
            return tryGetRestTypeOfSignature(signature) || anyType;
        }

        function tryGetRestTypeOfSignature(signature: Signature): Type | undefined {
            if (signature.hasRestParameter) {
                const sigRestType = getTypeOfSymbol(signature.parameters[signature.parameters.length - 1]);
                const restType = isTupleType(sigRestType) ? getRestTypeOfTupleType(sigRestType) : sigRestType;
                return restType && getIndexTypeOfType(restType, IndexKind.Number);
            }
            return undefined;
        }

        function getSignatureInstantiation(signature: Signature, typeArguments: Type[] | undefined, isJavascript: boolean): Signature {
            return getSignatureInstantiationWithoutFillingInTypeArguments(signature, fillMissingTypeArguments(typeArguments, signature.typeParameters, getMinTypeArgumentCount(signature.typeParameters), isJavascript));
        }
        function getSignatureInstantiationWithoutFillingInTypeArguments(signature: Signature, typeArguments: ReadonlyArray<Type> | undefined): Signature {
            const instantiations = signature.instantiations || (signature.instantiations = createMap<Signature>());
            const id = getTypeListId(typeArguments);
            let instantiation = instantiations.get(id);
            if (!instantiation) {
                instantiations.set(id, instantiation = createSignatureInstantiation(signature, typeArguments));
            }
            return instantiation;
        }

        function createSignatureInstantiation(signature: Signature, typeArguments: ReadonlyArray<Type> | undefined): Signature {
            return instantiateSignature(signature, createSignatureTypeMapper(signature, typeArguments), /*eraseTypeParameters*/ true);
        }
        function createSignatureTypeMapper(signature: Signature, typeArguments: ReadonlyArray<Type> | undefined): TypeMapper {
            return createTypeMapper(signature.typeParameters!, typeArguments);
        }

        function getErasedSignature(signature: Signature): Signature {
            return signature.typeParameters ?
                signature.erasedSignatureCache || (signature.erasedSignatureCache = createErasedSignature(signature)) :
                signature;
        }

        function createErasedSignature(signature: Signature) {
            // Create an instantiation of the signature where all type arguments are the any type.
            return instantiateSignature(signature, createTypeEraser(signature.typeParameters!), /*eraseTypeParameters*/ true);
        }

        function getCanonicalSignature(signature: Signature): Signature {
            return signature.typeParameters ?
                signature.canonicalSignatureCache || (signature.canonicalSignatureCache = createCanonicalSignature(signature)) :
                signature;
        }

        function createCanonicalSignature(signature: Signature) {
            // Create an instantiation of the signature where each unconstrained type parameter is replaced with
            // its original. When a generic class or interface is instantiated, each generic method in the class or
            // interface is instantiated with a fresh set of cloned type parameters (which we need to handle scenarios
            // where different generations of the same type parameter are in scope). This leads to a lot of new type
            // identities, and potentially a lot of work comparing those identities, so here we create an instantiation
            // that uses the original type identities for all unconstrained type parameters.
            return getSignatureInstantiation(
                signature,
                map(signature.typeParameters, tp => tp.target && !getConstraintOfTypeParameter(tp.target) ? tp.target : tp),
                isInJSFile(signature.declaration));
        }

        function getBaseSignature(signature: Signature) {
            const typeParameters = signature.typeParameters;
            if (typeParameters) {
                const typeEraser = createTypeEraser(typeParameters);
                const baseConstraints = map(typeParameters, tp => instantiateType(getBaseConstraintOfType(tp), typeEraser) || emptyObjectType);
                return instantiateSignature(signature, createTypeMapper(typeParameters, baseConstraints), /*eraseTypeParameters*/ true);
            }
            return signature;
        }

        function getOrCreateTypeFromSignature(signature: Signature): ObjectType {
            // There are two ways to declare a construct signature, one is by declaring a class constructor
            // using the constructor keyword, and the other is declaring a bare construct signature in an
            // object type literal or interface (using the new keyword). Each way of declaring a constructor
            // will result in a different declaration kind.
            if (!signature.isolatedSignatureType) {
                const isConstructor = signature.declaration!.kind === SyntaxKind.Constructor || signature.declaration!.kind === SyntaxKind.ConstructSignature; // TODO: GH#18217
                const type = createObjectType(ObjectFlags.Anonymous);
                type.members = emptySymbols;
                type.properties = emptyArray;
                type.callSignatures = !isConstructor ? [signature] : emptyArray;
                type.constructSignatures = isConstructor ? [signature] : emptyArray;
                signature.isolatedSignatureType = type;
            }

            return signature.isolatedSignatureType;
        }

        function getIndexSymbol(symbol: Symbol): Symbol | undefined {
            return symbol.members!.get(InternalSymbolName.Index);
        }

        function getIndexDeclarationOfSymbol(symbol: Symbol, kind: IndexKind): IndexSignatureDeclaration | undefined {
            const syntaxKind = kind === IndexKind.Number ? SyntaxKind.NumberKeyword : SyntaxKind.StringKeyword;
            const indexSymbol = getIndexSymbol(symbol);
            if (indexSymbol) {
                for (const decl of indexSymbol.declarations) {
                    const node = cast(decl, isIndexSignatureDeclaration);
                    if (node.parameters.length === 1) {
                        const parameter = node.parameters[0];
                        if (parameter.type && parameter.type.kind === syntaxKind) {
                            return node;
                        }
                    }
                }
            }

            return undefined;
        }

        function createIndexInfo(type: Type, isReadonly: boolean, declaration?: IndexSignatureDeclaration): IndexInfo {
            return { type, isReadonly, declaration };
        }

        function getIndexInfoOfSymbol(symbol: Symbol, kind: IndexKind): IndexInfo | undefined {
            const declaration = getIndexDeclarationOfSymbol(symbol, kind);
            if (declaration) {
                return createIndexInfo(declaration.type ? getTypeFromTypeNode(declaration.type) : anyType,
                    hasModifier(declaration, ModifierFlags.Readonly), declaration);
            }
            return undefined;
        }

        function getConstraintDeclaration(type: TypeParameter) {
            const decl = type.symbol && getDeclarationOfKind<TypeParameterDeclaration>(type.symbol, SyntaxKind.TypeParameter);
            return decl && getEffectiveConstraintOfTypeParameter(decl);
        }

        function getInferredTypeParameterConstraint(typeParameter: TypeParameter) {
            let inferences: Type[] | undefined;
            if (typeParameter.symbol) {
                for (const declaration of typeParameter.symbol.declarations) {
                    if (declaration.parent.kind === SyntaxKind.InferType) {
                        // When an 'infer T' declaration is immediately contained in a type reference node
                        // (such as 'Foo<infer T>'), T's constraint is inferred from the constraint of the
                        // corresponding type parameter in 'Foo'. When multiple 'infer T' declarations are
                        // present, we form an intersection of the inferred constraint types.
                        const grandParent = declaration.parent.parent;
                        if (grandParent.kind === SyntaxKind.TypeReference) {
                            const typeReference = <TypeReferenceNode>grandParent;
                            const typeParameters = getTypeParametersForTypeReference(typeReference);
                            if (typeParameters) {
                                const index = typeReference.typeArguments!.indexOf(<TypeNode>declaration.parent);
                                if (index < typeParameters.length) {
                                    const declaredConstraint = getConstraintOfTypeParameter(typeParameters[index]);
                                    if (declaredConstraint) {
                                        // Type parameter constraints can reference other type parameters so
                                        // constraints need to be instantiated. If instantiation produces the
                                        // type parameter itself, we discard that inference. For example, in
                                        //   type Foo<T extends string, U extends T> = [T, U];
                                        //   type Bar<T> = T extends Foo<infer X, infer X> ? Foo<X, X> : T;
                                        // the instantiated constraint for U is X, so we discard that inference.
                                        const mapper = createTypeMapper(typeParameters, getEffectiveTypeArguments(typeReference, typeParameters));
                                        const constraint = instantiateType(declaredConstraint, mapper);
                                        if (constraint !== typeParameter) {
                                            inferences = append(inferences, constraint);
                                        }
                                    }
                                }
                            }
                        }
                        // When an 'infer T' declaration is immediately contained in a rest parameter
                        // declaration, we infer an 'unknown[]' constraint.
                        else if (grandParent.kind === SyntaxKind.Parameter && (<ParameterDeclaration>grandParent).dotDotDotToken) {
                            inferences = append(inferences, createArrayType(unknownType));
                        }
                    }
                }
            }
            return inferences && getIntersectionType(inferences);
        }

        /** This is a worker function. Use getConstraintOfTypeParameter which guards against circular constraints. */
        function getConstraintFromTypeParameter(typeParameter: TypeParameter): Type | undefined {
            if (!typeParameter.constraint) {
                if (typeParameter.target) {
                    const targetConstraint = getConstraintOfTypeParameter(typeParameter.target);
                    typeParameter.constraint = targetConstraint ? instantiateType(targetConstraint, typeParameter.mapper!) : noConstraintType;
                }
                else {
                    const constraintDeclaration = getConstraintDeclaration(typeParameter);
                    typeParameter.constraint = constraintDeclaration ? getTypeFromTypeNode(constraintDeclaration) :
                        getInferredTypeParameterConstraint(typeParameter) || noConstraintType;
                }
            }
            return typeParameter.constraint === noConstraintType ? undefined : typeParameter.constraint;
        }

        function getParentSymbolOfTypeParameter(typeParameter: TypeParameter): Symbol | undefined {
            const tp = getDeclarationOfKind<TypeParameterDeclaration>(typeParameter.symbol, SyntaxKind.TypeParameter)!;
            const host = isJSDocTemplateTag(tp.parent) ? getHostSignatureFromJSDoc(tp.parent) : tp.parent;
            return host && getSymbolOfNode(host);
        }

        function getTypeListId(types: ReadonlyArray<Type> | undefined) {
            let result = "";
            if (types) {
                const length = types.length;
                let i = 0;
                while (i < length) {
                    const startId = types[i].id;
                    let count = 1;
                    while (i + count < length && types[i + count].id === startId + count) {
                        count++;
                    }
                    if (result.length) {
                        result += ",";
                    }
                    result += startId;
                    if (count > 1) {
                        result += ":" + count;
                    }
                    i += count;
                }
            }
            return result;
        }

        // This function is used to propagate certain flags when creating new object type references and union types.
        // It is only necessary to do so if a constituent type might be the undefined type, the null type, the type
        // of an object literal or the anyFunctionType. This is because there are operations in the type checker
        // that care about the presence of such types at arbitrary depth in a containing type.
        function getPropagatingFlagsOfTypes(types: ReadonlyArray<Type>, excludeKinds: TypeFlags): TypeFlags {
            let result: TypeFlags = 0;
            for (const type of types) {
                if (!(type.flags & excludeKinds)) {
                    result |= type.flags;
                }
            }
            return result & TypeFlags.PropagatingFlags;
        }

        function createTypeReference(target: GenericType, typeArguments: ReadonlyArray<Type> | undefined): TypeReference {
            const id = getTypeListId(typeArguments);
            let type = target.instantiations.get(id);
            if (!type) {
                type = <TypeReference>createObjectType(ObjectFlags.Reference, target.symbol);
                target.instantiations.set(id, type);
                type.flags |= typeArguments ? getPropagatingFlagsOfTypes(typeArguments, /*excludeKinds*/ 0) : 0;
                type.target = target;
                type.typeArguments = typeArguments;
            }
            return type;
        }

        function cloneTypeReference(source: TypeReference): TypeReference {
            const type = <TypeReference>createType(source.flags);
            type.symbol = source.symbol;
            type.objectFlags = source.objectFlags;
            type.target = source.target;
            type.typeArguments = source.typeArguments;
            return type;
        }

        function getTypeReferenceArity(type: TypeReference): number {
            return length(type.target.typeParameters);
        }

        /**
         * Get type from type-reference that reference to class or interface
         */
        function getTypeFromClassOrInterfaceReference(node: NodeWithTypeArguments, symbol: Symbol, typeArgs: Type[] | undefined): Type {
            const type = <InterfaceType>getDeclaredTypeOfSymbol(getMergedSymbol(symbol));
            const typeParameters = type.localTypeParameters;
            if (typeParameters) {
                const numTypeArguments = length(node.typeArguments);
                const minTypeArgumentCount = getMinTypeArgumentCount(typeParameters);
                const isJs = isInJSFile(node);
                const isJsImplicitAny = !noImplicitAny && isJs;
                if (!isJsImplicitAny && (numTypeArguments < minTypeArgumentCount || numTypeArguments > typeParameters.length)) {
                    const missingAugmentsTag = isJs && node.parent.kind !== SyntaxKind.JSDocAugmentsTag;
                    const diag = minTypeArgumentCount === typeParameters.length
                        ? missingAugmentsTag
                        ? Diagnostics.Expected_0_type_arguments_provide_these_with_an_extends_tag
                        : Diagnostics.Generic_type_0_requires_1_type_argument_s
                    : missingAugmentsTag
                        ? Diagnostics.Expected_0_1_type_arguments_provide_these_with_an_extends_tag
                        : Diagnostics.Generic_type_0_requires_between_1_and_2_type_arguments;
                    const typeStr = typeToString(type, /*enclosingDeclaration*/ undefined, TypeFormatFlags.WriteArrayAsGenericType);
                    error(node, diag, typeStr, minTypeArgumentCount, typeParameters.length);
                    if (!isJs) {
                        // TODO: Adopt same permissive behavior in TS as in JS to reduce follow-on editing experience failures (requires editing fillMissingTypeArguments)
                        return errorType;
                    }
                }
                // In a type reference, the outer type parameters of the referenced class or interface are automatically
                // supplied as type arguments and the type reference only specifies arguments for the local type parameters
                // of the class or interface.
                const typeArguments = concatenate(type.outerTypeParameters, fillMissingTypeArguments(typeArgs, typeParameters, minTypeArgumentCount, isJs));
                return createTypeReference(<GenericType>type, typeArguments);
            }
            return checkNoTypeArguments(node, symbol) ? type : errorType;
        }

        function getTypeAliasInstantiation(symbol: Symbol, typeArguments: Type[] | undefined): Type {
            const type = getDeclaredTypeOfSymbol(symbol);
            const links = getSymbolLinks(symbol);
            const typeParameters = links.typeParameters!;
            const id = getTypeListId(typeArguments);
            let instantiation = links.instantiations!.get(id);
            if (!instantiation) {
                links.instantiations!.set(id, instantiation = instantiateType(type, createTypeMapper(typeParameters, fillMissingTypeArguments(typeArguments, typeParameters, getMinTypeArgumentCount(typeParameters), isInJSFile(symbol.valueDeclaration)))));
            }
            return instantiation;
        }

        /**
         * Get type from reference to type alias. When a type alias is generic, the declared type of the type alias may include
         * references to the type parameters of the alias. We replace those with the actual type arguments by instantiating the
         * declared type. Instantiations are cached using the type identities of the type arguments as the key.
         */
        function getTypeFromTypeAliasReference(node: NodeWithTypeArguments, symbol: Symbol, typeArguments: Type[] | undefined): Type {
            const type = getDeclaredTypeOfSymbol(symbol);
            const typeParameters = getSymbolLinks(symbol).typeParameters;
            if (typeParameters) {
                const numTypeArguments = length(node.typeArguments);
                const minTypeArgumentCount = getMinTypeArgumentCount(typeParameters);
                if (numTypeArguments < minTypeArgumentCount || numTypeArguments > typeParameters.length) {
                    error(node,
                          minTypeArgumentCount === typeParameters.length
                          ? Diagnostics.Generic_type_0_requires_1_type_argument_s
                          : Diagnostics.Generic_type_0_requires_between_1_and_2_type_arguments,
                          symbolToString(symbol),
                          minTypeArgumentCount,
                          typeParameters.length);
                    return errorType;
                }
                return getTypeAliasInstantiation(symbol, typeArguments);
            }
            return checkNoTypeArguments(node, symbol) ? type : errorType;
        }

        function getTypeReferenceName(node: TypeReferenceType): EntityNameOrEntityNameExpression | undefined {
            switch (node.kind) {
                case SyntaxKind.TypeReference:
                    return node.typeName;
                case SyntaxKind.ExpressionWithTypeArguments:
                    // We only support expressions that are simple qualified names. For other
                    // expressions this produces undefined.
                    const expr = node.expression;
                    if (isEntityNameExpression(expr)) {
                        return expr;
                    }
                // fall through;
            }

            return undefined;
        }

        function resolveTypeReferenceName(typeReferenceName: EntityNameExpression | EntityName | undefined, meaning: SymbolFlags) {
            if (!typeReferenceName) {
                return unknownSymbol;
            }

            return resolveEntityName(typeReferenceName, meaning) || unknownSymbol;
        }

        function getTypeReferenceType(node: NodeWithTypeArguments, symbol: Symbol): Type {
            const typeArguments = typeArgumentsFromTypeReferenceNode(node); // Do unconditionally so we mark type arguments as referenced.
            if (symbol === unknownSymbol) {
                return errorType;
            }

            const type = getTypeReferenceTypeWorker(node, symbol, typeArguments);
            if (type) {
                return type;
            }

            // JS are 'string' or 'number', not an enum type.
            const enumTag = symbol.valueDeclaration && getJSDocEnumTag(symbol.valueDeclaration);
            if (enumTag) {
                return enumTag.typeExpression ? getTypeFromTypeNode(enumTag.typeExpression) : errorType;
            }

            // Get type from reference to named type that cannot be generic (enum or type parameter)
            const res = tryGetDeclaredTypeOfSymbol(symbol);
            if (res) {
                return checkNoTypeArguments(node, symbol) ?
                    res.flags & TypeFlags.TypeParameter ? getConstrainedTypeVariable(<TypeParameter>res, node) : getRegularTypeOfLiteralType(res) :
                    errorType;
            }

            if (!(symbol.flags & SymbolFlags.Value && isJSDocTypeReference(node))) {
                return errorType;
            }

            const jsdocType = getJSDocTypeReference(node, symbol, typeArguments);
            if (jsdocType) {
                return jsdocType;
            }

            // Resolve the type reference as a Type for the purpose of reporting errors.
            resolveTypeReferenceName(getTypeReferenceName(node), SymbolFlags.Type);
            return getTypeOfSymbol(symbol);
        }

        /**
         * A jsdoc TypeReference may have resolved to a value (as opposed to a type). If
         * the symbol is a constructor function, return the inferred class type; otherwise,
         * the type of this reference is just the type of the value we resolved to.
         */
        function getJSDocTypeReference(node: NodeWithTypeArguments, symbol: Symbol, typeArguments: Type[] | undefined): Type | undefined {
            const assignedType = getAssignedClassType(symbol);
            const valueType = getTypeOfSymbol(symbol);
            const referenceType = valueType.symbol && valueType.symbol !== symbol && !isInferredClassType(valueType) && getTypeReferenceTypeWorker(node, valueType.symbol, typeArguments);
            if (referenceType || assignedType) {
                // TODO: GH#18217 (should the `|| assignedType` be at a lower precedence?)
                return (referenceType && assignedType ? getIntersectionType([assignedType, referenceType]) : referenceType || assignedType)!;
            }
        }

        function getTypeReferenceTypeWorker(node: NodeWithTypeArguments, symbol: Symbol, typeArguments: Type[] | undefined): Type | undefined {
            if (symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                if (symbol.valueDeclaration && isBinaryExpression(symbol.valueDeclaration.parent)) {
                    const jsdocType = getJSDocTypeReference(node, symbol, typeArguments);
                    if (jsdocType) {
                        return jsdocType;
                    }
                }
                return getTypeFromClassOrInterfaceReference(node, symbol, typeArguments);
            }

            if (symbol.flags & SymbolFlags.TypeAlias) {
                return getTypeFromTypeAliasReference(node, symbol, typeArguments);
            }

            if (symbol.flags & SymbolFlags.Function &&
                isJSDocTypeReference(node) &&
                (symbol.members || getJSDocClassTag(symbol.valueDeclaration))) {
                return getInferredClassType(symbol);
            }
        }

        function getSubstitutionType(typeVariable: TypeVariable, substitute: Type) {
            const result = <SubstitutionType>createType(TypeFlags.Substitution);
            result.typeVariable = typeVariable;
            result.substitute = substitute;
            return result;
        }

        function isUnaryTupleTypeNode(node: TypeNode) {
            return node.kind === SyntaxKind.TupleType && (<TupleTypeNode>node).elementTypes.length === 1;
        }

        function getImpliedConstraint(typeVariable: TypeVariable, checkNode: TypeNode, extendsNode: TypeNode): Type | undefined {
            return isUnaryTupleTypeNode(checkNode) && isUnaryTupleTypeNode(extendsNode) ? getImpliedConstraint(typeVariable, (<TupleTypeNode>checkNode).elementTypes[0], (<TupleTypeNode>extendsNode).elementTypes[0]) :
                getActualTypeVariable(getTypeFromTypeNode(checkNode)) === typeVariable ? getTypeFromTypeNode(extendsNode) :
                undefined;
        }

        function getConstrainedTypeVariable(typeVariable: TypeVariable, node: Node) {
            let constraints: Type[] | undefined;
            while (node && !isStatement(node) && node.kind !== SyntaxKind.JSDocComment) {
                const parent = node.parent;
                if (parent.kind === SyntaxKind.ConditionalType && node === (<ConditionalTypeNode>parent).trueType) {
                    const constraint = getImpliedConstraint(typeVariable, (<ConditionalTypeNode>parent).checkType, (<ConditionalTypeNode>parent).extendsType);
                    if (constraint) {
                        constraints = append(constraints, constraint);
                    }
                }
                node = parent;
            }
            return constraints ? getSubstitutionType(typeVariable, getIntersectionType(append(constraints, typeVariable))) : typeVariable;
        }

        function isJSDocTypeReference(node: Node): node is TypeReferenceNode {
            return !!(node.flags & NodeFlags.JSDoc) && (node.kind === SyntaxKind.TypeReference || node.kind === SyntaxKind.ImportType);
        }

        function checkNoTypeArguments(node: NodeWithTypeArguments, symbol?: Symbol) {
            if (node.typeArguments) {
                error(node, Diagnostics.Type_0_is_not_generic, symbol ? symbolToString(symbol) : (<TypeReferenceNode>node).typeName ? declarationNameToString((<TypeReferenceNode>node).typeName) : "(anonymous)");
                return false;
            }
            return true;
        }

        function getIntendedTypeFromJSDocTypeReference(node: TypeReferenceNode): Type | undefined {
            if (isIdentifier(node.typeName)) {
                const typeArgs = node.typeArguments;
                switch (node.typeName.escapedText) {
                    case "String":
                        checkNoTypeArguments(node);
                        return stringType;
                    case "Number":
                        checkNoTypeArguments(node);
                        return numberType;
                    case "Boolean":
                        checkNoTypeArguments(node);
                        return booleanType;
                    case "Void":
                        checkNoTypeArguments(node);
                        return voidType;
                    case "Undefined":
                        checkNoTypeArguments(node);
                        return undefinedType;
                    case "Null":
                        checkNoTypeArguments(node);
                        return nullType;
                    case "Function":
                    case "function":
                        checkNoTypeArguments(node);
                        return globalFunctionType;
                    case "Array":
                    case "array":
                        return !typeArgs || !typeArgs.length ? anyArrayType : undefined;
                    case "Promise":
                    case "promise":
                        return !typeArgs || !typeArgs.length ? createPromiseType(anyType) : undefined;
                    case "Object":
                        if (typeArgs && typeArgs.length === 2) {
                            if (isJSDocIndexSignature(node)) {
                                const indexed = getTypeFromTypeNode(typeArgs[0]);
                                const target = getTypeFromTypeNode(typeArgs[1]);
                                const index = createIndexInfo(target, /*isReadonly*/ false);
                                return createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, indexed === stringType ? index : undefined, indexed === numberType ? index : undefined);
                            }
                            return anyType;
                        }
                        checkNoTypeArguments(node);
                        return anyType;
                }
            }
        }

        function getTypeFromJSDocNullableTypeNode(node: JSDocNullableType) {
            const type = getTypeFromTypeNode(node.type);
            return strictNullChecks ? getNullableType(type, TypeFlags.Null) : type;
        }

        function getTypeFromTypeReference(node: TypeReferenceType): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                let symbol: Symbol | undefined;
                let type: Type | undefined;
                let meaning = SymbolFlags.Type;
                if (isJSDocTypeReference(node)) {
                    type = getIntendedTypeFromJSDocTypeReference(node);
                    meaning |= SymbolFlags.Value;
                }
                if (!type) {
                    symbol = resolveTypeReferenceName(getTypeReferenceName(node), meaning);
                    type = getTypeReferenceType(node, symbol);
                }
                // Cache both the resolved symbol and the resolved type. The resolved symbol is needed in when we check the
                // type reference in checkTypeReferenceNode.
                links.resolvedSymbol = symbol;
                links.resolvedType = type;
            }
            return links.resolvedType;
        }

        function typeArgumentsFromTypeReferenceNode(node: NodeWithTypeArguments): Type[] | undefined {
            return map(node.typeArguments, getTypeFromTypeNode);
        }

        function getTypeFromTypeQueryNode(node: TypeQueryNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                // TypeScript 1.0 spec (April 2014): 3.6.3
                // The expression is processed as an identifier expression (section 4.3)
                // or property access expression(section 4.10),
                // the widened type(section 3.9) of which becomes the result.
                links.resolvedType = getRegularTypeOfLiteralType(getWidenedType(checkExpression(node.exprName)));
            }
            return links.resolvedType;
        }

        function getTypeOfGlobalSymbol(symbol: Symbol | undefined, arity: number): ObjectType {

            function getTypeDeclaration(symbol: Symbol): Declaration | undefined {
                const declarations = symbol.declarations;
                for (const declaration of declarations) {
                    switch (declaration.kind) {
                        case SyntaxKind.ClassDeclaration:
                        case SyntaxKind.InterfaceDeclaration:
                        case SyntaxKind.EnumDeclaration:
                            return declaration;
                    }
                }
            }

            if (!symbol) {
                return arity ? emptyGenericType : emptyObjectType;
            }
            const type = getDeclaredTypeOfSymbol(symbol);
            if (!(type.flags & TypeFlags.Object)) {
                error(getTypeDeclaration(symbol), Diagnostics.Global_type_0_must_be_a_class_or_interface_type, symbolName(symbol));
                return arity ? emptyGenericType : emptyObjectType;
            }
            if (length((<InterfaceType>type).typeParameters) !== arity) {
                error(getTypeDeclaration(symbol), Diagnostics.Global_type_0_must_have_1_type_parameter_s, symbolName(symbol), arity);
                return arity ? emptyGenericType : emptyObjectType;
            }
            return <ObjectType>type;
        }

        function getGlobalValueSymbol(name: __String, reportErrors: boolean): Symbol | undefined {
            return getGlobalSymbol(name, SymbolFlags.Value, reportErrors ? Diagnostics.Cannot_find_global_value_0 : undefined);
        }

        function getGlobalTypeSymbol(name: __String, reportErrors: boolean): Symbol | undefined {
            return getGlobalSymbol(name, SymbolFlags.Type, reportErrors ? Diagnostics.Cannot_find_global_type_0 : undefined);
        }

        function getGlobalSymbol(name: __String, meaning: SymbolFlags, diagnostic: DiagnosticMessage | undefined): Symbol | undefined {
            // Don't track references for global symbols anyway, so value if `isReference` is arbitrary
            return resolveName(undefined, name, meaning, diagnostic, name, /*isUse*/ false);
        }

        function getGlobalType(name: __String, arity: 0, reportErrors: boolean): ObjectType;
        function getGlobalType(name: __String, arity: number, reportErrors: boolean): GenericType;
        function getGlobalType(name: __String, arity: number, reportErrors: boolean): ObjectType | undefined {
            const symbol = getGlobalTypeSymbol(name, reportErrors);
            return symbol || reportErrors ? getTypeOfGlobalSymbol(symbol, arity) : undefined;
        }

        function getGlobalTypedPropertyDescriptorType() {
            return deferredGlobalTypedPropertyDescriptorType || (deferredGlobalTypedPropertyDescriptorType = getGlobalType("TypedPropertyDescriptor" as __String, /*arity*/ 1, /*reportErrors*/ true)) || emptyGenericType;
        }

        function getGlobalTemplateStringsArrayType() {
            return deferredGlobalTemplateStringsArrayType || (deferredGlobalTemplateStringsArrayType = getGlobalType("TemplateStringsArray" as __String, /*arity*/ 0, /*reportErrors*/ true)) || emptyObjectType;
        }

        function getGlobalImportMetaType() {
            return deferredGlobalImportMetaType || (deferredGlobalImportMetaType = getGlobalType("ImportMeta" as __String, /*arity*/ 0, /*reportErrors*/ true)) || emptyObjectType;
        }

        function getGlobalESSymbolConstructorSymbol(reportErrors: boolean) {
            return deferredGlobalESSymbolConstructorSymbol || (deferredGlobalESSymbolConstructorSymbol = getGlobalValueSymbol("Symbol" as __String, reportErrors));
        }

        function getGlobalESSymbolType(reportErrors: boolean) {
            return deferredGlobalESSymbolType || (deferredGlobalESSymbolType = getGlobalType("Symbol" as __String, /*arity*/ 0, reportErrors)) || emptyObjectType;
        }

        function getGlobalPromiseType(reportErrors: boolean) {
            return deferredGlobalPromiseType || (deferredGlobalPromiseType = getGlobalType("Promise" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalPromiseConstructorSymbol(reportErrors: boolean): Symbol | undefined {
            return deferredGlobalPromiseConstructorSymbol || (deferredGlobalPromiseConstructorSymbol = getGlobalValueSymbol("Promise" as __String, reportErrors));
        }

        function getGlobalPromiseConstructorLikeType(reportErrors: boolean) {
            return deferredGlobalPromiseConstructorLikeType || (deferredGlobalPromiseConstructorLikeType = getGlobalType("PromiseConstructorLike" as __String, /*arity*/ 0, reportErrors)) || emptyObjectType;
        }

        function getGlobalAsyncIterableType(reportErrors: boolean) {
            return deferredGlobalAsyncIterableType || (deferredGlobalAsyncIterableType = getGlobalType("AsyncIterable" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalAsyncIteratorType(reportErrors: boolean) {
            return deferredGlobalAsyncIteratorType || (deferredGlobalAsyncIteratorType = getGlobalType("AsyncIterator" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalAsyncIterableIteratorType(reportErrors: boolean) {
            return deferredGlobalAsyncIterableIteratorType || (deferredGlobalAsyncIterableIteratorType = getGlobalType("AsyncIterableIterator" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalIterableType(reportErrors: boolean) {
            return deferredGlobalIterableType || (deferredGlobalIterableType = getGlobalType("Iterable" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalIteratorType(reportErrors: boolean) {
            return deferredGlobalIteratorType || (deferredGlobalIteratorType = getGlobalType("Iterator" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalIterableIteratorType(reportErrors: boolean) {
            return deferredGlobalIterableIteratorType || (deferredGlobalIterableIteratorType = getGlobalType("IterableIterator" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
        }

        function getGlobalTypeOrUndefined(name: __String, arity = 0): ObjectType | undefined {
            const symbol = getGlobalSymbol(name, SymbolFlags.Type, /*diagnostic*/ undefined);
            return symbol && <GenericType>getTypeOfGlobalSymbol(symbol, arity);
        }

        function getGlobalExtractSymbol(): Symbol {
            return deferredGlobalExtractSymbol || (deferredGlobalExtractSymbol = getGlobalSymbol("Extract" as __String, SymbolFlags.TypeAlias, Diagnostics.Cannot_find_global_type_0)!); // TODO: GH#18217
        }

        /**
         * Instantiates a global type that is generic with some element type, and returns that instantiation.
         */
        function createTypeFromGenericGlobalType(genericGlobalType: GenericType, typeArguments: ReadonlyArray<Type>): ObjectType {
            return genericGlobalType !== emptyGenericType ? createTypeReference(genericGlobalType, typeArguments) : emptyObjectType;
        }

        function createTypedPropertyDescriptorType(propertyType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalTypedPropertyDescriptorType(), [propertyType]);
        }

        function createAsyncIterableType(iteratedType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalAsyncIterableType(/*reportErrors*/ true), [iteratedType]);
        }

        function createAsyncIterableIteratorType(iteratedType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalAsyncIterableIteratorType(/*reportErrors*/ true), [iteratedType]);
        }

        function createIterableType(iteratedType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalIterableType(/*reportErrors*/ true), [iteratedType]);
        }

        function createIterableIteratorType(iteratedType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalIterableIteratorType(/*reportErrors*/ true), [iteratedType]);
        }

        function createArrayType(elementType: Type): ObjectType {
            return createTypeFromGenericGlobalType(globalArrayType, [elementType]);
        }

        function createReadonlyArrayType(elementType: Type): ObjectType {
            return createTypeFromGenericGlobalType(globalReadonlyArrayType, [elementType]);
        }

        function getTypeFromArrayTypeNode(node: ArrayTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = createArrayType(getTypeFromTypeNode(node.elementType));
            }
            return links.resolvedType;
        }

        // We represent tuple types as type references to synthesized generic interface types created by
        // this function. The types are of the form:
        //
        //   interface Tuple<T0, T1, T2, ...> extends Array<T0 | T1 | T2 | ...> { 0: T0, 1: T1, 2: T2, ... }
        //
        // Note that the generic type created by this function has no symbol associated with it. The same
        // is true for each of the synthesized type parameters.
        function createTupleTypeOfArity(arity: number, minLength: number, hasRestElement: boolean, associatedNames: __String[] | undefined): TupleType {
            let typeParameters: TypeParameter[] | undefined;
            const properties: Symbol[] = [];
            const maxLength = hasRestElement ? arity - 1 : arity;
            if (arity) {
                typeParameters = new Array(arity);
                for (let i = 0; i < arity; i++) {
                    const typeParameter = typeParameters[i] = <TypeParameter>createType(TypeFlags.TypeParameter);
                    if (i < maxLength) {
                        const property = createSymbol(SymbolFlags.Property | (i >= minLength ? SymbolFlags.Optional : 0), "" + i as __String);
                        property.type = typeParameter;
                        properties.push(property);
                    }
                }
            }
            const literalTypes = [];
            for (let i = minLength; i <= maxLength; i++) literalTypes.push(getLiteralType(i));
            const lengthSymbol = createSymbol(SymbolFlags.Property, "length" as __String);
            lengthSymbol.type = hasRestElement ? numberType : getUnionType(literalTypes);
            properties.push(lengthSymbol);
            const type = <TupleType & InterfaceTypeWithDeclaredMembers>createObjectType(ObjectFlags.Tuple | ObjectFlags.Reference);
            type.typeParameters = typeParameters;
            type.outerTypeParameters = undefined;
            type.localTypeParameters = typeParameters;
            type.instantiations = createMap<TypeReference>();
            type.instantiations.set(getTypeListId(type.typeParameters), <GenericType>type);
            type.target = <GenericType>type;
            type.typeArguments = type.typeParameters;
            type.thisType = <TypeParameter>createType(TypeFlags.TypeParameter);
            type.thisType.isThisType = true;
            type.thisType.constraint = type;
            type.declaredProperties = properties;
            type.declaredCallSignatures = emptyArray;
            type.declaredConstructSignatures = emptyArray;
            type.declaredStringIndexInfo = undefined;
            type.declaredNumberIndexInfo = undefined;
            type.minLength = minLength;
            type.hasRestElement = hasRestElement;
            type.associatedNames = associatedNames;
            return type;
        }

        function getTupleTypeOfArity(arity: number, minLength: number, hasRestElement: boolean, associatedNames?: __String[]): GenericType {
            const key = arity + (hasRestElement ? "+" : ",") + minLength + (associatedNames && associatedNames.length ? "," + associatedNames.join(",") : "");
            let type = tupleTypes.get(key);
            if (!type) {
                tupleTypes.set(key, type = createTupleTypeOfArity(arity, minLength, hasRestElement, associatedNames));
            }
            return type;
        }

        function createTupleType(elementTypes: ReadonlyArray<Type>, minLength = elementTypes.length, hasRestElement = false, associatedNames?: __String[]) {
            const arity = elementTypes.length;
            if (arity === 1 && hasRestElement) {
                return createArrayType(elementTypes[0]);
            }
            const tupleType = getTupleTypeOfArity(arity, minLength, arity > 0 && hasRestElement, associatedNames);
            return elementTypes.length ? createTypeReference(tupleType, elementTypes) : tupleType;
        }

        function getTypeFromTupleTypeNode(node: TupleTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const lastElement = lastOrUndefined(node.elementTypes);
                const restElement = lastElement && lastElement.kind === SyntaxKind.RestType ? lastElement : undefined;
                const minLength = findLastIndex(node.elementTypes, n => n.kind !== SyntaxKind.OptionalType && n !== restElement) + 1;
                const elementTypes = map(node.elementTypes, n => {
                    const type = getTypeFromTypeNode(n);
                    return n === restElement && getIndexTypeOfType(type, IndexKind.Number) || type;
                });
                links.resolvedType = createTupleType(elementTypes, minLength, !!restElement);
            }
            return links.resolvedType;
        }

        function getTypeFromOptionalTypeNode(node: OptionalTypeNode): Type {
            const type = getTypeFromTypeNode(node.type);
            return strictNullChecks ? getOptionalType(type) : type;
        }

        function getTypeId(type: Type) {
            return type.id;
        }

        function containsType(types: ReadonlyArray<Type>, type: Type): boolean {
            return binarySearch(types, type, getTypeId, compareValues) >= 0;
        }

        function insertType(types: Type[], type: Type): boolean {
            const index = binarySearch(types, type, getTypeId, compareValues);
            if (index < 0) {
                types.splice(~index, 0, type);
                return true;
            }
            return false;
        }

        // Return true if the given intersection type contains
        // more than one unit type or,
        // an object type and a nullable type (null or undefined), or
        // a string-like type and a type known to be non-string-like, or
        // a number-like type and a type known to be non-number-like, or
        // a symbol-like type and a type known to be non-symbol-like, or
        // a void-like type and a type known to be non-void-like, or
        // a non-primitive type and a type known to be primitive.
        function isEmptyIntersectionType(type: IntersectionType) {
            let combined: TypeFlags = 0;
            for (const t of type.types) {
                if (t.flags & TypeFlags.Unit && combined & TypeFlags.Unit) {
                    return true;
                }
                combined |= t.flags;
                if (combined & TypeFlags.Nullable && combined & (TypeFlags.Object | TypeFlags.NonPrimitive) ||
                    combined & TypeFlags.NonPrimitive && combined & (TypeFlags.DisjointDomains & ~TypeFlags.NonPrimitive) ||
                    combined & TypeFlags.StringLike && combined & (TypeFlags.DisjointDomains & ~TypeFlags.StringLike) ||
                    combined & TypeFlags.NumberLike && combined & (TypeFlags.DisjointDomains & ~TypeFlags.NumberLike) ||
                    combined & TypeFlags.ESSymbolLike && combined & (TypeFlags.DisjointDomains & ~TypeFlags.ESSymbolLike) ||
                    combined & TypeFlags.VoidLike && combined & (TypeFlags.DisjointDomains & ~TypeFlags.VoidLike)) {
                    return true;
                }
            }
            return false;
        }

        function addTypeToUnion(typeSet: Type[], includes: TypeFlags, type: Type) {
            const flags = type.flags;
            if (flags & TypeFlags.Union) {
                return addTypesToUnion(typeSet, includes, (<UnionType>type).types);
            }
            // We ignore 'never' types in unions. Likewise, we ignore intersections of unit types as they are
            // another form of 'never' (in that they have an empty value domain). We could in theory turn
            // intersections of unit types into 'never' upon construction, but deferring the reduction makes it
            // easier to reason about their origin.
            if (!(flags & TypeFlags.Never || flags & TypeFlags.Intersection && isEmptyIntersectionType(<IntersectionType>type))) {
                includes |= flags & ~TypeFlags.ConstructionFlags;
                if (flags & TypeFlags.AnyOrUnknown) {
                    if (type === wildcardType) includes |= TypeFlags.Wildcard;
                }
                else if (!strictNullChecks && flags & TypeFlags.Nullable) {
                    if (!(flags & TypeFlags.ContainsWideningType)) includes |= TypeFlags.NonWideningType;
                }
                else {
                    const len = typeSet.length;
                    const index = len && type.id > typeSet[len - 1].id ? ~len : binarySearch(typeSet, type, getTypeId, compareValues);
                    if (index < 0) {
                        typeSet.splice(~index, 0, type);
                    }
                }
            }
            return includes;
        }

        // Add the given types to the given type set. Order is preserved, duplicates are removed,
        // and nested types of the given kind are flattened into the set.
        function addTypesToUnion(typeSet: Type[], includes: TypeFlags, types: ReadonlyArray<Type>): TypeFlags {
            for (const type of types) {
                includes = addTypeToUnion(typeSet, includes, type);
            }
            return includes;
        }

        function isSubtypeOfAny(source: Type, targets: ReadonlyArray<Type>): boolean {
            for (const target of targets) {
                if (source !== target && isTypeSubtypeOf(source, target) && (
                    !(getObjectFlags(getTargetType(source)) & ObjectFlags.Class) ||
                    !(getObjectFlags(getTargetType(target)) & ObjectFlags.Class) ||
                    isTypeDerivedFrom(source, target))) {
                    return true;
                }
            }
            return false;
        }

        function isSetOfLiteralsFromSameEnum(types: ReadonlyArray<Type>): boolean {
            const first = types[0];
            if (first.flags & TypeFlags.EnumLiteral) {
                const firstEnum = getParentOfSymbol(first.symbol);
                for (let i = 1; i < types.length; i++) {
                    const other = types[i];
                    if (!(other.flags & TypeFlags.EnumLiteral) || (firstEnum !== getParentOfSymbol(other.symbol))) {
                        return false;
                    }
                }
                return true;
            }

            return false;
        }

        function removeSubtypes(types: Type[]) {
            if (types.length === 0 || isSetOfLiteralsFromSameEnum(types)) {
                return;
            }
            let i = types.length;
            while (i > 0) {
                i--;
                if (isSubtypeOfAny(types[i], types)) {
                    orderedRemoveItemAt(types, i);
                }
            }
        }

        function removeRedundantLiteralTypes(types: Type[], includes: TypeFlags) {
            let i = types.length;
            while (i > 0) {
                i--;
                const t = types[i];
                const remove =
                    t.flags & TypeFlags.StringLiteral && includes & TypeFlags.String ||
                    t.flags & TypeFlags.NumberLiteral && includes & TypeFlags.Number ||
                    t.flags & TypeFlags.UniqueESSymbol && includes & TypeFlags.ESSymbol ||
                    t.flags & TypeFlags.StringOrNumberLiteral && t.flags & TypeFlags.FreshLiteral && containsType(types, (<LiteralType>t).regularType);
                if (remove) {
                    orderedRemoveItemAt(types, i);
                }
            }
        }

        // We sort and deduplicate the constituent types based on object identity. If the subtypeReduction
        // flag is specified we also reduce the constituent type set to only include types that aren't subtypes
        // of other types. Subtype reduction is expensive for large union types and is possible only when union
        // types are known not to circularly reference themselves (as is the case with union types created by
        // expression constructs such as array literals and the || and ?: operators). Named types can
        // circularly reference themselves and therefore cannot be subtype reduced during their declaration.
        // For example, "type Item = string | (() => Item" is a named type that circularly references itself.
        function getUnionType(types: ReadonlyArray<Type>, unionReduction: UnionReduction = UnionReduction.Literal, aliasSymbol?: Symbol, aliasTypeArguments?: ReadonlyArray<Type>): Type {
            if (types.length === 0) {
                return neverType;
            }
            if (types.length === 1) {
                return types[0];
            }
            const typeSet: Type[] = [];
            const includes = addTypesToUnion(typeSet, 0, types);
            if (includes & TypeFlags.AnyOrUnknown) {
                return includes & TypeFlags.Any ? includes & TypeFlags.Wildcard ? wildcardType : anyType : unknownType;
            }
            switch (unionReduction) {
                case UnionReduction.Literal:
                    if (includes & TypeFlags.StringOrNumberLiteralOrUnique) {
                        removeRedundantLiteralTypes(typeSet, includes);
                    }
                    break;
                case UnionReduction.Subtype:
                    removeSubtypes(typeSet);
                    break;
            }
            if (typeSet.length === 0) {
                return includes & TypeFlags.Null ? includes & TypeFlags.NonWideningType ? nullType : nullWideningType :
                    includes & TypeFlags.Undefined ? includes & TypeFlags.NonWideningType ? undefinedType : undefinedWideningType :
                        neverType;
            }
            return getUnionTypeFromSortedList(typeSet, includes & TypeFlags.NotPrimitiveUnion ? 0 : TypeFlags.UnionOfPrimitiveTypes, aliasSymbol, aliasTypeArguments);
        }

        function getUnionTypePredicate(signatures: ReadonlyArray<Signature>): TypePredicate | undefined {
            let first: TypePredicate | undefined;
            const types: Type[] = [];
            for (const sig of signatures) {
                const pred = getTypePredicateOfSignature(sig);
                if (!pred) {
                    continue;
                }

                if (first) {
                    if (!typePredicateKindsMatch(first, pred)) {
                        // No common type predicate.
                        return undefined;
                    }
                }
                else {
                    first = pred;
                }
                types.push(pred.type);
            }
            if (!first) {
                // No union signatures had a type predicate.
                return undefined;
            }
            const unionType = getUnionType(types);
            return isIdentifierTypePredicate(first)
                ? createIdentifierTypePredicate(first.parameterName, first.parameterIndex, unionType)
                : createThisTypePredicate(unionType);
        }

        function typePredicateKindsMatch(a: TypePredicate, b: TypePredicate): boolean {
            return isIdentifierTypePredicate(a)
                ? isIdentifierTypePredicate(b) && a.parameterIndex === b.parameterIndex
                : !isIdentifierTypePredicate(b);
        }

        // This function assumes the constituent type list is sorted and deduplicated.
        function getUnionTypeFromSortedList(types: Type[], unionOfUnitTypes: TypeFlags, aliasSymbol?: Symbol, aliasTypeArguments?: ReadonlyArray<Type>): Type {
            if (types.length === 0) {
                return neverType;
            }
            if (types.length === 1) {
                return types[0];
            }
            const id = getTypeListId(types);
            let type = unionTypes.get(id);
            if (!type) {
                const propagatedFlags = getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
                type = <UnionType>createType(TypeFlags.Union | propagatedFlags | unionOfUnitTypes);
                unionTypes.set(id, type);
                type.types = types;
                /*
                Note: This is the alias symbol (or lack thereof) that we see when we first encounter this union type.
                For aliases of identical unions, eg `type T = A | B; type U = A | B`, the symbol of the first alias encountered is the aliasSymbol.
                (In the language service, the order may depend on the order in which a user takes actions, such as hovering over symbols.)
                It's important that we create equivalent union types only once, so that's an unfortunate side effect.
                */
                type.aliasSymbol = aliasSymbol;
                type.aliasTypeArguments = aliasTypeArguments;
            }
            return type;
        }

        function getTypeFromUnionTypeNode(node: UnionTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const aliasSymbol = getAliasSymbolForTypeNode(node);
                links.resolvedType = getUnionType(map(node.types, getTypeFromTypeNode), UnionReduction.Literal,
                    aliasSymbol, getTypeArgumentsForAliasSymbol(aliasSymbol));
            }
            return links.resolvedType;
        }

        function addTypeToIntersection(typeSet: Type[], includes: TypeFlags, type: Type) {
            const flags = type.flags;
            if (flags & TypeFlags.Intersection) {
                return addTypesToIntersection(typeSet, includes, (<IntersectionType>type).types);
            }
            if (getObjectFlags(type) & ObjectFlags.Anonymous && isEmptyObjectType(type)) {
                includes |= TypeFlags.EmptyObject;
            }
            else {
                includes |= flags & ~TypeFlags.ConstructionFlags;
                if (flags & TypeFlags.AnyOrUnknown) {
                    if (type === wildcardType) includes |= TypeFlags.Wildcard;
                }
                else if ((strictNullChecks || !(flags & TypeFlags.Nullable)) && !contains(typeSet, type)) {
                    typeSet.push(type);
                }
            }
            return includes;
        }

        // Add the given types to the given type set. Order is preserved, freshness is removed from literal
        // types, duplicates are removed, and nested types of the given kind are flattened into the set.
        function addTypesToIntersection(typeSet: Type[], includes: TypeFlags, types: ReadonlyArray<Type>) {
            for (const type of types) {
                includes = addTypeToIntersection(typeSet, includes, getRegularTypeOfLiteralType(type));
            }
            return includes;
        }

        function removeRedundantPrimitiveTypes(types: Type[], includes: TypeFlags) {
            let i = types.length;
            while (i > 0) {
                i--;
                const t = types[i];
                const remove =
                    t.flags & TypeFlags.String && includes & TypeFlags.StringLiteral ||
                    t.flags & TypeFlags.Number && includes & TypeFlags.NumberLiteral ||
                    t.flags & TypeFlags.ESSymbol && includes & TypeFlags.UniqueESSymbol;
                if (remove) {
                    orderedRemoveItemAt(types, i);
                }
            }
        }

        // Check that the given type has a match in every union. A given type is matched by
        // an identical type, and a literal type is additionally matched by its corresponding
        // primitive type.
        function eachUnionContains(unionTypes: UnionType[], type: Type) {
            for (const u of unionTypes) {
                if (!containsType(u.types, type)) {
                    const primitive = type.flags & TypeFlags.StringLiteral ? stringType :
                        type.flags & TypeFlags.NumberLiteral ? numberType :
                        type.flags & TypeFlags.UniqueESSymbol ? esSymbolType :
                        undefined;
                    if (!primitive || !containsType(u.types, primitive)) {
                        return false;
                    }
                }
            }
            return true;
        }

        // If the given list of types contains more than one union of primitive types, replace the
        // first with a union containing an intersection of those primitive types, then remove the
        // other unions and return true. Otherwise, do nothing and return false.
        function intersectUnionsOfPrimitiveTypes(types: Type[]) {
            let unionTypes: UnionType[] | undefined;
            const index = findIndex(types, t => (t.flags & TypeFlags.UnionOfPrimitiveTypes) !== 0);
            let i = index + 1;
            // Remove all but the first union of primitive types and collect them in
            // the unionTypes array.
            while (i < types.length) {
                const t = types[i];
                if (t.flags & TypeFlags.UnionOfPrimitiveTypes) {
                    (unionTypes || (unionTypes = [<UnionType>types[index]])).push(<UnionType>t);
                    orderedRemoveItemAt(types, i);
                }
                else {
                    i++;
                }
            }
            // Return false if there was only one union of primitive types
            if (!unionTypes) {
                return false;
            }
            // We have more than one union of primitive types, now intersect them. For each
            // type in each union we check if the type is matched in every union and if so
            // we include it in the result.
            const checked: Type[] = [];
            const result: Type[] = [];
            for (const u of unionTypes) {
                for (const t of u.types) {
                    if (insertType(checked, t)) {
                        if (eachUnionContains(unionTypes, t)) {
                            insertType(result, t);
                        }
                    }
                }
            }
            // Finally replace the first union with the result
            types[index] = getUnionTypeFromSortedList(result, TypeFlags.UnionOfPrimitiveTypes);
            return true;
        }

        // We normalize combinations of intersection and union types based on the distributive property of the '&'
        // operator. Specifically, because X & (A | B) is equivalent to X & A | X & B, we can transform intersection
        // types with union type constituents into equivalent union types with intersection type constituents and
        // effectively ensure that union types are always at the top level in type representations.
        //
        // We do not perform structural deduplication on intersection types. Intersection types are created only by the &
        // type operator and we can't reduce those because we want to support recursive intersection types. For example,
        // a type alias of the form "type List<T> = T & { next: List<T> }" cannot be reduced during its declaration.
        // Also, unlike union types, the order of the constituent types is preserved in order that overload resolution
        // for intersections of types with signatures can be deterministic.
        function getIntersectionType(types: ReadonlyArray<Type>, aliasSymbol?: Symbol, aliasTypeArguments?: ReadonlyArray<Type>): Type {
            const typeSet: Type[] = [];
            const includes = addTypesToIntersection(typeSet, 0, types);
            if (includes & TypeFlags.Never) {
                return neverType;
            }
            if (includes & TypeFlags.Any) {
                return includes & TypeFlags.Wildcard ? wildcardType : anyType;
            }
            if (!strictNullChecks && includes & TypeFlags.Nullable) {
                return includes & TypeFlags.Undefined ? undefinedType : nullType;
            }
            if (includes & TypeFlags.String && includes & TypeFlags.StringLiteral ||
                includes & TypeFlags.Number && includes & TypeFlags.NumberLiteral ||
                includes & TypeFlags.ESSymbol && includes & TypeFlags.UniqueESSymbol) {
                removeRedundantPrimitiveTypes(typeSet, includes);
            }
            if (includes & TypeFlags.EmptyObject && !(includes & TypeFlags.Object)) {
                typeSet.push(emptyObjectType);
            }
            if (typeSet.length === 0) {
                return unknownType;
            }
            if (typeSet.length === 1) {
                return typeSet[0];
            }
            if (includes & TypeFlags.Union) {
                if (includes & TypeFlags.UnionOfPrimitiveTypes && intersectUnionsOfPrimitiveTypes(typeSet)) {
                    // When the intersection creates a reduced set (which might mean that *all* union types have
                    // disappeared), we restart the operation to get a new set of combined flags. Once we have
                    // reduced we'll never reduce again, so this occurs at most once.
                    return getIntersectionType(typeSet, aliasSymbol, aliasTypeArguments);
                }
                // We are attempting to construct a type of the form X & (A | B) & Y. Transform this into a type of
                // the form X & A & Y | X & B & Y and recursively reduce until no union type constituents remain.
                const unionIndex = findIndex(typeSet, t => (t.flags & TypeFlags.Union) !== 0);
                const unionType = <UnionType>typeSet[unionIndex];
                return getUnionType(map(unionType.types, t => getIntersectionType(replaceElement(typeSet, unionIndex, t))),
                    UnionReduction.Literal, aliasSymbol, aliasTypeArguments);
            }
            const id = getTypeListId(typeSet);
            let type = intersectionTypes.get(id);
            if (!type) {
                const propagatedFlags = getPropagatingFlagsOfTypes(typeSet, /*excludeKinds*/ TypeFlags.Nullable);
                type = <IntersectionType>createType(TypeFlags.Intersection | propagatedFlags);
                intersectionTypes.set(id, type);
                type.types = typeSet;
                type.aliasSymbol = aliasSymbol; // See comment in `getUnionTypeFromSortedList`.
                type.aliasTypeArguments = aliasTypeArguments;
            }
            return type;
        }

        function getTypeFromIntersectionTypeNode(node: IntersectionTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const aliasSymbol = getAliasSymbolForTypeNode(node);
                links.resolvedType = getIntersectionType(map(node.types, getTypeFromTypeNode),
                    aliasSymbol, getTypeArgumentsForAliasSymbol(aliasSymbol));
            }
            return links.resolvedType;
        }

        function createIndexType(type: InstantiableType | UnionOrIntersectionType, stringsOnly: boolean) {
            const result = <IndexType>createType(TypeFlags.Index);
            result.type = type;
            result.stringsOnly = stringsOnly;
            return result;
        }

        function getIndexTypeForGenericType(type: InstantiableType | UnionOrIntersectionType, stringsOnly: boolean) {
            return stringsOnly ?
                type.resolvedStringIndexType || (type.resolvedStringIndexType = createIndexType(type, /*stringsOnly*/ true)) :
                type.resolvedIndexType || (type.resolvedIndexType = createIndexType(type, /*stringsOnly*/ false));
        }

        function getLiteralTypeFromPropertyName(prop: Symbol, include: TypeFlags) {
            if (!(getDeclarationModifierFlagsFromSymbol(prop) & ModifierFlags.NonPublicAccessibilityModifier)) {
                let type = getLateBoundSymbol(prop).nameType;
                if (!type && !isKnownSymbol(prop)) {
                    const name = prop.valueDeclaration && getNameOfDeclaration(prop.valueDeclaration);
                    type = name && isNumericLiteral(name) ? getLiteralType(+name.text) :
                        name && name.kind === SyntaxKind.ComputedPropertyName && isNumericLiteral(name.expression) ? getLiteralType(+name.expression.text) :
                        getLiteralType(symbolName(prop));
                }
                if (type && type.flags & include) {
                    return type;
                }
            }
            return neverType;
        }

        function getLiteralTypeFromPropertyNames(type: Type, include: TypeFlags) {
            return getUnionType(map(getPropertiesOfType(type), t => getLiteralTypeFromPropertyName(t, include)));
        }

        function getNonEnumNumberIndexInfo(type: Type) {
            const numberIndexInfo = getIndexInfoOfType(type, IndexKind.Number);
            return numberIndexInfo !== enumNumberIndexInfo ? numberIndexInfo : undefined;
        }

        function getIndexType(type: Type, stringsOnly = keyofStringsOnly): Type {
            return type.flags & TypeFlags.Union ? getIntersectionType(map((<IntersectionType>type).types, t => getIndexType(t, stringsOnly))) :
                type.flags & TypeFlags.Intersection ? getUnionType(map((<IntersectionType>type).types, t => getIndexType(t, stringsOnly))) :
                maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive) ? getIndexTypeForGenericType(<InstantiableType | UnionOrIntersectionType>type, stringsOnly) :
                getObjectFlags(type) & ObjectFlags.Mapped ? getConstraintTypeFromMappedType(<MappedType>type) :
                type === wildcardType ? wildcardType :
                type.flags & TypeFlags.Any ? keyofConstraintType :
                stringsOnly ? getIndexInfoOfType(type, IndexKind.String) ? stringType : getLiteralTypeFromPropertyNames(type, TypeFlags.StringLiteral) :
                getIndexInfoOfType(type, IndexKind.String) ? getUnionType([stringType, numberType, getLiteralTypeFromPropertyNames(type, TypeFlags.UniqueESSymbol)]) :
                getNonEnumNumberIndexInfo(type) ? getUnionType([numberType, getLiteralTypeFromPropertyNames(type, TypeFlags.StringLiteral | TypeFlags.UniqueESSymbol)]) :
                getLiteralTypeFromPropertyNames(type, TypeFlags.StringOrNumberLiteralOrUnique);
        }

        function getExtractStringType(type: Type) {
            if (keyofStringsOnly) {
                return type;
            }
            const extractTypeAlias = getGlobalExtractSymbol();
            return extractTypeAlias ? getTypeAliasInstantiation(extractTypeAlias, [type, stringType]) : stringType;
        }

        function getIndexTypeOrString(type: Type): Type {
            const indexType = getExtractStringType(getIndexType(type));
            return indexType.flags & TypeFlags.Never ? stringType : indexType;
        }

        function getTypeFromTypeOperatorNode(node: TypeOperatorNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                switch (node.operator) {
                    case SyntaxKind.KeyOfKeyword:
                        links.resolvedType = getIndexType(getTypeFromTypeNode(node.type));
                        break;
                    case SyntaxKind.UniqueKeyword:
                        links.resolvedType = node.type.kind === SyntaxKind.SymbolKeyword
                            ? getESSymbolLikeTypeForNode(walkUpParenthesizedTypes(node.parent))
                            : errorType;
                        break;
                }
            }
            return links.resolvedType!; // TODO: GH#18217
        }

        function createIndexedAccessType(objectType: Type, indexType: Type) {
            const type = <IndexedAccessType>createType(TypeFlags.IndexedAccess);
            type.objectType = objectType;
            type.indexType = indexType;
            return type;
        }

        /**
         * Returns if a type is or consists of a JSLiteral object type
         * In addition to objects which are directly literals,
         * * unions where every element is a jsliteral
         * * intersections where at least one element is a jsliteral
         * * and instantiable types constrained to a jsliteral
         * Should all count as literals and not print errors on access or assignment of possibly existing properties.
         * This mirrors the behavior of the index signature propagation, to which this behaves similarly (but doesn't affect assignability or inference).
         */
        function isJSLiteralType(type: Type): boolean {
            if (noImplicitAny) {
                return false; // Flag is meaningless under `noImplicitAny` mode
            }
            if (getObjectFlags(type) & ObjectFlags.JSLiteral) {
                return true;
            }
            if (type.flags & TypeFlags.Union) {
                return every((type as UnionType).types, isJSLiteralType);
            }
            if (type.flags & TypeFlags.Intersection) {
                return some((type as IntersectionType).types, isJSLiteralType);
            }
            if (type.flags & TypeFlags.Instantiable) {
                return isJSLiteralType(getResolvedBaseConstraint(type));
            }
            return false;
        }

        function getPropertyTypeForIndexType(objectType: Type, indexType: Type, accessNode: ElementAccessExpression | IndexedAccessTypeNode | undefined, cacheSymbol: boolean, missingType: Type) {
            const accessExpression = accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode : undefined;
            const propName = isTypeUsableAsLateBoundName(indexType) ? getLateBoundNameFromType(indexType) :
                accessExpression && checkThatExpressionIsProperSymbolReference(accessExpression.argumentExpression, indexType, /*reportError*/ false) ?
                    getPropertyNameForKnownSymbolName(idText((<PropertyAccessExpression>accessExpression.argumentExpression).name)) :
                    undefined;
            if (propName !== undefined) {
                const prop = getPropertyOfType(objectType, propName);
                if (prop) {
                    if (accessExpression) {
                        markPropertyAsReferenced(prop, accessExpression, /*isThisAccess*/ accessExpression.expression.kind === SyntaxKind.ThisKeyword);
                        if (isAssignmentTarget(accessExpression) && (isReferenceToReadonlyEntity(accessExpression, prop) || isReferenceThroughNamespaceImport(accessExpression))) {
                            error(accessExpression.argumentExpression, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, symbolToString(prop));
                            return missingType;
                        }
                        if (cacheSymbol) {
                            getNodeLinks(accessNode!).resolvedSymbol = prop;
                        }
                    }
                    const propType = getTypeOfSymbol(prop);
                    return accessExpression ? getFlowTypeOfReference(accessExpression, propType) : propType;
                }
                if (isTupleType(objectType)) {
                    const restType = getRestTypeOfTupleType(objectType);
                    if (restType && isNumericLiteralName(propName) && +propName >= 0) {
                        return restType;
                    }
                }
            }
            if (!(indexType.flags & TypeFlags.Nullable) && isTypeAssignableToKind(indexType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbolLike)) {
                if (objectType.flags & (TypeFlags.Any | TypeFlags.Never)) {
                    return objectType;
                }
                const indexInfo = isTypeAssignableToKind(indexType, TypeFlags.NumberLike) && getIndexInfoOfType(objectType, IndexKind.Number) ||
                    getIndexInfoOfType(objectType, IndexKind.String) ||
                    undefined;
                if (indexInfo) {
                    if (accessNode && !isTypeAssignableToKind(indexType, TypeFlags.String | TypeFlags.Number)) {
                        const indexNode = accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode.argumentExpression : accessNode.indexType;
                        error(indexNode, Diagnostics.Type_0_cannot_be_used_as_an_index_type, typeToString(indexType));
                    }
                    else if (accessExpression && indexInfo.isReadonly && (isAssignmentTarget(accessExpression) || isDeleteTarget(accessExpression))) {
                        error(accessExpression, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(objectType));
                    }
                    return indexInfo.type;
                }
                if (indexType.flags & TypeFlags.Never) {
                    return neverType;
                }
                if (isJSLiteralType(objectType)) {
                    return anyType;
                }
                if (accessExpression && !isConstEnumObjectType(objectType)) {
                    if (noImplicitAny && !compilerOptions.suppressImplicitAnyIndexErrors) {
                        if (propName !== undefined && typeHasStaticProperty(propName, objectType)) {
                            error(accessExpression, Diagnostics.Property_0_is_a_static_member_of_type_1, propName as string, typeToString(objectType));
                        }
                        else if (getIndexTypeOfType(objectType, IndexKind.Number)) {
                            error(accessExpression.argumentExpression, Diagnostics.Element_implicitly_has_an_any_type_because_index_expression_is_not_of_type_number);
                        }
                        else {
                            let suggestion: string | undefined;
                            if (propName !== undefined && (suggestion = getSuggestionForNonexistentProperty(propName as string, objectType))) {
                                if (suggestion !== undefined) {
                                    error(accessExpression.argumentExpression, Diagnostics.Property_0_does_not_exist_on_type_1_Did_you_mean_2, propName as string, typeToString(objectType), suggestion);
                                }
                            }
                            else {
                                error(accessExpression, Diagnostics.Element_implicitly_has_an_any_type_because_type_0_has_no_index_signature, typeToString(objectType));
                            }
                        }
                    }
                    return missingType;
                }
            }
            if (isJSLiteralType(objectType)) {
                return anyType;
            }
            if (accessNode) {
                const indexNode = accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode.argumentExpression : accessNode.indexType;
                if (indexType.flags & (TypeFlags.StringLiteral | TypeFlags.NumberLiteral)) {
                    error(indexNode, Diagnostics.Property_0_does_not_exist_on_type_1, "" + (<LiteralType>indexType).value, typeToString(objectType));
                }
                else if (indexType.flags & (TypeFlags.String | TypeFlags.Number)) {
                    error(indexNode, Diagnostics.Type_0_has_no_matching_index_signature_for_type_1, typeToString(objectType), typeToString(indexType));
                }
                else {
                    error(indexNode, Diagnostics.Type_0_cannot_be_used_as_an_index_type, typeToString(indexType));
                }
            }
            if (isTypeAny(indexType)) {
                return indexType;
            }
            return missingType;
        }

        function isGenericObjectType(type: Type): boolean {
            return maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive | TypeFlags.GenericMappedType);
        }

        function isGenericIndexType(type: Type): boolean {
            return maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive | TypeFlags.Index);
        }

        function getSimplifiedType(type: Type): Type {
            return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(<IndexedAccessType>type) : type;
        }

        // Transform an indexed access to a simpler form, if possible. Return the simpler form, or return
        // the type itself if no transformation is possible.
        function getSimplifiedIndexedAccessType(type: IndexedAccessType): Type {
            if (type.simplified) {
                return type.simplified === circularConstraintType ? type : type.simplified;
            }
            type.simplified = circularConstraintType;
            // We recursively simplify the object type as it may in turn be an indexed access type. For example, with
            // '{ [P in T]: { [Q in U]: number } }[T][U]' we want to first simplify the inner indexed access type.
            const objectType = getSimplifiedType(type.objectType);
            const indexType = getSimplifiedType(type.indexType);
            if (objectType.flags & TypeFlags.Union) {
                return type.simplified = mapType(objectType, t => getSimplifiedType(getIndexedAccessType(t, indexType)));
            }
            if (objectType.flags & TypeFlags.Intersection) {
                return type.simplified = getIntersectionType(map((objectType as IntersectionType).types, t => getSimplifiedType(getIndexedAccessType(t, indexType))));
            }
            // If the object type is a mapped type { [P in K]: E }, where K is generic, instantiate E using a mapper
            // that substitutes the index type for P. For example, for an index access { [P in K]: Box<T[P]> }[X], we
            // construct the type Box<T[X]>. We do not further simplify the result because mapped types can be recursive
            // and we might never terminate.
            if (isGenericMappedType(objectType)) {
                return type.simplified = substituteIndexedMappedType(objectType, type);
            }
            if (objectType.flags & TypeFlags.TypeParameter) {
                const constraint = getConstraintOfTypeParameter(objectType as TypeParameter);
                if (constraint && isGenericMappedType(constraint)) {
                    return type.simplified = substituteIndexedMappedType(constraint, type);
                }
            }
            return type.simplified = type;
        }

        function substituteIndexedMappedType(objectType: MappedType, type: IndexedAccessType) {
            const mapper = createTypeMapper([getTypeParameterFromMappedType(objectType)], [type.indexType]);
            const templateMapper = combineTypeMappers(objectType.mapper, mapper);
            return instantiateType(getTemplateTypeFromMappedType(objectType), templateMapper);
        }

        function getIndexedAccessType(objectType: Type, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode, missingType = accessNode ? errorType : unknownType): Type {
            if (objectType === wildcardType || indexType === wildcardType) {
                return wildcardType;
            }
            // If the index type is generic, or if the object type is generic and doesn't originate in an expression,
            // we are performing a higher-order index access where we cannot meaningfully access the properties of the
            // object type. Note that for a generic T and a non-generic K, we eagerly resolve T[K] if it originates in
            // an expression. This is to preserve backwards compatibility. For example, an element access 'this["foo"]'
            // has always been resolved eagerly using the constraint type of 'this' at the given location.
            if (isGenericIndexType(indexType) || !(accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression) && isGenericObjectType(objectType)) {
                if (objectType.flags & TypeFlags.AnyOrUnknown) {
                    return objectType;
                }
                // Defer the operation by creating an indexed access type.
                const id = objectType.id + "," + indexType.id;
                let type = indexedAccessTypes.get(id);
                if (!type) {
                    indexedAccessTypes.set(id, type = createIndexedAccessType(objectType, indexType));
                }
                return type;
            }
            // In the following we resolve T[K] to the type of the property in T selected by K.
            // We treat boolean as different from other unions to improve errors;
            // skipping straight to getPropertyTypeForIndexType gives errors with 'boolean' instead of 'true'.
            const apparentObjectType = getApparentType(objectType);
            if (indexType.flags & TypeFlags.Union && !(indexType.flags & TypeFlags.Boolean)) {
                const propTypes: Type[] = [];
                let wasMissingProp = false;
                for (const t of (<UnionType>indexType).types) {
                    const propType = getPropertyTypeForIndexType(apparentObjectType, t, accessNode, /*cacheSymbol*/ false, missingType);
                    if (propType === missingType) {
                        if (!accessNode) {
                            // If there's no error node, we can immeditely stop, since error reporting is off
                            return missingType;
                        }
                        else {
                            // Otherwise we set a flag and return at the end of the loop so we still mark all errors
                            wasMissingProp = true;
                        }
                    }
                    propTypes.push(propType);
                }
                if (wasMissingProp) {
                    return missingType;
                }
                return getUnionType(propTypes);
            }
            return getPropertyTypeForIndexType(apparentObjectType, indexType, accessNode, /*cacheSymbol*/ true, missingType);
        }

        function getTypeFromIndexedAccessTypeNode(node: IndexedAccessTypeNode) {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const objectType = getTypeFromTypeNode(node.objectType);
                const indexType = getTypeFromTypeNode(node.indexType);
                const resolved = getIndexedAccessType(objectType, indexType, node);
                links.resolvedType = resolved.flags & TypeFlags.IndexedAccess &&
                    (<IndexedAccessType>resolved).objectType === objectType &&
                    (<IndexedAccessType>resolved).indexType === indexType ?
                    getConstrainedTypeVariable(<IndexedAccessType>resolved, node) : resolved;
            }
            return links.resolvedType;
        }

        function getTypeFromMappedTypeNode(node: MappedTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const type = <MappedType>createObjectType(ObjectFlags.Mapped, node.symbol);
                type.declaration = node;
                type.aliasSymbol = getAliasSymbolForTypeNode(node);
                type.aliasTypeArguments = getTypeArgumentsForAliasSymbol(type.aliasSymbol);
                links.resolvedType = type;
                // Eagerly resolve the constraint type which forces an error if the constraint type circularly
                // references itself through one or more type aliases.
                getConstraintTypeFromMappedType(type);
            }
            return links.resolvedType;
        }

        function getActualTypeVariable(type: Type) {
            return type.flags & TypeFlags.Substitution ? (<SubstitutionType>type).typeVariable : type;
        }

        function getConditionalType(root: ConditionalRoot, mapper: TypeMapper | undefined): Type {
            const checkType = instantiateType(root.checkType, mapper);
            const extendsType = instantiateType(root.extendsType, mapper);
            if (checkType === wildcardType || extendsType === wildcardType) {
                return wildcardType;
            }
            // If this is a distributive conditional type and the check type is generic we need to defer
            // resolution of the conditional type such that a later instantiation will properly distribute
            // over union types.
            const isDeferred = root.isDistributive && maybeTypeOfKind(checkType, TypeFlags.Instantiable);
            let combinedMapper: TypeMapper | undefined;
            if (root.inferTypeParameters) {
                const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
                if (!isDeferred) {
                    // We don't want inferences from constraints as they may cause us to eagerly resolve the
                    // conditional type instead of deferring resolution. Also, we always want strict function
                    // types rules (i.e. proper contravariance) for inferences.
                    inferTypes(context.inferences, checkType, extendsType, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
                }
                combinedMapper = combineTypeMappers(mapper, context);
            }
            if (!isDeferred) {
                if (extendsType.flags & TypeFlags.AnyOrUnknown) {
                    return instantiateType(root.trueType, mapper);
                }
                // Return union of trueType and falseType for 'any' since it matches anything
                if (checkType.flags & TypeFlags.Any) {
                    return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), instantiateType(root.falseType, mapper)]);
                }
                // Instantiate the extends type including inferences for 'infer T' type parameters
                const inferredExtendsType = combinedMapper ? instantiateType(root.extendsType, combinedMapper) : extendsType;
                // Return falseType for a definitely false extends check. We check an instantations of the two
                // types with type parameters mapped to the wildcard type, the most permissive instantiations
                // possible (the wildcard type is assignable to and from all types). If those are not related,
                // then no instatiations will be and we can just return the false branch type.
                if (!isTypeAssignableTo(getWildcardInstantiation(checkType), getWildcardInstantiation(inferredExtendsType))) {
                    return instantiateType(root.falseType, mapper);
                }
                // Return trueType for a definitely true extends check. The definitely assignable relation excludes
                // type variable constraints from consideration. Without the definitely assignable relation, the type
                //   type Foo<T extends { x: any }> = T extends { x: string } ? string : number
                // would immediately resolve to 'string' instead of being deferred.
                if (checkTypeRelatedTo(checkType, inferredExtendsType, definitelyAssignableRelation, /*errorNode*/ undefined)) {
                    return instantiateType(root.trueType, combinedMapper || mapper);
                }
            }
            // Return a deferred type for a check that is neither definitely true nor definitely false
            const erasedCheckType = getActualTypeVariable(checkType);
            const result = <ConditionalType>createType(TypeFlags.Conditional);
            result.root = root;
            result.checkType = erasedCheckType;
            result.extendsType = extendsType;
            result.mapper = mapper;
            result.combinedMapper = combinedMapper;
            result.aliasSymbol = root.aliasSymbol;
            result.aliasTypeArguments = instantiateTypes(root.aliasTypeArguments, mapper!); // TODO: GH#18217
            return result;
        }

        function getTrueTypeFromConditionalType(type: ConditionalType) {
            return type.resolvedTrueType || (type.resolvedTrueType = instantiateType(type.root.trueType, type.mapper));
        }

        function getFalseTypeFromConditionalType(type: ConditionalType) {
            return type.resolvedFalseType || (type.resolvedFalseType = instantiateType(type.root.falseType, type.mapper));
        }

        function getInferTypeParameters(node: ConditionalTypeNode): TypeParameter[] | undefined {
            let result: TypeParameter[] | undefined;
            if (node.locals) {
                node.locals.forEach(symbol => {
                    if (symbol.flags & SymbolFlags.TypeParameter) {
                        result = append(result, getDeclaredTypeOfSymbol(symbol));
                    }
                });
            }
            return result;
        }

        function isPossiblyReferencedInConditionalType(tp: TypeParameter, node: Node) {
            if (isTypeParameterPossiblyReferenced(tp, node)) {
                return true;
            }
            while (node) {
                if (node.kind === SyntaxKind.ConditionalType) {
                    if (isTypeParameterPossiblyReferenced(tp, (<ConditionalTypeNode>node).extendsType)) {
                        return true;
                    }
                }
                node = node.parent;
            }
            return false;
        }

        function getTypeFromConditionalTypeNode(node: ConditionalTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const checkType = getTypeFromTypeNode(node.checkType);
                const aliasSymbol = getAliasSymbolForTypeNode(node);
                const aliasTypeArguments = getTypeArgumentsForAliasSymbol(aliasSymbol);
                const allOuterTypeParameters = getOuterTypeParameters(node, /*includeThisTypes*/ true);
                const outerTypeParameters = aliasTypeArguments ? allOuterTypeParameters : filter(allOuterTypeParameters, tp => isPossiblyReferencedInConditionalType(tp, node));
                const root: ConditionalRoot = {
                    node,
                    checkType,
                    extendsType: getTypeFromTypeNode(node.extendsType),
                    trueType: getTypeFromTypeNode(node.trueType),
                    falseType: getTypeFromTypeNode(node.falseType),
                    isDistributive: !!(checkType.flags & TypeFlags.TypeParameter),
                    inferTypeParameters: getInferTypeParameters(node),
                    outerTypeParameters,
                    instantiations: undefined,
                    aliasSymbol,
                    aliasTypeArguments
                };
                links.resolvedType = getConditionalType(root, /*mapper*/ undefined);
                if (outerTypeParameters) {
                    root.instantiations = createMap<Type>();
                    root.instantiations.set(getTypeListId(outerTypeParameters), links.resolvedType);
                }
            }
            return links.resolvedType;
        }

        function getTypeFromInferTypeNode(node: InferTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = getDeclaredTypeOfTypeParameter(getSymbolOfNode(node.typeParameter));
            }
            return links.resolvedType;
        }

        function getIdentifierChain(node: EntityName): Identifier[] {
            if (isIdentifier(node)) {
                return [node];
            }
            else {
                return append(getIdentifierChain(node.left), node.right);
            }
        }

        function getTypeFromImportTypeNode(node: ImportTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                if (node.isTypeOf && node.typeArguments) { // Only the non-typeof form can make use of type arguments
                    error(node, Diagnostics.Type_arguments_cannot_be_used_here);
                    links.resolvedSymbol = unknownSymbol;
                    return links.resolvedType = errorType;
                }
                if (!isLiteralImportTypeNode(node)) {
                    error(node.argument, Diagnostics.String_literal_expected);
                    links.resolvedSymbol = unknownSymbol;
                    return links.resolvedType = errorType;
                }
                const targetMeaning = node.isTypeOf ? SymbolFlags.Value : node.flags & NodeFlags.JSDoc ? SymbolFlags.Value | SymbolFlags.Type : SymbolFlags.Type;
                // TODO: Future work: support unions/generics/whatever via a deferred import-type
                const innerModuleSymbol = resolveExternalModuleName(node, node.argument.literal);
                if (!innerModuleSymbol) {
                    links.resolvedSymbol = unknownSymbol;
                    return links.resolvedType = errorType;
                }
                const moduleSymbol = resolveExternalModuleSymbol(innerModuleSymbol, /*dontResolveAlias*/ false);
                if (!nodeIsMissing(node.qualifier)) {
                    const nameStack: Identifier[] = getIdentifierChain(node.qualifier!);
                    let currentNamespace = moduleSymbol;
                    let current: Identifier | undefined;
                    while (current = nameStack.shift()) {
                        const meaning = nameStack.length ? SymbolFlags.Namespace : targetMeaning;
                        const next = getSymbol(getExportsOfSymbol(getMergedSymbol(resolveSymbol(currentNamespace))), current.escapedText, meaning);
                        if (!next) {
                            error(current, Diagnostics.Namespace_0_has_no_exported_member_1, getFullyQualifiedName(currentNamespace), declarationNameToString(current));
                            return links.resolvedType = errorType;
                        }
                        getNodeLinks(current).resolvedSymbol = next;
                        getNodeLinks(current.parent).resolvedSymbol = next;
                        currentNamespace = next;
                    }
                    resolveImportSymbolType(node, links, currentNamespace, targetMeaning);
                }
                else {
                    if (moduleSymbol.flags & targetMeaning) {
                        resolveImportSymbolType(node, links, moduleSymbol, targetMeaning);
                    }
                    else {
                        const errorMessage = targetMeaning === SymbolFlags.Value
                            ? Diagnostics.Module_0_does_not_refer_to_a_value_but_is_used_as_a_value_here
                            : Diagnostics.Module_0_does_not_refer_to_a_type_but_is_used_as_a_type_here_Did_you_mean_typeof_import_0;

                        error(node, errorMessage, node.argument.literal.text);

                        links.resolvedSymbol = unknownSymbol;
                        links.resolvedType = errorType;
                    }
                }
            }
            return links.resolvedType!; // TODO: GH#18217
        }

        function resolveImportSymbolType(node: ImportTypeNode, links: NodeLinks, symbol: Symbol, meaning: SymbolFlags) {
            const resolvedSymbol = resolveSymbol(symbol);
            links.resolvedSymbol = resolvedSymbol;
            if (meaning === SymbolFlags.Value) {
                return links.resolvedType = getTypeOfSymbol(symbol); // intentionally doesn't use resolved symbol so type is cached as expected on the alias
            }
            else {
                return links.resolvedType = getTypeReferenceType(node, resolvedSymbol); // getTypeReferenceType doesn't handle aliases - it must get the resolved symbol
            }
        }

        function getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode(node: TypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                // Deferred resolution of members is handled by resolveObjectTypeMembers
                const aliasSymbol = getAliasSymbolForTypeNode(node);
                if (getMembersOfSymbol(node.symbol).size === 0 && !aliasSymbol) {
                    links.resolvedType = emptyTypeLiteralType;
                }
                else {
                    let type = createObjectType(ObjectFlags.Anonymous, node.symbol);
                    type.aliasSymbol = aliasSymbol;
                    type.aliasTypeArguments = getTypeArgumentsForAliasSymbol(aliasSymbol);
                    if (isJSDocTypeLiteral(node) && node.isArrayType) {
                        type = createArrayType(type);
                    }
                    links.resolvedType = type;
                }
            }
            return links.resolvedType;
        }

        function getAliasSymbolForTypeNode(node: TypeNode) {
            return isTypeAlias(node.parent) ? getSymbolOfNode(node.parent) : undefined;
        }

        function getTypeArgumentsForAliasSymbol(symbol: Symbol | undefined) {
            return symbol ? getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol) : undefined;
        }

        /**
         * Since the source of spread types are object literals, which are not binary,
         * this function should be called in a left folding style, with left = previous result of getSpreadType
         * and right = the new element to be spread.
         */
        function getSpreadType(left: Type, right: Type, symbol: Symbol | undefined, typeFlags: TypeFlags, objectFlags: ObjectFlags): Type {
            if (left.flags & TypeFlags.Any || right.flags & TypeFlags.Any) {
                return anyType;
            }
            if (left.flags & TypeFlags.Unknown || right.flags & TypeFlags.Unknown) {
                return unknownType;
            }
            if (left.flags & TypeFlags.Never) {
                return right;
            }
            if (right.flags & TypeFlags.Never) {
                return left;
            }
            if (left.flags & TypeFlags.Union) {
                return mapType(left, t => getSpreadType(t, right, symbol, typeFlags, objectFlags));
            }
            if (right.flags & TypeFlags.Union) {
                return mapType(right, t => getSpreadType(left, t, symbol, typeFlags, objectFlags));
            }
            if (right.flags & (TypeFlags.BooleanLike | TypeFlags.NumberLike | TypeFlags.StringLike | TypeFlags.EnumLike | TypeFlags.NonPrimitive | TypeFlags.Index)) {
                return left;
            }

            const members = createSymbolTable();
            const skippedPrivateMembers = createUnderscoreEscapedMap<boolean>();
            let stringIndexInfo: IndexInfo | undefined;
            let numberIndexInfo: IndexInfo | undefined;
            if (left === emptyObjectType) {
                // for the first spread element, left === emptyObjectType, so take the right's string indexer
                stringIndexInfo = getIndexInfoOfType(right, IndexKind.String);
                numberIndexInfo = getIndexInfoOfType(right, IndexKind.Number);
            }
            else {
                stringIndexInfo = unionSpreadIndexInfos(getIndexInfoOfType(left, IndexKind.String), getIndexInfoOfType(right, IndexKind.String));
                numberIndexInfo = unionSpreadIndexInfos(getIndexInfoOfType(left, IndexKind.Number), getIndexInfoOfType(right, IndexKind.Number));
            }

            for (const rightProp of getPropertiesOfType(right)) {
                if (getDeclarationModifierFlagsFromSymbol(rightProp) & (ModifierFlags.Private | ModifierFlags.Protected)) {
                    skippedPrivateMembers.set(rightProp.escapedName, true);
                }
                else if (isSpreadableProperty(rightProp)) {
                    members.set(rightProp.escapedName, getNonReadonlySymbol(rightProp));
                }
            }

            for (const leftProp of getPropertiesOfType(left)) {
                if (skippedPrivateMembers.has(leftProp.escapedName) || !isSpreadableProperty(leftProp)) {
                    continue;
                }
                if (members.has(leftProp.escapedName)) {
                    const rightProp = members.get(leftProp.escapedName)!;
                    const rightType = getTypeOfSymbol(rightProp);
                    if (rightProp.flags & SymbolFlags.Optional) {
                        const declarations = concatenate(leftProp.declarations, rightProp.declarations);
                        const flags = SymbolFlags.Property | (leftProp.flags & SymbolFlags.Optional);
                        const result = createSymbol(flags, leftProp.escapedName);
                        result.type = getUnionType([getTypeOfSymbol(leftProp), getTypeWithFacts(rightType, TypeFacts.NEUndefined)]);
                        result.leftSpread = leftProp;
                        result.rightSpread = rightProp;
                        result.declarations = declarations;
                        result.nameType = leftProp.nameType;
                        members.set(leftProp.escapedName, result);
                    }
                }
                else {
                    members.set(leftProp.escapedName, getNonReadonlySymbol(leftProp));
                }
            }

            const spread = createAnonymousType(
                symbol,
                members,
                emptyArray,
                emptyArray,
                getNonReadonlyIndexSignature(stringIndexInfo),
                getNonReadonlyIndexSignature(numberIndexInfo));
            spread.flags |= typeFlags | TypeFlags.ContainsObjectLiteral;
            (spread as ObjectType).objectFlags |= objectFlags | (ObjectFlags.ObjectLiteral | ObjectFlags.ContainsSpread);
            return spread;
        }

        /** We approximate own properties as non-methods plus methods that are inside the object literal */
        function isSpreadableProperty(prop: Symbol): boolean {
            return prop.flags & (SymbolFlags.Method | SymbolFlags.GetAccessor)
                ? !prop.declarations.some(decl => isClassLike(decl.parent))
                : !(prop.flags & SymbolFlags.SetAccessor); // Setter without getter is not spreadable
        }

        function getNonReadonlySymbol(prop: Symbol) {
            if (!isReadonlySymbol(prop)) {
                return prop;
            }
            const flags = SymbolFlags.Property | (prop.flags & SymbolFlags.Optional);
            const result = createSymbol(flags, prop.escapedName);
            result.type = getTypeOfSymbol(prop);
            result.declarations = prop.declarations;
            result.nameType = prop.nameType;
            result.syntheticOrigin = prop;
            return result;
        }

        function getNonReadonlyIndexSignature(index: IndexInfo | undefined) {
            if (index && index.isReadonly) {
                return createIndexInfo(index.type, /*isReadonly*/ false, index.declaration);
            }
            return index;
        }

        function createLiteralType(flags: TypeFlags, value: string | number, symbol: Symbol | undefined) {
            const type = <LiteralType>createType(flags);
            type.symbol = symbol!;
            type.value = value;
            return type;
        }

        function getFreshTypeOfLiteralType(type: Type): Type {
            if (type.flags & TypeFlags.StringOrNumberLiteral && !(type.flags & TypeFlags.FreshLiteral)) {
                if (!(<LiteralType>type).freshType) {
                    const freshType = createLiteralType(type.flags | TypeFlags.FreshLiteral, (<LiteralType>type).value, (<LiteralType>type).symbol);
                    freshType.regularType = <LiteralType>type;
                    (<LiteralType>type).freshType = freshType;
                }
                return (<LiteralType>type).freshType;
            }
            return type;
        }

        function getRegularTypeOfLiteralType(type: Type): Type {
            return type.flags & TypeFlags.StringOrNumberLiteral && type.flags & TypeFlags.FreshLiteral ? (<LiteralType>type).regularType :
                type.flags & TypeFlags.Union ? getUnionType(sameMap((<UnionType>type).types, getRegularTypeOfLiteralType)) :
                type;
        }

        function getLiteralType(value: string | number, enumId?: number, symbol?: Symbol) {
            // We store all literal types in a single map with keys of the form '#NNN' and '@SSS',
            // where NNN is the text representation of a numeric literal and SSS are the characters
            // of a string literal. For literal enum members we use 'EEE#NNN' and 'EEE@SSS', where
            // EEE is a unique id for the containing enum type.
            const qualifier = typeof value === "number" ? "#" : "@";
            const key = enumId ? enumId + qualifier + value : qualifier + value;
            let type = literalTypes.get(key);
            if (!type) {
                const flags = (typeof value === "number" ? TypeFlags.NumberLiteral : TypeFlags.StringLiteral) | (enumId ? TypeFlags.EnumLiteral : 0);
                literalTypes.set(key, type = createLiteralType(flags, value, symbol));
            }
            return type;
        }

        function getTypeFromLiteralTypeNode(node: LiteralTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = getRegularTypeOfLiteralType(checkExpression(node.literal));
            }
            return links.resolvedType;
        }

        function createUniqueESSymbolType(symbol: Symbol) {
            const type = <UniqueESSymbolType>createType(TypeFlags.UniqueESSymbol);
            type.symbol = symbol;
            return type;
        }

        function getESSymbolLikeTypeForNode(node: Node) {
            if (isValidESSymbolDeclaration(node)) {
                const symbol = getSymbolOfNode(node);
                const links = getSymbolLinks(symbol);
                return links.uniqueESSymbolType || (links.uniqueESSymbolType = createUniqueESSymbolType(symbol));
            }
            return esSymbolType;
        }

        function getThisType(node: Node): Type {
            const container = getThisContainer(node, /*includeArrowFunctions*/ false);
            const parent = container && container.parent;
            if (parent && (isClassLike(parent) || parent.kind === SyntaxKind.InterfaceDeclaration)) {
                if (!hasModifier(container, ModifierFlags.Static) &&
                    (container.kind !== SyntaxKind.Constructor || isNodeDescendantOf(node, (<ConstructorDeclaration>container).body!))) {
                    return getDeclaredTypeOfClassOrInterface(getSymbolOfNode(parent as ClassLikeDeclaration | InterfaceDeclaration)).thisType!;
                }
            }
            error(node, Diagnostics.A_this_type_is_available_only_in_a_non_static_member_of_a_class_or_interface);
            return errorType;
        }

        function getTypeFromThisTypeNode(node: ThisExpression | ThisTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = getThisType(node);
            }
            return links.resolvedType;
        }

        function getTypeFromTypeNode(node: TypeNode): Type {
            switch (node.kind) {
                case SyntaxKind.AnyKeyword:
                case SyntaxKind.JSDocAllType:
                case SyntaxKind.JSDocUnknownType:
                    return anyType;
                case SyntaxKind.UnknownKeyword:
                    return unknownType;
                case SyntaxKind.StringKeyword:
                    return stringType;
                case SyntaxKind.NumberKeyword:
                    return numberType;
                case SyntaxKind.BooleanKeyword:
                    return booleanType;
                case SyntaxKind.SymbolKeyword:
                    return esSymbolType;
                case SyntaxKind.VoidKeyword:
                    return voidType;
                case SyntaxKind.UndefinedKeyword:
                    return undefinedType;
                case SyntaxKind.NullKeyword:
                    return nullType;
                case SyntaxKind.NeverKeyword:
                    return neverType;
                case SyntaxKind.ObjectKeyword:
                    return node.flags & NodeFlags.JavaScriptFile ? anyType : nonPrimitiveType;
                case SyntaxKind.ThisType:
                case SyntaxKind.ThisKeyword:
                    return getTypeFromThisTypeNode(node as ThisExpression | ThisTypeNode);
                case SyntaxKind.LiteralType:
                    return getTypeFromLiteralTypeNode(<LiteralTypeNode>node);
                case SyntaxKind.TypeReference:
                    return getTypeFromTypeReference(<TypeReferenceNode>node);
                case SyntaxKind.TypePredicate:
                    return booleanType;
                case SyntaxKind.ExpressionWithTypeArguments:
                    return getTypeFromTypeReference(<ExpressionWithTypeArguments>node);
                case SyntaxKind.TypeQuery:
                    return getTypeFromTypeQueryNode(<TypeQueryNode>node);
                case SyntaxKind.ArrayType:
                    return getTypeFromArrayTypeNode(<ArrayTypeNode>node);
                case SyntaxKind.TupleType:
                    return getTypeFromTupleTypeNode(<TupleTypeNode>node);
                case SyntaxKind.OptionalType:
                    return getTypeFromOptionalTypeNode(<OptionalTypeNode>node);
                case SyntaxKind.UnionType:
                    return getTypeFromUnionTypeNode(<UnionTypeNode>node);
                case SyntaxKind.IntersectionType:
                    return getTypeFromIntersectionTypeNode(<IntersectionTypeNode>node);
                case SyntaxKind.JSDocNullableType:
                    return getTypeFromJSDocNullableTypeNode(<JSDocNullableType>node);
                case SyntaxKind.JSDocOptionalType:
                    return addOptionality(getTypeFromTypeNode((node as JSDocOptionalType).type));
                case SyntaxKind.ParenthesizedType:
                case SyntaxKind.RestType:
                case SyntaxKind.JSDocNonNullableType:
                case SyntaxKind.JSDocTypeExpression:
                    return getTypeFromTypeNode((<ParenthesizedTypeNode | RestTypeNode | JSDocTypeReferencingNode | JSDocTypeExpression>node).type);
                case SyntaxKind.JSDocVariadicType:
                    return getTypeFromJSDocVariadicType(node as JSDocVariadicType);
                case SyntaxKind.FunctionType:
                case SyntaxKind.ConstructorType:
                case SyntaxKind.TypeLiteral:
                case SyntaxKind.JSDocTypeLiteral:
                case SyntaxKind.JSDocFunctionType:
                case SyntaxKind.JSDocSignature:
                    return getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode(node);
                case SyntaxKind.TypeOperator:
                    return getTypeFromTypeOperatorNode(<TypeOperatorNode>node);
                case SyntaxKind.IndexedAccessType:
                    return getTypeFromIndexedAccessTypeNode(<IndexedAccessTypeNode>node);
                case SyntaxKind.MappedType:
                    return getTypeFromMappedTypeNode(<MappedTypeNode>node);
                case SyntaxKind.ConditionalType:
                    return getTypeFromConditionalTypeNode(<ConditionalTypeNode>node);
                case SyntaxKind.InferType:
                    return getTypeFromInferTypeNode(<InferTypeNode>node);
                case SyntaxKind.ImportType:
                    return getTypeFromImportTypeNode(<ImportTypeNode>node);
                // This function assumes that an identifier or qualified name is a type expression
                // Callers should first ensure this by calling isTypeNode
                case SyntaxKind.Identifier:
                case SyntaxKind.QualifiedName:
                    const symbol = getSymbolAtLocation(node);
                    return symbol ? getDeclaredTypeOfSymbol(symbol) : errorType;
                default:
                    return errorType;
            }
        }

        function instantiateList<T>(items: ReadonlyArray<T>, mapper: TypeMapper, instantiator: (item: T, mapper: TypeMapper) => T): ReadonlyArray<T>;
        function instantiateList<T>(items: ReadonlyArray<T> | undefined, mapper: TypeMapper, instantiator: (item: T, mapper: TypeMapper) => T): ReadonlyArray<T> | undefined;
        function instantiateList<T>(items: ReadonlyArray<T> | undefined, mapper: TypeMapper, instantiator: (item: T, mapper: TypeMapper) => T): ReadonlyArray<T> | undefined {
            if (items && items.length) {
                for (let i = 0; i < items.length; i++) {
                    const item = items[i];
                    const mapped = instantiator(item, mapper);
                    if (item !== mapped) {
                        const result = i === 0 ? [] : items.slice(0, i);
                        result.push(mapped);
                        for (i++; i < items.length; i++) {
                            result.push(instantiator(items[i], mapper));
                        }
                        return result;
                    }
                }
            }
            return items;
        }

        function instantiateTypes(types: ReadonlyArray<Type>, mapper: TypeMapper): ReadonlyArray<Type>;
        function instantiateTypes(types: ReadonlyArray<Type> | undefined, mapper: TypeMapper): ReadonlyArray<Type> | undefined;
        function instantiateTypes(types: ReadonlyArray<Type> | undefined, mapper: TypeMapper): ReadonlyArray<Type> | undefined {
            return instantiateList<Type>(types, mapper, instantiateType);
        }

        function instantiateSignatures(signatures: ReadonlyArray<Signature>, mapper: TypeMapper): ReadonlyArray<Signature> {
            return instantiateList<Signature>(signatures, mapper, instantiateSignature);
        }

        function makeUnaryTypeMapper(source: Type, target: Type) {
            return (t: Type) => t === source ? target : t;
        }

        function makeBinaryTypeMapper(source1: Type, target1: Type, source2: Type, target2: Type) {
            return (t: Type) => t === source1 ? target1 : t === source2 ? target2 : t;
        }

        function makeArrayTypeMapper(sources: ReadonlyArray<Type>, targets: ReadonlyArray<Type> | undefined) {
            return (t: Type) => {
                for (let i = 0; i < sources.length; i++) {
                    if (t === sources[i]) {
                        return targets ? targets[i] : anyType;
                    }
                }
                return t;
            };
        }

        function createTypeMapper(sources: ReadonlyArray<TypeParameter>, targets: ReadonlyArray<Type> | undefined): TypeMapper {
            Debug.assert(targets === undefined || sources.length === targets.length);
            return sources.length === 1 ? makeUnaryTypeMapper(sources[0], targets ? targets[0] : anyType) :
                sources.length === 2 ? makeBinaryTypeMapper(sources[0], targets ? targets[0] : anyType, sources[1], targets ? targets[1] : anyType) :
                makeArrayTypeMapper(sources, targets);
        }

        function createTypeEraser(sources: ReadonlyArray<TypeParameter>): TypeMapper {
            return createTypeMapper(sources, /*targets*/ undefined);
        }

        /**
         * Maps forward-references to later types parameters to the empty object type.
         * This is used during inference when instantiating type parameter defaults.
         */
        function createBackreferenceMapper(typeParameters: ReadonlyArray<TypeParameter>, index: number): TypeMapper {
            return t => typeParameters.indexOf(t) >= index ? emptyObjectType : t;
        }

        function isInferenceContext(mapper: TypeMapper): mapper is InferenceContext {
            return !!(<InferenceContext>mapper).typeParameters;
        }

        function cloneTypeMapper(mapper: TypeMapper): TypeMapper {
            return mapper && isInferenceContext(mapper) ?
                createInferenceContext(mapper.typeParameters, mapper.signature, mapper.flags | InferenceFlags.NoDefault, mapper.compareTypes, mapper.inferences) :
                mapper;
        }

        function combineTypeMappers(mapper1: TypeMapper | undefined, mapper2: TypeMapper): TypeMapper;
        function combineTypeMappers(mapper1: TypeMapper, mapper2: TypeMapper | undefined): TypeMapper;
        function combineTypeMappers(mapper1: TypeMapper, mapper2: TypeMapper): TypeMapper {
            if (!mapper1) return mapper2;
            if (!mapper2) return mapper1;
            return t => instantiateType(mapper1(t), mapper2);
        }

        function createReplacementMapper(source: Type, target: Type, baseMapper: TypeMapper): TypeMapper {
            return t => t === source ? target : baseMapper(t);
        }

        function wildcardMapper(type: Type) {
            return type.flags & TypeFlags.TypeParameter ? wildcardType : type;
        }

        function cloneTypeParameter(typeParameter: TypeParameter): TypeParameter {
            const result = <TypeParameter>createType(TypeFlags.TypeParameter);
            result.symbol = typeParameter.symbol;
            result.target = typeParameter;
            return result;
        }

        function instantiateTypePredicate(predicate: TypePredicate, mapper: TypeMapper): ThisTypePredicate | IdentifierTypePredicate {
            if (isIdentifierTypePredicate(predicate)) {
                return {
                    kind: TypePredicateKind.Identifier,
                    parameterName: predicate.parameterName,
                    parameterIndex: predicate.parameterIndex,
                    type: instantiateType(predicate.type, mapper)
                };
            }
            else {
                return {
                    kind: TypePredicateKind.This,
                    type: instantiateType(predicate.type, mapper)
                };
            }
        }

        function instantiateSignature(signature: Signature, mapper: TypeMapper, eraseTypeParameters?: boolean): Signature {
            let freshTypeParameters: TypeParameter[] | undefined;
            if (signature.typeParameters && !eraseTypeParameters) {
                // First create a fresh set of type parameters, then include a mapping from the old to the
                // new type parameters in the mapper function. Finally store this mapper in the new type
                // parameters such that we can use it when instantiating constraints.
                freshTypeParameters = map(signature.typeParameters, cloneTypeParameter);
                mapper = combineTypeMappers(createTypeMapper(signature.typeParameters, freshTypeParameters), mapper);
                for (const tp of freshTypeParameters) {
                    tp.mapper = mapper;
                }
            }
            // Don't compute resolvedReturnType and resolvedTypePredicate now,
            // because using `mapper` now could trigger inferences to become fixed. (See `createInferenceContext`.)
            // See GH#17600.
            const result = createSignature(signature.declaration, freshTypeParameters,
                signature.thisParameter && instantiateSymbol(signature.thisParameter, mapper),
                instantiateList(signature.parameters, mapper, instantiateSymbol),
                /*resolvedReturnType*/ undefined,
                /*resolvedTypePredicate*/ undefined,
                signature.minArgumentCount,
                signature.hasRestParameter,
                signature.hasLiteralTypes);
            result.target = signature;
            result.mapper = mapper;
            return result;
        }

        function instantiateSymbol(symbol: Symbol, mapper: TypeMapper): Symbol {
            const links = getSymbolLinks(symbol);
            if (links.type && !maybeTypeOfKind(links.type, TypeFlags.Object | TypeFlags.Instantiable)) {
                // If the type of the symbol is already resolved, and if that type could not possibly
                // be affected by instantiation, simply return the symbol itself.
                return symbol;
            }
            if (getCheckFlags(symbol) & CheckFlags.Instantiated) {
                // If symbol being instantiated is itself a instantiation, fetch the original target and combine the
                // type mappers. This ensures that original type identities are properly preserved and that aliases
                // always reference a non-aliases.
                symbol = links.target!;
                mapper = combineTypeMappers(links.mapper!, mapper);
            }
            // Keep the flags from the symbol we're instantiating.  Mark that is instantiated, and
            // also transient so that we can just store data on it directly.
            const result = createSymbol(symbol.flags, symbol.escapedName, CheckFlags.Instantiated | getCheckFlags(symbol) & (CheckFlags.Late | CheckFlags.OptionalParameter | CheckFlags.RestParameter));
            result.declarations = symbol.declarations;
            result.parent = symbol.parent;
            result.target = symbol;
            result.mapper = mapper;
            if (symbol.valueDeclaration) {
                result.valueDeclaration = symbol.valueDeclaration;
            }
            if (symbol.nameType) {
                result.nameType = symbol.nameType;
            }
            return result;
        }

        function getAnonymousTypeInstantiation(type: AnonymousType, mapper: TypeMapper) {
            const target = type.objectFlags & ObjectFlags.Instantiated ? type.target! : type;
            const { symbol } = target;
            const links = getSymbolLinks(symbol);
            let typeParameters = links.outerTypeParameters;
            if (!typeParameters) {
                // The first time an anonymous type is instantiated we compute and store a list of the type
                // parameters that are in scope (and therefore potentially referenced). For type literals that
                // aren't the right hand side of a generic type alias declaration we optimize by reducing the
                // set of type parameters to those that are possibly referenced in the literal.
                let declaration = symbol.declarations[0];
                if (isInJSFile(declaration)) {
                    const paramTag = findAncestor(declaration, isJSDocParameterTag);
                    if (paramTag) {
                        const paramSymbol = getParameterSymbolFromJSDoc(paramTag);
                        if (paramSymbol) {
                            declaration = paramSymbol.valueDeclaration;
                        }
                    }
                }
                let outerTypeParameters = getOuterTypeParameters(declaration, /*includeThisTypes*/ true);
                if (isJavascriptConstructor(declaration)) {
                    const templateTagParameters = getTypeParametersFromDeclaration(declaration as DeclarationWithTypeParameters);
                    outerTypeParameters = addRange(outerTypeParameters, templateTagParameters);
                }
                typeParameters = outerTypeParameters || emptyArray;
                typeParameters = symbol.flags & SymbolFlags.TypeLiteral && !target.aliasTypeArguments ?
                    filter(typeParameters, tp => isTypeParameterPossiblyReferenced(tp, declaration)) :
                    typeParameters;
                links.outerTypeParameters = typeParameters;
                if (typeParameters.length) {
                    links.instantiations = createMap<Type>();
                    links.instantiations.set(getTypeListId(typeParameters), target);
                }
            }
            if (typeParameters.length) {
                // We are instantiating an anonymous type that has one or more type parameters in scope. Apply the
                // mapper to the type parameters to produce the effective list of type arguments, and compute the
                // instantiation cache key from the type IDs of the type arguments.
                const combinedMapper = type.objectFlags & ObjectFlags.Instantiated ? combineTypeMappers(type.mapper!, mapper) : mapper;
                const typeArguments: Type[] = map(typeParameters, combinedMapper);
                const id = getTypeListId(typeArguments);
                let result = links.instantiations!.get(id);
                if (!result) {
                    const newMapper = createTypeMapper(typeParameters, typeArguments);
                    result = target.objectFlags & ObjectFlags.Mapped ? instantiateMappedType(<MappedType>target, newMapper) : instantiateAnonymousType(target, newMapper);
                    links.instantiations!.set(id, result);
                }
                return result;
            }
            return type;
        }

        function maybeTypeParameterReference(node: Node) {
            return !(node.kind === SyntaxKind.QualifiedName ||
                node.parent.kind === SyntaxKind.TypeReference && (<TypeReferenceNode>node.parent).typeArguments && node === (<TypeReferenceNode>node.parent).typeName);
        }

        function isTypeParameterPossiblyReferenced(tp: TypeParameter, node: Node) {
            // If the type parameter doesn't have exactly one declaration, if there are invening statement blocks
            // between the node and the type parameter declaration, if the node contains actual references to the
            // type parameter, or if the node contains type queries, we consider the type parameter possibly referenced.
            if (tp.symbol && tp.symbol.declarations && tp.symbol.declarations.length === 1) {
                const container = tp.symbol.declarations[0].parent;
                if (findAncestor(node, n => n.kind === SyntaxKind.Block ? "quit" : n === container)) {
                    return !!forEachChild(node, containsReference);
                }
            }
            return true;
            function containsReference(node: Node): boolean {
                switch (node.kind) {
                    case SyntaxKind.ThisType:
                        return !!tp.isThisType;
                    case SyntaxKind.Identifier:
                        return !tp.isThisType && isPartOfTypeNode(node) && maybeTypeParameterReference(node) &&
                            getTypeFromTypeNode(<TypeNode>node) === tp;
                    case SyntaxKind.TypeQuery:
                        return true;
                }
                return !!forEachChild(node, containsReference);
            }
        }

        function getHomomorphicTypeVariable(type: MappedType) {
            const constraintType = getConstraintTypeFromMappedType(type);
            if (constraintType.flags & TypeFlags.Index) {
                const typeVariable = (<IndexType>constraintType).type;
                if (typeVariable.flags & TypeFlags.TypeParameter) {
                    return <TypeParameter>typeVariable;
                }
            }
            return undefined;
        }

        function instantiateMappedType(type: MappedType, mapper: TypeMapper): Type {
            // For a homomorphic mapped type { [P in keyof T]: X }, where T is some type variable, the mapping
            // operation depends on T as follows:
            // * If T is a primitive type no mapping is performed and the result is simply T.
            // * If T is a union type we distribute the mapped type over the union.
            // * If T is an array we map to an array where the element type has been transformed.
            // * If T is a tuple we map to a tuple where the element types have been transformed.
            // * Otherwise we map to an object type where the type of each property has been transformed.
            // For example, when T is instantiated to a union type A | B, we produce { [P in keyof A]: X } |
            // { [P in keyof B]: X }, and when when T is instantiated to a union type A | undefined, we produce
            // { [P in keyof A]: X } | undefined.
            const typeVariable = getHomomorphicTypeVariable(type);
            if (typeVariable) {
                const mappedTypeVariable = instantiateType(typeVariable, mapper);
                if (typeVariable !== mappedTypeVariable) {
                    return mapType(mappedTypeVariable, t => {
                        if (t.flags & (TypeFlags.AnyOrUnknown | TypeFlags.InstantiableNonPrimitive | TypeFlags.Object | TypeFlags.Intersection) && t !== wildcardType) {
                            const replacementMapper = createReplacementMapper(typeVariable, t, mapper);
                            return isArrayType(t) ? createArrayType(instantiateMappedTypeTemplate(type, numberType, /*isOptional*/ true, replacementMapper)) :
                                isReadonlyArrayType(t) ? createReadonlyArrayType(instantiateMappedTypeTemplate(type, numberType, /*isOptional*/ true, replacementMapper)) :
                                isTupleType(t) ? instantiateMappedTupleType(t, type, replacementMapper) :
                                instantiateAnonymousType(type, replacementMapper);
                        }
                        return t;
                    });
                }
            }
            return instantiateAnonymousType(type, mapper);
        }

        function instantiateMappedTupleType(tupleType: TupleTypeReference, mappedType: MappedType, mapper: TypeMapper) {
            const minLength = tupleType.target.minLength;
            const elementTypes = map(tupleType.typeArguments || emptyArray, (_, i) =>
                instantiateMappedTypeTemplate(mappedType, getLiteralType("" + i), i >= minLength, mapper));
            const modifiers = getMappedTypeModifiers(mappedType);
            const newMinLength = modifiers & MappedTypeModifiers.IncludeOptional ? 0 :
                modifiers & MappedTypeModifiers.ExcludeOptional ? getTypeReferenceArity(tupleType) - (tupleType.target.hasRestElement ? 1 : 0) :
                minLength;
            return createTupleType(elementTypes, newMinLength, tupleType.target.hasRestElement, tupleType.target.associatedNames);
        }

        function instantiateMappedTypeTemplate(type: MappedType, key: Type, isOptional: boolean, mapper: TypeMapper) {
            const templateMapper = combineTypeMappers(mapper, createTypeMapper([getTypeParameterFromMappedType(type)], [key]));
            const propType = instantiateType(getTemplateTypeFromMappedType(<MappedType>type.target || type), templateMapper);
            const modifiers = getMappedTypeModifiers(type);
            return strictNullChecks && modifiers & MappedTypeModifiers.IncludeOptional && !isTypeAssignableTo(undefinedType, propType) ? getOptionalType(propType) :
                strictNullChecks && modifiers & MappedTypeModifiers.ExcludeOptional && isOptional ? getTypeWithFacts(propType, TypeFacts.NEUndefined) :
                propType;
        }

        function instantiateAnonymousType(type: AnonymousType, mapper: TypeMapper): AnonymousType {
            const result = <AnonymousType>createObjectType(type.objectFlags | ObjectFlags.Instantiated, type.symbol);
            if (type.objectFlags & ObjectFlags.Mapped) {
                (<MappedType>result).declaration = (<MappedType>type).declaration;
            }
            result.target = type;
            result.mapper = mapper;
            result.aliasSymbol = type.aliasSymbol;
            result.aliasTypeArguments = instantiateTypes(type.aliasTypeArguments, mapper);
            return result;
        }

        function getConditionalTypeInstantiation(type: ConditionalType, mapper: TypeMapper): Type {
            const root = type.root;
            if (root.outerTypeParameters) {
                // We are instantiating a conditional type that has one or more type parameters in scope. Apply the
                // mapper to the type parameters to produce the effective list of type arguments, and compute the
                // instantiation cache key from the type IDs of the type arguments.
                const typeArguments = map(root.outerTypeParameters, mapper);
                const id = getTypeListId(typeArguments);
                let result = root.instantiations!.get(id);
                if (!result) {
                    const newMapper = createTypeMapper(root.outerTypeParameters, typeArguments);
                    result = instantiateConditionalType(root, newMapper);
                    root.instantiations!.set(id, result);
                }
                return result;
            }
            return type;
        }

        function instantiateConditionalType(root: ConditionalRoot, mapper: TypeMapper): Type {
            // Check if we have a conditional type where the check type is a naked type parameter. If so,
            // the conditional type is distributive over union types and when T is instantiated to a union
            // type A | B, we produce (A extends U ? X : Y) | (B extends U ? X : Y).
            if (root.isDistributive) {
                const checkType = <TypeParameter>root.checkType;
                const instantiatedType = mapper(checkType);
                if (checkType !== instantiatedType && instantiatedType.flags & (TypeFlags.Union | TypeFlags.Never)) {
                    return mapType(instantiatedType, t => getConditionalType(root, createReplacementMapper(checkType, t, mapper)));
                }
            }
            return getConditionalType(root, mapper);
        }

        function instantiateType(type: Type, mapper: TypeMapper | undefined): Type;
        function instantiateType(type: Type | undefined, mapper: TypeMapper | undefined): Type | undefined;
        function instantiateType(type: Type | undefined, mapper: TypeMapper | undefined): Type | undefined {
            if (!type || !mapper || mapper === identityMapper) {
                return type;
            }
            if (instantiationDepth === 50) {
                // We have reached 50 recursive type instantiations and there is a very high likelyhood we're dealing
                // with a combination of infinite generic types that perpetually generate new type identities. We stop
                // the recursion here by yielding the error type.
                return errorType;
            }
            instantiationDepth++;
            const result = instantiateTypeWorker(type, mapper);
            instantiationDepth--;
            return result;
        }

        function instantiateTypeWorker(type: Type, mapper: TypeMapper): Type {
            const flags = type.flags;
            if (flags & TypeFlags.TypeParameter) {
                return mapper(type);
            }
            if (flags & TypeFlags.Object) {
                const objectFlags = (<ObjectType>type).objectFlags;
                if (objectFlags & ObjectFlags.Anonymous) {
                    // If the anonymous type originates in a declaration of a function, method, class, or
                    // interface, in an object type literal, or in an object literal expression, we may need
                    // to instantiate the type because it might reference a type parameter.
                    return type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.TypeLiteral | SymbolFlags.ObjectLiteral) && type.symbol.declarations ?
                        getAnonymousTypeInstantiation(<AnonymousType>type, mapper) : type;
                }
                if (objectFlags & ObjectFlags.Mapped) {
                    return getAnonymousTypeInstantiation(<AnonymousType>type, mapper);
                }
                if (objectFlags & ObjectFlags.Reference) {
                    const typeArguments = (<TypeReference>type).typeArguments;
                    const newTypeArguments = instantiateTypes(typeArguments, mapper);
                    return newTypeArguments !== typeArguments ? createTypeReference((<TypeReference>type).target, newTypeArguments) : type;
                }
                return type;
            }
            if (flags & TypeFlags.Union && !(flags & TypeFlags.Primitive)) {
                const types = (<UnionType>type).types;
                const newTypes = instantiateTypes(types, mapper);
                return newTypes !== types ? getUnionType(newTypes, UnionReduction.Literal, type.aliasSymbol, instantiateTypes(type.aliasTypeArguments, mapper)) : type;
            }
            if (flags & TypeFlags.Intersection) {
                const types = (<IntersectionType>type).types;
                const newTypes = instantiateTypes(types, mapper);
                return newTypes !== types ? getIntersectionType(newTypes, type.aliasSymbol, instantiateTypes(type.aliasTypeArguments, mapper)) : type;
            }
            if (flags & TypeFlags.Index) {
                return getIndexType(instantiateType((<IndexType>type).type, mapper));
            }
            if (flags & TypeFlags.IndexedAccess) {
                return getIndexedAccessType(instantiateType((<IndexedAccessType>type).objectType, mapper), instantiateType((<IndexedAccessType>type).indexType, mapper));
            }
            if (flags & TypeFlags.Conditional) {
                return getConditionalTypeInstantiation(<ConditionalType>type, combineTypeMappers((<ConditionalType>type).mapper, mapper));
            }
            if (flags & TypeFlags.Substitution) {
                return instantiateType((<SubstitutionType>type).typeVariable, mapper);
            }
            return type;
        }

        function getWildcardInstantiation(type: Type) {
            return type.flags & (TypeFlags.Primitive | TypeFlags.AnyOrUnknown | TypeFlags.Never) ? type :
                type.wildcardInstantiation || (type.wildcardInstantiation = instantiateType(type, wildcardMapper));
        }

        function instantiateIndexInfo(info: IndexInfo | undefined, mapper: TypeMapper): IndexInfo | undefined {
            return info && createIndexInfo(instantiateType(info.type, mapper), info.isReadonly, info.declaration);
        }

        // Returns true if the given expression contains (at any level of nesting) a function or arrow expression
        // that is subject to contextual typing.
        function isContextSensitive(node: Expression | MethodDeclaration | ObjectLiteralElementLike | JsxAttributeLike): boolean {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));
            switch (node.kind) {
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.MethodDeclaration:
                    return isContextSensitiveFunctionLikeDeclaration(<FunctionExpression | ArrowFunction | MethodDeclaration>node);
                case SyntaxKind.ObjectLiteralExpression:
                    return some((<ObjectLiteralExpression>node).properties, isContextSensitive);
                case SyntaxKind.ArrayLiteralExpression:
                    return some((<ArrayLiteralExpression>node).elements, isContextSensitive);
                case SyntaxKind.ConditionalExpression:
                    return isContextSensitive((<ConditionalExpression>node).whenTrue) ||
                        isContextSensitive((<ConditionalExpression>node).whenFalse);
                case SyntaxKind.BinaryExpression:
                    return (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken &&
                        (isContextSensitive((<BinaryExpression>node).left) || isContextSensitive((<BinaryExpression>node).right));
                case SyntaxKind.PropertyAssignment:
                    return isContextSensitive((<PropertyAssignment>node).initializer);
                case SyntaxKind.ParenthesizedExpression:
                    return isContextSensitive((<ParenthesizedExpression>node).expression);
                case SyntaxKind.JsxAttributes:
                    return some((<JsxAttributes>node).properties, isContextSensitive);
                case SyntaxKind.JsxAttribute: {
                    // If there is no initializer, JSX attribute has a boolean value of true which is not context sensitive.
                    const { initializer } = node as JsxAttribute;
                    return !!initializer && isContextSensitive(initializer);
                }
                case SyntaxKind.JsxExpression: {
                    // It is possible to that node.expression is undefined (e.g <div x={} />)
                    const { expression } = node as JsxExpression;
                    return !!expression && isContextSensitive(expression);
                }
            }

            return false;
        }

        function isContextSensitiveFunctionLikeDeclaration(node: FunctionLikeDeclaration): boolean {
            // Functions with type parameters are not context sensitive.
            if (node.typeParameters) {
                return false;
            }
            // Functions with any parameters that lack type annotations are context sensitive.
            if (some(node.parameters, p => !getEffectiveTypeAnnotationNode(p))) {
                return true;
            }
            if (node.kind !== SyntaxKind.ArrowFunction) {
                // If the first parameter is not an explicit 'this' parameter, then the function has
                // an implicit 'this' parameter which is subject to contextual typing.
                const parameter = firstOrUndefined(node.parameters);
                if (!(parameter && parameterIsThisKeyword(parameter))) {
                    return true;
                }
            }
            return hasContextSensitiveReturnExpression(node);
        }

        function hasContextSensitiveReturnExpression(node: FunctionLikeDeclaration) {
            // TODO(anhans): A block should be context-sensitive if it has a context-sensitive return value.
            const body = node.body!;
            return body.kind === SyntaxKind.Block ? false : isContextSensitive(body);
        }

        function isContextSensitiveFunctionOrObjectLiteralMethod(func: Node): func is FunctionExpression | ArrowFunction | MethodDeclaration {
            return (isInJSFile(func) && isFunctionDeclaration(func) || isFunctionExpressionOrArrowFunction(func) || isObjectLiteralMethod(func)) &&
                isContextSensitiveFunctionLikeDeclaration(func);
        }

        function getTypeWithoutSignatures(type: Type): Type {
            if (type.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                if (resolved.constructSignatures.length || resolved.callSignatures.length) {
                    const result = createObjectType(ObjectFlags.Anonymous, type.symbol);
                    result.members = resolved.members;
                    result.properties = resolved.properties;
                    result.callSignatures = emptyArray;
                    result.constructSignatures = emptyArray;
                    return result;
                }
            }
            else if (type.flags & TypeFlags.Intersection) {
                return getIntersectionType(map((<IntersectionType>type).types, getTypeWithoutSignatures));
            }
            return type;
        }

        // TYPE CHECKING

        function isTypeIdenticalTo(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, identityRelation);
        }

        function compareTypesIdentical(source: Type, target: Type): Ternary {
            return isTypeRelatedTo(source, target, identityRelation) ? Ternary.True : Ternary.False;
        }

        function compareTypesAssignable(source: Type, target: Type): Ternary {
            return isTypeRelatedTo(source, target, assignableRelation) ? Ternary.True : Ternary.False;
        }

        function isTypeSubtypeOf(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, subtypeRelation);
        }

        function isTypeAssignableTo(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, assignableRelation);
        }

        // An object type S is considered to be derived from an object type T if
        // S is a union type and every constituent of S is derived from T,
        // T is a union type and S is derived from at least one constituent of T, or
        // S is a type variable with a base constraint that is derived from T,
        // T is one of the global types Object and Function and S is a subtype of T, or
        // T occurs directly or indirectly in an 'extends' clause of S.
        // Note that this check ignores type parameters and only considers the
        // inheritance hierarchy.
        function isTypeDerivedFrom(source: Type, target: Type): boolean {
            return source.flags & TypeFlags.Union ? every((<UnionType>source).types, t => isTypeDerivedFrom(t, target)) :
                target.flags & TypeFlags.Union ? some((<UnionType>target).types, t => isTypeDerivedFrom(source, t)) :
                source.flags & TypeFlags.InstantiableNonPrimitive ? isTypeDerivedFrom(getBaseConstraintOfType(source) || emptyObjectType, target) :
                target === globalObjectType || target === globalFunctionType ? isTypeSubtypeOf(source, target) :
                hasBaseType(source, getTargetType(target));
            }

        /**
         * This is *not* a bi-directional relationship.
         * If one needs to check both directions for comparability, use a second call to this function or 'checkTypeComparableTo'.
         *
         * A type S is comparable to a type T if some (but not necessarily all) of the possible values of S are also possible values of T.
         * It is used to check following cases:
         *   - the types of the left and right sides of equality/inequality operators (`===`, `!==`, `==`, `!=`).
         *   - the types of `case` clause expressions and their respective `switch` expressions.
         *   - the type of an expression in a type assertion with the type being asserted.
         */
        function isTypeComparableTo(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, comparableRelation);
        }

        function areTypesComparable(type1: Type, type2: Type): boolean {
            return isTypeComparableTo(type1, type2) || isTypeComparableTo(type2, type1);
        }

        function checkTypeAssignableTo(source: Type, target: Type, errorNode: Node | undefined, headMessage?: DiagnosticMessage, containingMessageChain?: () => DiagnosticMessageChain | undefined, errorOutputObject?: { error?: Diagnostic }): boolean {
            return checkTypeRelatedTo(source, target, assignableRelation, errorNode, headMessage, containingMessageChain, errorOutputObject);
        }

        /**
         * Like `checkTypeAssignableTo`, but if it would issue an error, instead performs structural comparisons of the types using the given expression node to
         * attempt to issue more specific errors on, for example, specific object literal properties or tuple members.
         */
        function checkTypeAssignableToAndOptionallyElaborate(source: Type, target: Type, errorNode: Node | undefined, expr: Expression | undefined, headMessage?: DiagnosticMessage, containingMessageChain?: () => DiagnosticMessageChain | undefined): boolean {
            return checkTypeRelatedToAndOptionallyElaborate(source, target, assignableRelation, errorNode, expr, headMessage, containingMessageChain);
        }

        function checkTypeRelatedToAndOptionallyElaborate(source: Type, target: Type, relation: Map<RelationComparisonResult>, errorNode: Node | undefined, expr: Expression | undefined, headMessage?: DiagnosticMessage, containingMessageChain?: () => DiagnosticMessageChain | undefined): boolean {
            if (isTypeRelatedTo(source, target, relation)) return true;
            if (!errorNode || !elaborateError(expr, source, target, relation)) {
                return checkTypeRelatedTo(source, target, relation, errorNode, headMessage, containingMessageChain);
            }
            return false;
        }

        function isOrHasGenericConditional(type: Type): boolean {
            return !!(type.flags & TypeFlags.Conditional || (type.flags & TypeFlags.Intersection && some((type as IntersectionType).types, isOrHasGenericConditional)));
        }

        function elaborateError(node: Expression | undefined, source: Type, target: Type, relation: Map<RelationComparisonResult>): boolean {
            if (!node || isOrHasGenericConditional(target)) return false;
            if (!checkTypeRelatedTo(source, target, relation, /*errorNode*/ undefined) && elaborateDidYouMeanToCallOrConstruct(node, source, target, relation)) {
                return true;
            }
            switch (node.kind) {
                case SyntaxKind.JsxExpression:
                case SyntaxKind.ParenthesizedExpression:
                    return elaborateError((node as ParenthesizedExpression | JsxExpression).expression, source, target, relation);
                case SyntaxKind.BinaryExpression:
                    switch ((node as BinaryExpression).operatorToken.kind) {
                        case SyntaxKind.EqualsToken:
                        case SyntaxKind.CommaToken:
                            return elaborateError((node as BinaryExpression).right, source, target, relation);
                    }
                    break;
                case SyntaxKind.ObjectLiteralExpression:
                    return elaborateObjectLiteral(node as ObjectLiteralExpression, source, target, relation);
                case SyntaxKind.ArrayLiteralExpression:
                    return elaborateArrayLiteral(node as ArrayLiteralExpression, source, target, relation);
                case SyntaxKind.JsxAttributes:
                    return elaborateJsxAttributes(node as JsxAttributes, source, target, relation);
            }
            return false;
        }

        function elaborateDidYouMeanToCallOrConstruct(node: Expression, source: Type, target: Type, relation: Map<RelationComparisonResult>): boolean {
            const callSignatures = getSignaturesOfType(source, SignatureKind.Call);
            const constructSignatures = getSignaturesOfType(source, SignatureKind.Construct);
            for (const signatures of [constructSignatures, callSignatures]) {
                if (some(signatures, s => {
                    const returnType = getReturnTypeOfSignature(s);
                    return !(returnType.flags & (TypeFlags.Any | TypeFlags.Never)) && checkTypeRelatedTo(returnType, target, relation, /*errorNode*/ undefined);
                })) {
                    const resultObj: { error?: Diagnostic } = {};
                    checkTypeAssignableTo(source, target, node, /*errorMessage*/ undefined, /*containingChain*/ undefined, resultObj);
                    const diagnostic = resultObj.error!;
                    addRelatedInfo(diagnostic, createDiagnosticForNode(
                        node,
                        signatures === constructSignatures ? Diagnostics.Did_you_mean_to_use_new_with_this_expression : Diagnostics.Did_you_mean_to_call_this_expression
                    ));
                    return true;
                }
            }
            return false;
        }

        type ElaborationIterator = IterableIterator<{ errorNode: Node, innerExpression: Expression | undefined, nameType: Type, errorMessage?: DiagnosticMessage | undefined }>;
        /**
         * For every element returned from the iterator, checks that element to issue an error on a property of that element's type
         * If that element would issue an error, we first attempt to dive into that element's inner expression and issue a more specific error by recuring into `elaborateError`
         * Otherwise, we issue an error on _every_ element which fail the assignability check
         */
        function elaborateElementwise(iterator: ElaborationIterator, source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            // Assignability failure - check each prop individually, and if that fails, fall back on the bad error span
            let reportedError = false;
            for (let status = iterator.next(); !status.done; status = iterator.next()) {
                const { errorNode: prop, innerExpression: next, nameType, errorMessage } = status.value;
                const sourcePropType = getIndexedAccessType(source, nameType, /*accessNode*/ undefined, errorType);
                const targetPropType = getIndexedAccessType(target, nameType, /*accessNode*/ undefined, errorType);
                if (sourcePropType !== errorType && targetPropType !== errorType && !isTypeAssignableTo(sourcePropType, targetPropType)) {
                    const elaborated = next && elaborateError(next, sourcePropType, targetPropType, relation);
                    if (elaborated) {
                        reportedError = true;
                    }
                    else {
                        // Issue error on the prop itself, since the prop couldn't elaborate the error
                        const resultObj: { error?: Diagnostic } = {};
                        // Use the expression type, if available
                        const specificSource = next ? checkExpressionForMutableLocation(next, CheckMode.Normal, sourcePropType) : sourcePropType;
                        const result = checkTypeRelatedTo(specificSource, targetPropType, relation, prop, errorMessage, /*containingChain*/ undefined, resultObj);
                        if (result && specificSource !== sourcePropType) {
                            // If for whatever reason the expression type doesn't yield an error, make sure we still issue an error on the sourcePropType
                            checkTypeRelatedTo(sourcePropType, targetPropType, relation, prop, errorMessage, /*containingChain*/ undefined, resultObj);
                        }
                        if (resultObj.error) {
                            const reportedDiag = resultObj.error;
                            const propertyName = isTypeUsableAsLateBoundName(nameType) ? getLateBoundNameFromType(nameType) : undefined;
                            const targetProp = propertyName !== undefined ? getPropertyOfType(target, propertyName) : undefined;

                            let issuedElaboration = false;
                            if (!targetProp) {
                                const indexInfo = isTypeAssignableToKind(nameType, TypeFlags.NumberLike) && getIndexInfoOfType(target, IndexKind.Number) ||
                                    getIndexInfoOfType(target, IndexKind.String) ||
                                    undefined;
                                if (indexInfo && indexInfo.declaration) {
                                    issuedElaboration = true;
                                    addRelatedInfo(reportedDiag, createDiagnosticForNode(indexInfo.declaration, Diagnostics.The_expected_type_comes_from_this_index_signature));
                                }
                            }

                            if (!issuedElaboration && (targetProp && length(targetProp.declarations) || target.symbol && length(target.symbol.declarations))) {
                                addRelatedInfo(reportedDiag, createDiagnosticForNode(
                                    targetProp && length(targetProp.declarations) ? targetProp.declarations[0] : target.symbol.declarations[0],
                                    Diagnostics.The_expected_type_comes_from_property_0_which_is_declared_here_on_type_1,
                                    propertyName && !(nameType.flags & TypeFlags.UniqueESSymbol) ? unescapeLeadingUnderscores(propertyName) : typeToString(nameType),
                                    typeToString(target)
                                ));
                            }
                        }
                        reportedError = true;
                    }
                }
            }
            return reportedError;
        }

        function *generateJsxAttributes(node: JsxAttributes): ElaborationIterator {
            if (!length(node.properties)) return;
            for (const prop of node.properties) {
                if (isJsxSpreadAttribute(prop)) continue;
                yield { errorNode: prop.name, innerExpression: prop.initializer, nameType: getLiteralType(idText(prop.name)) };
            }
        }

        function elaborateJsxAttributes(node: JsxAttributes, source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            return elaborateElementwise(generateJsxAttributes(node), source, target, relation);
        }

        function *generateLimitedTupleElements(node: ArrayLiteralExpression, target: Type): ElaborationIterator {
            const len = length(node.elements);
            if (!len) return;
            for (let i = 0; i < len; i++) {
                // Skip elements which do not exist in the target - a length error on the tuple overall is likely better than an error on a mismatched index signature
                if (isTupleLikeType(target) && !getPropertyOfType(target, ("" + i) as __String)) continue;
                const elem = node.elements[i];
                if (isOmittedExpression(elem)) continue;
                const nameType = getLiteralType(i);
                yield { errorNode: elem, innerExpression: elem, nameType };
            }
        }

        function elaborateArrayLiteral(node: ArrayLiteralExpression, source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            if (isTupleLikeType(source)) {
                return elaborateElementwise(generateLimitedTupleElements(node, target), source, target, relation);
            }
            return false;
        }

        function *generateObjectLiteralElements(node: ObjectLiteralExpression): ElaborationIterator {
            if (!length(node.properties)) return;
            for (const prop of node.properties) {
                if (isSpreadAssignment(prop)) continue;
                const type = getLiteralTypeFromPropertyName(getSymbolOfNode(prop), TypeFlags.StringOrNumberLiteralOrUnique);
                if (!type || (type.flags & TypeFlags.Never)) {
                    continue;
                }
                switch (prop.kind) {
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.ShorthandPropertyAssignment:
                        yield { errorNode: prop.name, innerExpression: undefined, nameType: type };
                        break;
                    case SyntaxKind.PropertyAssignment:
                        yield { errorNode: prop.name, innerExpression: prop.initializer, nameType: type, errorMessage: isComputedNonLiteralName(prop.name) ? Diagnostics.Type_of_computed_property_s_value_is_0_which_is_not_assignable_to_type_1 : undefined };
                        break;
                    default:
                        Debug.assertNever(prop);
                }
            }
        }

        function elaborateObjectLiteral(node: ObjectLiteralExpression, source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            return elaborateElementwise(generateObjectLiteralElements(node), source, target, relation);
        }

        /**
         * This is *not* a bi-directional relationship.
         * If one needs to check both directions for comparability, use a second call to this function or 'isTypeComparableTo'.
         */
        function checkTypeComparableTo(source: Type, target: Type, errorNode: Node, headMessage?: DiagnosticMessage, containingMessageChain?: () => DiagnosticMessageChain | undefined): boolean {
            return checkTypeRelatedTo(source, target, comparableRelation, errorNode, headMessage, containingMessageChain);
        }

        function isSignatureAssignableTo(source: Signature,
            target: Signature,
            ignoreReturnTypes: boolean): boolean {
            return compareSignaturesRelated(source, target, CallbackCheck.None, ignoreReturnTypes, /*reportErrors*/ false,
                /*errorReporter*/ undefined, compareTypesAssignable) !== Ternary.False;
        }

        type ErrorReporter = (message: DiagnosticMessage, arg0?: string, arg1?: string) => void;

        /**
         * See signatureRelatedTo, compareSignaturesIdentical
         */
        function compareSignaturesRelated(source: Signature,
            target: Signature,
            callbackCheck: CallbackCheck,
            ignoreReturnTypes: boolean,
            reportErrors: boolean,
            errorReporter: ErrorReporter | undefined,
            compareTypes: TypeComparer): Ternary {
            // TODO (drosen): De-duplicate code between related functions.
            if (source === target) {
                return Ternary.True;
            }

            const targetCount = getParameterCount(target);
            if (!hasEffectiveRestParameter(target) && getMinArgumentCount(source) > targetCount) {
                return Ternary.False;
            }

            if (source.typeParameters && source.typeParameters !== target.typeParameters) {
                target = getCanonicalSignature(target);
                source = instantiateSignatureInContextOf(source, target, /*contextualMapper*/ undefined, compareTypes);
            }

            const sourceCount = getParameterCount(source);
            const sourceRestType = getNonArrayRestType(source);
            const targetRestType = getNonArrayRestType(target);
            if (sourceRestType && targetRestType && sourceCount !== targetCount) {
                // We're not able to relate misaligned complex rest parameters
                return Ternary.False;
            }

            const kind = target.declaration ? target.declaration.kind : SyntaxKind.Unknown;
            const strictVariance = !callbackCheck && strictFunctionTypes && kind !== SyntaxKind.MethodDeclaration &&
                kind !== SyntaxKind.MethodSignature && kind !== SyntaxKind.Constructor;
            let result = Ternary.True;

            const sourceThisType = getThisTypeOfSignature(source);
            if (sourceThisType && sourceThisType !== voidType) {
                const targetThisType = getThisTypeOfSignature(target);
                if (targetThisType) {
                    // void sources are assignable to anything.
                    const related = !strictVariance && compareTypes(sourceThisType, targetThisType, /*reportErrors*/ false)
                        || compareTypes(targetThisType, sourceThisType, reportErrors);
                    if (!related) {
                        if (reportErrors) {
                            errorReporter!(Diagnostics.The_this_types_of_each_signature_are_incompatible);
                        }
                        return Ternary.False;
                    }
                    result &= related;
                }
            }

            const paramCount = sourceRestType || targetRestType ? Math.min(sourceCount, targetCount) : Math.max(sourceCount, targetCount);
            const restIndex = sourceRestType || targetRestType ? paramCount - 1 : -1;

            for (let i = 0; i < paramCount; i++) {
                const sourceType = i === restIndex ? getRestTypeAtPosition(source, i) : getTypeAtPosition(source, i);
                const targetType = i === restIndex ? getRestTypeAtPosition(target, i) : getTypeAtPosition(target, i);
                // In order to ensure that any generic type Foo<T> is at least co-variant with respect to T no matter
                // how Foo uses T, we need to relate parameters bi-variantly (given that parameters are input positions,
                // they naturally relate only contra-variantly). However, if the source and target parameters both have
                // function types with a single call signature, we know we are relating two callback parameters. In
                // that case it is sufficient to only relate the parameters of the signatures co-variantly because,
                // similar to return values, callback parameters are output positions. This means that a Promise<T>,
                // where T is used only in callback parameter positions, will be co-variant (as opposed to bi-variant)
                // with respect to T.
                const sourceSig = callbackCheck ? undefined : getSingleCallSignature(getNonNullableType(sourceType));
                const targetSig = callbackCheck ? undefined : getSingleCallSignature(getNonNullableType(targetType));
                const callbacks = sourceSig && targetSig && !signatureHasTypePredicate(sourceSig) && !signatureHasTypePredicate(targetSig) &&
                    (getFalsyFlags(sourceType) & TypeFlags.Nullable) === (getFalsyFlags(targetType) & TypeFlags.Nullable);
                const related = callbacks ?
                    // TODO: GH#18217 It will work if they're both `undefined`, but not if only one is
                    compareSignaturesRelated(targetSig!, sourceSig!, strictVariance ? CallbackCheck.Strict : CallbackCheck.Bivariant, /*ignoreReturnTypes*/ false, reportErrors, errorReporter, compareTypes) :
                    !callbackCheck && !strictVariance && compareTypes(sourceType, targetType, /*reportErrors*/ false) || compareTypes(targetType, sourceType, reportErrors);
                if (!related) {
                    if (reportErrors) {
                        errorReporter!(Diagnostics.Types_of_parameters_0_and_1_are_incompatible,
                            unescapeLeadingUnderscores(getParameterNameAtPosition(source, i)),
                            unescapeLeadingUnderscores(getParameterNameAtPosition(target, i)));
                    }
                    return Ternary.False;
                }
                result &= related;
            }

            if (!ignoreReturnTypes) {
                const targetReturnType = (target.declaration && isJavascriptConstructor(target.declaration)) ?
                    getJavascriptClassType(target.declaration.symbol)! : getReturnTypeOfSignature(target);
                if (targetReturnType === voidType) {
                    return result;
                }
                const sourceReturnType = (source.declaration && isJavascriptConstructor(source.declaration)) ?
                    getJavascriptClassType(source.declaration.symbol)! : getReturnTypeOfSignature(source);

                // The following block preserves behavior forbidding boolean returning functions from being assignable to type guard returning functions
                const targetTypePredicate = getTypePredicateOfSignature(target);
                if (targetTypePredicate) {
                    const sourceTypePredicate = getTypePredicateOfSignature(source);
                    if (sourceTypePredicate) {
                        result &= compareTypePredicateRelatedTo(sourceTypePredicate, targetTypePredicate, source.declaration!, target.declaration!, reportErrors, errorReporter, compareTypes); // TODO: GH#18217
                    }
                    else if (isIdentifierTypePredicate(targetTypePredicate)) {
                        if (reportErrors) {
                            errorReporter!(Diagnostics.Signature_0_must_be_a_type_predicate, signatureToString(source));
                        }
                        return Ternary.False;
                    }
                }
                else {
                    // When relating callback signatures, we still need to relate return types bi-variantly as otherwise
                    // the containing type wouldn't be co-variant. For example, interface Foo<T> { add(cb: () => T): void }
                    // wouldn't be co-variant for T without this rule.
                    result &= callbackCheck === CallbackCheck.Bivariant && compareTypes(targetReturnType, sourceReturnType, /*reportErrors*/ false) ||
                        compareTypes(sourceReturnType, targetReturnType, reportErrors);
                }

            }

            return result;
        }

        function compareTypePredicateRelatedTo(
            source: TypePredicate,
            target: TypePredicate,
            sourceDeclaration: SignatureDeclaration | JSDocSignature,
            targetDeclaration: SignatureDeclaration | JSDocSignature,
            reportErrors: boolean,
            errorReporter: ErrorReporter | undefined,
            compareTypes: (s: Type, t: Type, reportErrors?: boolean) => Ternary): Ternary {
            if (source.kind !== target.kind) {
                if (reportErrors) {
                    errorReporter!(Diagnostics.A_this_based_type_guard_is_not_compatible_with_a_parameter_based_type_guard);
                    errorReporter!(Diagnostics.Type_predicate_0_is_not_assignable_to_1, typePredicateToString(source), typePredicateToString(target));
                }
                return Ternary.False;
            }

            if (source.kind === TypePredicateKind.Identifier) {
                const targetPredicate = target as IdentifierTypePredicate;
                const sourceIndex = source.parameterIndex - (getThisParameter(sourceDeclaration) ? 1 : 0);
                const targetIndex = targetPredicate.parameterIndex - (getThisParameter(targetDeclaration) ? 1 : 0);
                if (sourceIndex !== targetIndex) {
                    if (reportErrors) {
                        errorReporter!(Diagnostics.Parameter_0_is_not_in_the_same_position_as_parameter_1, source.parameterName, targetPredicate.parameterName);
                        errorReporter!(Diagnostics.Type_predicate_0_is_not_assignable_to_1, typePredicateToString(source), typePredicateToString(target));
                    }
                    return Ternary.False;
                }
            }

            const related = compareTypes(source.type, target.type, reportErrors);
            if (related === Ternary.False && reportErrors) {
                errorReporter!(Diagnostics.Type_predicate_0_is_not_assignable_to_1, typePredicateToString(source), typePredicateToString(target));
            }
            return related;
        }

        function isImplementationCompatibleWithOverload(implementation: Signature, overload: Signature): boolean {
            const erasedSource = getErasedSignature(implementation);
            const erasedTarget = getErasedSignature(overload);

            // First see if the return types are compatible in either direction.
            const sourceReturnType = getReturnTypeOfSignature(erasedSource);
            const targetReturnType = getReturnTypeOfSignature(erasedTarget);
            if (targetReturnType === voidType
                || isTypeRelatedTo(targetReturnType, sourceReturnType, assignableRelation)
                || isTypeRelatedTo(sourceReturnType, targetReturnType, assignableRelation)) {

                return isSignatureAssignableTo(erasedSource, erasedTarget, /*ignoreReturnTypes*/ true);
            }

            return false;
        }

        function isEmptyResolvedType(t: ResolvedType) {
            return t.properties.length === 0 &&
                t.callSignatures.length === 0 &&
                t.constructSignatures.length === 0 &&
                !t.stringIndexInfo &&
                !t.numberIndexInfo;
        }

        function isEmptyObjectType(type: Type): boolean {
            return type.flags & TypeFlags.Object ? isEmptyResolvedType(resolveStructuredTypeMembers(<ObjectType>type)) :
                type.flags & TypeFlags.NonPrimitive ? true :
                type.flags & TypeFlags.Union ? some((<UnionType>type).types, isEmptyObjectType) :
                type.flags & TypeFlags.Intersection ? every((<UnionType>type).types, isEmptyObjectType) :
                false;
        }

        function isEnumTypeRelatedTo(sourceSymbol: Symbol, targetSymbol: Symbol, errorReporter?: ErrorReporter) {
            if (sourceSymbol === targetSymbol) {
                return true;
            }
            const id = getSymbolId(sourceSymbol) + "," + getSymbolId(targetSymbol);
            const relation = enumRelation.get(id);
            if (relation !== undefined && !(relation === RelationComparisonResult.Failed && errorReporter)) {
                return relation === RelationComparisonResult.Succeeded;
            }
            if (sourceSymbol.escapedName !== targetSymbol.escapedName || !(sourceSymbol.flags & SymbolFlags.RegularEnum) || !(targetSymbol.flags & SymbolFlags.RegularEnum)) {
                enumRelation.set(id, RelationComparisonResult.FailedAndReported);
                return false;
            }
            const targetEnumType = getTypeOfSymbol(targetSymbol);
            for (const property of getPropertiesOfType(getTypeOfSymbol(sourceSymbol))) {
                if (property.flags & SymbolFlags.EnumMember) {
                    const targetProperty = getPropertyOfType(targetEnumType, property.escapedName);
                    if (!targetProperty || !(targetProperty.flags & SymbolFlags.EnumMember)) {
                        if (errorReporter) {
                            errorReporter(Diagnostics.Property_0_is_missing_in_type_1, symbolName(property),
                                typeToString(getDeclaredTypeOfSymbol(targetSymbol), /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType));
                            enumRelation.set(id, RelationComparisonResult.FailedAndReported);
                        }
                        else {
                            enumRelation.set(id, RelationComparisonResult.Failed);
                        }
                        return false;
                    }
                }
            }
            enumRelation.set(id, RelationComparisonResult.Succeeded);
            return true;
        }

        function isSimpleTypeRelatedTo(source: Type, target: Type, relation: Map<RelationComparisonResult>, errorReporter?: ErrorReporter) {
            const s = source.flags;
            const t = target.flags;
            if (t & TypeFlags.AnyOrUnknown || s & TypeFlags.Never || source === wildcardType) return true;
            if (t & TypeFlags.Never) return false;
            if (s & TypeFlags.StringLike && t & TypeFlags.String) return true;
            if (s & TypeFlags.StringLiteral && s & TypeFlags.EnumLiteral &&
                t & TypeFlags.StringLiteral && !(t & TypeFlags.EnumLiteral) &&
                (<LiteralType>source).value === (<LiteralType>target).value) return true;
            if (s & TypeFlags.NumberLike && t & TypeFlags.Number) return true;
            if (s & TypeFlags.NumberLiteral && s & TypeFlags.EnumLiteral &&
                t & TypeFlags.NumberLiteral && !(t & TypeFlags.EnumLiteral) &&
                (<LiteralType>source).value === (<LiteralType>target).value) return true;
            if (s & TypeFlags.BooleanLike && t & TypeFlags.Boolean) return true;
            if (s & TypeFlags.ESSymbolLike && t & TypeFlags.ESSymbol) return true;
            if (s & TypeFlags.Enum && t & TypeFlags.Enum && isEnumTypeRelatedTo(source.symbol, target.symbol, errorReporter)) return true;
            if (s & TypeFlags.EnumLiteral && t & TypeFlags.EnumLiteral) {
                if (s & TypeFlags.Union && t & TypeFlags.Union && isEnumTypeRelatedTo(source.symbol, target.symbol, errorReporter)) return true;
                if (s & TypeFlags.Literal && t & TypeFlags.Literal &&
                    (<LiteralType>source).value === (<LiteralType>target).value &&
                    isEnumTypeRelatedTo(getParentOfSymbol(source.symbol)!, getParentOfSymbol(target.symbol)!, errorReporter)) return true;
            }
            if (s & TypeFlags.Undefined && (!strictNullChecks || t & (TypeFlags.Undefined | TypeFlags.Void))) return true;
            if (s & TypeFlags.Null && (!strictNullChecks || t & TypeFlags.Null)) return true;
            if (s & TypeFlags.Object && t & TypeFlags.NonPrimitive) return true;
            if (s & TypeFlags.UniqueESSymbol || t & TypeFlags.UniqueESSymbol) return false;
            if (relation === assignableRelation || relation === definitelyAssignableRelation || relation === comparableRelation) {
                if (s & TypeFlags.Any) return true;
                // Type number or any numeric literal type is assignable to any numeric enum type or any
                // numeric enum literal type. This rule exists for backwards compatibility reasons because
                // bit-flag enum types sometimes look like literal enum types with numeric literal values.
                if (s & (TypeFlags.Number | TypeFlags.NumberLiteral) && !(s & TypeFlags.EnumLiteral) && (
                    t & TypeFlags.Enum || t & TypeFlags.NumberLiteral && t & TypeFlags.EnumLiteral)) return true;
            }
            return false;
        }

        function isTypeRelatedTo(source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            if (source.flags & TypeFlags.StringOrNumberLiteral && source.flags & TypeFlags.FreshLiteral) {
                source = (<LiteralType>source).regularType;
            }
            if (target.flags & TypeFlags.StringOrNumberLiteral && target.flags & TypeFlags.FreshLiteral) {
                target = (<LiteralType>target).regularType;
            }
            if (source === target ||
                relation === comparableRelation && !(target.flags & TypeFlags.Never) && isSimpleTypeRelatedTo(target, source, relation) ||
                relation !== identityRelation && isSimpleTypeRelatedTo(source, target, relation)) {
                return true;
            }
            if (source.flags & TypeFlags.Object && target.flags & TypeFlags.Object) {
                const related = relation.get(getRelationKey(source, target, relation));
                if (related !== undefined) {
                    return related === RelationComparisonResult.Succeeded;
                }
            }
            if (source.flags & TypeFlags.StructuredOrInstantiable || target.flags & TypeFlags.StructuredOrInstantiable) {
                return checkTypeRelatedTo(source, target, relation, /*errorNode*/ undefined);
            }
            return false;
        }

        function isIgnoredJsxProperty(source: Type, sourceProp: Symbol, targetMemberType: Type | undefined) {
            return getObjectFlags(source) & ObjectFlags.JsxAttributes && !(isUnhyphenatedJsxName(sourceProp.escapedName) || targetMemberType);
        }

        /**
         * Checks if 'source' is related to 'target' (e.g.: is a assignable to).
         * @param source The left-hand-side of the relation.
         * @param target The right-hand-side of the relation.
         * @param relation The relation considered. One of 'identityRelation', 'subtypeRelation', 'assignableRelation', or 'comparableRelation'.
         * Used as both to determine which checks are performed and as a cache of previously computed results.
         * @param errorNode The suggested node upon which all errors will be reported, if defined. This may or may not be the actual node used.
         * @param headMessage If the error chain should be prepended by a head message, then headMessage will be used.
         * @param containingMessageChain A chain of errors to prepend any new errors found.
         */
        function checkTypeRelatedTo(
            source: Type,
            target: Type,
            relation: Map<RelationComparisonResult>,
            errorNode: Node | undefined,
            headMessage?: DiagnosticMessage,
            containingMessageChain?: () => DiagnosticMessageChain | undefined,
            errorOutputContainer?: { error?: Diagnostic }
        ): boolean {

            let errorInfo: DiagnosticMessageChain | undefined;
            let maybeKeys: string[];
            let sourceStack: Type[];
            let targetStack: Type[];
            let maybeCount = 0;
            let depth = 0;
            let expandingFlags = ExpandingFlags.None;
            let overflow = false;
            let isIntersectionConstituent = false;

            Debug.assert(relation !== identityRelation || !errorNode, "no error reporting in identity checking");

            const result = isRelatedTo(source, target, /*reportErrors*/ !!errorNode, headMessage);
            if (overflow) {
                error(errorNode, Diagnostics.Excessive_stack_depth_comparing_types_0_and_1, typeToString(source), typeToString(target));
            }
            else if (errorInfo) {
                if (containingMessageChain) {
                    const chain = containingMessageChain();
                    if (chain) {
                        errorInfo = concatenateDiagnosticMessageChains(chain, errorInfo);
                    }
                }

                let relatedInformation: DiagnosticRelatedInformation[] | undefined;
                // Check if we should issue an extra diagnostic to produce a quickfix for a slightly incorrect import statement
                if (headMessage && errorNode && !result && source.symbol) {
                    const links = getSymbolLinks(source.symbol);
                    if (links.originatingImport && !isImportCall(links.originatingImport)) {
                        const helpfulRetry = checkTypeRelatedTo(getTypeOfSymbol(links.target!), target, relation, /*errorNode*/ undefined);
                        if (helpfulRetry) {
                            // Likely an incorrect import. Issue a helpful diagnostic to produce a quickfix to change the import
                            const diag = createDiagnosticForNode(links.originatingImport, Diagnostics.Type_originates_at_this_import_A_namespace_style_import_cannot_be_called_or_constructed_and_will_cause_a_failure_at_runtime_Consider_using_a_default_import_or_import_require_here_instead);
                            relatedInformation = append(relatedInformation, diag); // Cause the error to appear with the error that triggered it
                        }
                    }
                }

                const diag = createDiagnosticForNodeFromMessageChain(errorNode!, errorInfo, relatedInformation);
                if (errorOutputContainer) {
                    errorOutputContainer.error = diag;
                }
                diagnostics.add(diag); // TODO: GH#18217
            }
            return result !== Ternary.False;

            function reportError(message: DiagnosticMessage, arg0?: string, arg1?: string, arg2?: string): void {
                Debug.assert(!!errorNode);
                errorInfo = chainDiagnosticMessages(errorInfo, message, arg0, arg1, arg2);
            }

            function reportRelationError(message: DiagnosticMessage | undefined, source: Type, target: Type) {
                let sourceType = typeToString(source);
                let targetType = typeToString(target);
                if (sourceType === targetType) {
                    sourceType = typeToString(source, /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType);
                    targetType = typeToString(target, /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType);
                }

                if (!message) {
                    if (relation === comparableRelation) {
                        message = Diagnostics.Type_0_is_not_comparable_to_type_1;
                    }
                    else if (sourceType === targetType) {
                        message = Diagnostics.Type_0_is_not_assignable_to_type_1_Two_different_types_with_this_name_exist_but_they_are_unrelated;
                    }
                    else {
                        message = Diagnostics.Type_0_is_not_assignable_to_type_1;
                    }
                }

                reportError(message, sourceType, targetType);
            }

            function tryElaborateErrorsForPrimitivesAndObjects(source: Type, target: Type) {
                const sourceType = typeToString(source);
                const targetType = typeToString(target);

                if ((globalStringType === source && stringType === target) ||
                    (globalNumberType === source && numberType === target) ||
                    (globalBooleanType === source && booleanType === target) ||
                    (getGlobalESSymbolType(/*reportErrors*/ false) === source && esSymbolType === target)) {
                    reportError(Diagnostics._0_is_a_primitive_but_1_is_a_wrapper_object_Prefer_using_0_when_possible, targetType, sourceType);
                }
            }

            function isUnionOrIntersectionTypeWithoutNullableConstituents(type: Type): boolean {
                if (!(type.flags & TypeFlags.UnionOrIntersection)) {
                    return false;
                }
                // at this point we know that this is union or intersection type possibly with nullable constituents.
                // check if we still will have compound type if we ignore nullable components.
                let seenNonNullable = false;
                for (const t of (<UnionOrIntersectionType>type).types) {
                    if (t.flags & TypeFlags.Nullable) {
                        continue;
                    }
                    if (seenNonNullable) {
                        return true;
                    }
                    seenNonNullable = true;
                }
                return false;
            }

            /**
             * Compare two types and return
             * * Ternary.True if they are related with no assumptions,
             * * Ternary.Maybe if they are related with assumptions of other relationships, or
             * * Ternary.False if they are not related.
             */
            function isRelatedTo(source: Type, target: Type, reportErrors = false, headMessage?: DiagnosticMessage): Ternary {
                if (source.flags & TypeFlags.StringOrNumberLiteral && source.flags & TypeFlags.FreshLiteral) {
                    source = (<LiteralType>source).regularType;
                }
                if (target.flags & TypeFlags.StringOrNumberLiteral && target.flags & TypeFlags.FreshLiteral) {
                    target = (<LiteralType>target).regularType;
                }
                if (source.flags & TypeFlags.Substitution) {
                    source = relation === definitelyAssignableRelation ? (<SubstitutionType>source).typeVariable : (<SubstitutionType>source).substitute;
                }
                if (target.flags & TypeFlags.Substitution) {
                    target = (<SubstitutionType>target).typeVariable;
                }
                if (source.flags & TypeFlags.IndexedAccess) {
                    source = getSimplifiedType(source);
                }
                if (target.flags & TypeFlags.IndexedAccess) {
                    target = getSimplifiedType(target);
                }

                // Try to see if we're relating something like `Foo` -> `Bar | null | undefined`.
                // If so, reporting the `null` and `undefined` in the type is hardly useful.
                // First, see if we're even relating an object type to a union.
                // Then see if the target is stripped down to a single non-union type.
                // Note
                //  * We actually want to remove null and undefined naively here (rather than using getNonNullableType),
                //    since we don't want to end up with a worse error like "`Foo` is not assignable to `NonNullable<T>`"
                //    when dealing with generics.
                //  * We also don't deal with primitive source types, since we already halt elaboration below.
                if (target.flags & TypeFlags.Union && source.flags & TypeFlags.Object &&
                    (target as UnionType).types.length <= 3 && maybeTypeOfKind(target, TypeFlags.Nullable)) {
                    const nullStrippedTarget = extractTypesOfKind(target, ~TypeFlags.Nullable);
                    if (!(nullStrippedTarget.flags & (TypeFlags.Union | TypeFlags.Never))) {
                        target = nullStrippedTarget;
                    }
                }

                // both types are the same - covers 'they are the same primitive type or both are Any' or the same type parameter cases
                if (source === target) return Ternary.True;

                if (relation === identityRelation) {
                    return isIdenticalTo(source, target);
                }

                if (relation === comparableRelation && !(target.flags & TypeFlags.Never) && isSimpleTypeRelatedTo(target, source, relation) ||
                    isSimpleTypeRelatedTo(source, target, relation, reportErrors ? reportError : undefined)) return Ternary.True;

                if (isObjectLiteralType(source) && source.flags & TypeFlags.FreshLiteral) {
                    const discriminantType = target.flags & TypeFlags.Union ? findMatchingDiscriminantType(source, target as UnionType) : undefined;
                    if (hasExcessProperties(<FreshObjectLiteralType>source, target, discriminantType, reportErrors)) {
                        if (reportErrors) {
                            reportRelationError(headMessage, source, target);
                        }
                        return Ternary.False;
                    }
                    // Above we check for excess properties with respect to the entire target type. When union
                    // and intersection types are further deconstructed on the target side, we don't want to
                    // make the check again (as it might fail for a partial target type). Therefore we obtain
                    // the regular source type and proceed with that.
                    if (isUnionOrIntersectionTypeWithoutNullableConstituents(target) && !discriminantType) {
                        source = getRegularTypeOfObjectLiteral(source);
                    }
                }

                if (relation !== comparableRelation && !isIntersectionConstituent &&
                    source.flags & (TypeFlags.Primitive | TypeFlags.Object | TypeFlags.Intersection) && source !== globalObjectType &&
                    target.flags & (TypeFlags.Object | TypeFlags.Intersection) && isWeakType(target) &&
                    (getPropertiesOfType(source).length > 0 || typeHasCallOrConstructSignatures(source)) &&
                    !hasCommonProperties(source, target)) {
                    if (reportErrors) {
                        const calls = getSignaturesOfType(source, SignatureKind.Call);
                        const constructs = getSignaturesOfType(source, SignatureKind.Construct);
                        if (calls.length > 0 && isRelatedTo(getReturnTypeOfSignature(calls[0]), target, /*reportErrors*/ false) ||
                            constructs.length > 0 && isRelatedTo(getReturnTypeOfSignature(constructs[0]), target, /*reportErrors*/ false)) {
                            reportError(Diagnostics.Value_of_type_0_has_no_properties_in_common_with_type_1_Did_you_mean_to_call_it, typeToString(source), typeToString(target));
                        }
                        else {
                            reportError(Diagnostics.Type_0_has_no_properties_in_common_with_type_1, typeToString(source), typeToString(target));
                        }
                    }
                    return Ternary.False;
                }

                let result = Ternary.False;
                const saveErrorInfo = errorInfo;
                const saveIsIntersectionConstituent = isIntersectionConstituent;
                isIntersectionConstituent = false;

                // Note that these checks are specifically ordered to produce correct results. In particular,
                // we need to deconstruct unions before intersections (because unions are always at the top),
                // and we need to handle "each" relations before "some" relations for the same kind of type.
                if (source.flags & TypeFlags.Union) {
                    result = relation === comparableRelation ?
                        someTypeRelatedToType(source as UnionType, target, reportErrors && !(source.flags & TypeFlags.Primitive)) :
                        eachTypeRelatedToType(source as UnionType, target, reportErrors && !(source.flags & TypeFlags.Primitive));
                }
                else {
                    if (target.flags & TypeFlags.Union) {
                        result = typeRelatedToSomeType(source, <UnionType>target, reportErrors && !(source.flags & TypeFlags.Primitive) && !(target.flags & TypeFlags.Primitive));
                    }
                    else if (target.flags & TypeFlags.Intersection) {
                        isIntersectionConstituent = true;
                        result = typeRelatedToEachType(source, target as IntersectionType, reportErrors);
                    }
                    else if (source.flags & TypeFlags.Intersection) {
                        // Check to see if any constituents of the intersection are immediately related to the target.
                        //
                        // Don't report errors though. Checking whether a constituent is related to the source is not actually
                        // useful and leads to some confusing error messages. Instead it is better to let the below checks
                        // take care of this, or to not elaborate at all. For instance,
                        //
                        //    - For an object type (such as 'C = A & B'), users are usually more interested in structural errors.
                        //
                        //    - For a union type (such as '(A | B) = (C & D)'), it's better to hold onto the whole intersection
                        //          than to report that 'D' is not assignable to 'A' or 'B'.
                        //
                        //    - For a primitive type or type parameter (such as 'number = A & B') there is no point in
                        //          breaking the intersection apart.
                        result = someTypeRelatedToType(<IntersectionType>source, target, /*reportErrors*/ false);
                    }
                    if (!result && (source.flags & TypeFlags.StructuredOrInstantiable || target.flags & TypeFlags.StructuredOrInstantiable)) {
                        if (result = recursiveTypeRelatedTo(source, target, reportErrors)) {
                            errorInfo = saveErrorInfo;
                        }
                    }
                }
                if (!result && source.flags & TypeFlags.Intersection) {
                    // The combined constraint of an intersection type is the intersection of the constraints of
                    // the constituents. When an intersection type contains instantiable types with union type
                    // constraints, there are situations where we need to examine the combined constraint. One is
                    // when the target is a union type. Another is when the intersection contains types belonging
                    // to one of the disjoint domains. For example, given type variables T and U, each with the
                    // constraint 'string | number', the combined constraint of 'T & U' is 'string | number' and
                    // we need to check this constraint against a union on the target side. Also, given a type
                    // variable V constrained to 'string | number', 'V & number' has a combined constraint of
                    // 'string & number | number & number' which reduces to just 'number'.
                    const constraint = getUnionConstraintOfIntersection(<IntersectionType>source, !!(target.flags & TypeFlags.Union));
                    if (constraint) {
                        if (result = isRelatedTo(constraint, target, reportErrors)) {
                            errorInfo = saveErrorInfo;
                        }
                    }
                }

                isIntersectionConstituent = saveIsIntersectionConstituent;

                if (!result && reportErrors) {
                    if (source.flags & TypeFlags.Object && target.flags & TypeFlags.Primitive) {
                        tryElaborateErrorsForPrimitivesAndObjects(source, target);
                    }
                    else if (source.symbol && source.flags & TypeFlags.Object && globalObjectType === source) {
                        reportError(Diagnostics.The_Object_type_is_assignable_to_very_few_other_types_Did_you_mean_to_use_the_any_type_instead);
                    }
                    else if (getObjectFlags(source) & ObjectFlags.JsxAttributes && target.flags & TypeFlags.Intersection) {
                        const targetTypes = (target as IntersectionType).types;
                        const intrinsicAttributes = getJsxType(JsxNames.IntrinsicAttributes, errorNode);
                        const intrinsicClassAttributes = getJsxType(JsxNames.IntrinsicClassAttributes, errorNode);
                        if (intrinsicAttributes !== errorType && intrinsicClassAttributes !== errorType &&
                            (contains(targetTypes, intrinsicAttributes) || contains(targetTypes, intrinsicClassAttributes))) {
                            // do not report top error
                            return result;
                        }
                    }
                    reportRelationError(headMessage, source, target);
                }
                return result;
            }

            function isIdenticalTo(source: Type, target: Type): Ternary {
                let result: Ternary;
                const flags = source.flags & target.flags;
                if (flags & TypeFlags.Object || flags & TypeFlags.IndexedAccess || flags & TypeFlags.Conditional || flags & TypeFlags.Index || flags & TypeFlags.Substitution) {
                    return recursiveTypeRelatedTo(source, target, /*reportErrors*/ false);
                }
                if (flags & (TypeFlags.Union | TypeFlags.Intersection)) {
                    if (result = eachTypeRelatedToSomeType(<UnionOrIntersectionType>source, <UnionOrIntersectionType>target)) {
                        if (result &= eachTypeRelatedToSomeType(<UnionOrIntersectionType>target, <UnionOrIntersectionType>source)) {
                            return result;
                        }
                    }
                }
                return Ternary.False;
            }

            function hasExcessProperties(source: FreshObjectLiteralType, target: Type, discriminant: Type | undefined, reportErrors: boolean): boolean {
                if (!noImplicitAny && getObjectFlags(target) & ObjectFlags.JSLiteral) {
                    return false; // Disable excess property checks on JS literals to simulate having an implicit "index signature" - but only outside of noImplicitAny
                }
                if (maybeTypeOfKind(target, TypeFlags.Object) && !(getObjectFlags(target) & ObjectFlags.ObjectLiteralPatternWithComputedProperties)) {
                    const isComparingJsxAttributes = !!(getObjectFlags(source) & ObjectFlags.JsxAttributes);
                    if ((relation === assignableRelation || relation === definitelyAssignableRelation || relation === comparableRelation) &&
                        (isTypeSubsetOf(globalObjectType, target) || (!isComparingJsxAttributes && isEmptyObjectType(target)))) {
                        return false;
                    }
                    if (discriminant) {
                        // check excess properties against discriminant type only, not the entire union
                        return hasExcessProperties(source, discriminant, /*discriminant*/ undefined, reportErrors);
                    }
                    for (const prop of getPropertiesOfObjectType(source)) {
                        if (!isPropertyFromSpread(prop, source.symbol) && !isKnownProperty(target, prop.escapedName, isComparingJsxAttributes)) {
                            if (reportErrors) {
                                // We know *exactly* where things went wrong when comparing the types.
                                // Use this property as the error node as this will be more helpful in
                                // reasoning about what went wrong.
                                if (!errorNode) return Debug.fail();
                                if (isJsxAttributes(errorNode) || isJsxOpeningLikeElement(errorNode)) {
                                    // JsxAttributes has an object-literal flag and undergo same type-assignablity check as normal object-literal.
                                    // However, using an object-literal error message will be very confusing to the users so we give different a message.
                                    reportError(Diagnostics.Property_0_does_not_exist_on_type_1, symbolToString(prop), typeToString(target));
                                }
                                else {
                                    // use the property's value declaration if the property is assigned inside the literal itself
                                    const objectLiteralDeclaration = source.symbol && firstOrUndefined(source.symbol.declarations);
                                    let suggestion;
                                    if (prop.valueDeclaration && findAncestor(prop.valueDeclaration, d => d === objectLiteralDeclaration)) {
                                        const propDeclaration = prop.valueDeclaration as ObjectLiteralElementLike;
                                        Debug.assertNode(propDeclaration, isObjectLiteralElementLike);

                                        errorNode = propDeclaration;

                                        const name = propDeclaration.name!;
                                        if (isIdentifier(name)) {
                                            suggestion = getSuggestionForNonexistentProperty(name, target);
                                        }
                                    }

                                    if (suggestion !== undefined) {
                                        reportError(Diagnostics.Object_literal_may_only_specify_known_properties_but_0_does_not_exist_in_type_1_Did_you_mean_to_write_2,
                                            symbolToString(prop), typeToString(target), suggestion);
                                    }
                                    else {
                                        reportError(Diagnostics.Object_literal_may_only_specify_known_properties_and_0_does_not_exist_in_type_1,
                                            symbolToString(prop), typeToString(target));
                                    }
                                }
                            }
                            return true;
                        }
                    }
                }
                return false;
            }

            function isPropertyFromSpread(prop: Symbol, container: Symbol) {
                return prop.valueDeclaration && container.valueDeclaration && prop.valueDeclaration.parent !== container.valueDeclaration;
            }

            function eachTypeRelatedToSomeType(source: UnionOrIntersectionType, target: UnionOrIntersectionType): Ternary {
                let result = Ternary.True;
                const sourceTypes = source.types;
                for (const sourceType of sourceTypes) {
                    const related = typeRelatedToSomeType(sourceType, target, /*reportErrors*/ false);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function typeRelatedToSomeType(source: Type, target: UnionOrIntersectionType, reportErrors: boolean): Ternary {
                const targetTypes = target.types;
                if (target.flags & TypeFlags.Union && containsType(targetTypes, source)) {
                    return Ternary.True;
                }
                for (const type of targetTypes) {
                    const related = isRelatedTo(source, type, /*reportErrors*/ false);
                    if (related) {
                        return related;
                    }
                }
                if (reportErrors) {
                    const bestMatchingType =
                        findMatchingDiscriminantType(source, target) ||
                        findMatchingTypeReferenceOrTypeAliasReference(source, target) ||
                        findBestTypeForObjectLiteral(source, target) ||
                        findBestTypeForInvokable(source, target);

                    isRelatedTo(source, bestMatchingType || targetTypes[targetTypes.length - 1], /*reportErrors*/ true);
                }
                return Ternary.False;
            }

            function findMatchingTypeReferenceOrTypeAliasReference(source: Type, unionTarget: UnionOrIntersectionType) {
                const sourceObjectFlags = getObjectFlags(source);
                if (sourceObjectFlags & (ObjectFlags.Reference | ObjectFlags.Anonymous) && unionTarget.flags & TypeFlags.Union) {
                    return find(unionTarget.types, target => {
                        if (target.flags & TypeFlags.Object) {
                            const overlapObjFlags = sourceObjectFlags & getObjectFlags(target);
                            if (overlapObjFlags & ObjectFlags.Reference) {
                                return (source as TypeReference).target === (target as TypeReference).target;
                            }
                            if (overlapObjFlags & ObjectFlags.Anonymous) {
                                return !!(source as AnonymousType).aliasSymbol && (source as AnonymousType).aliasSymbol === (target as AnonymousType).aliasSymbol;
                            }
                        }
                        return false;
                    });
                }
            }

            function findBestTypeForObjectLiteral(source: Type, unionTarget: UnionOrIntersectionType) {
                if (getObjectFlags(source) & ObjectFlags.ObjectLiteral && forEachType(unionTarget, isArrayLikeType)) {
                    return find(unionTarget.types, t => !isArrayLikeType(t));
                }
            }

            function findBestTypeForInvokable(source: Type, unionTarget: UnionOrIntersectionType) {
                let signatureKind = SignatureKind.Call;
                const hasSignatures = getSignaturesOfType(source, signatureKind).length > 0 ||
                    (signatureKind = SignatureKind.Construct, getSignaturesOfType(source, signatureKind).length > 0);
                if (hasSignatures) {
                    return find(unionTarget.types, t => getSignaturesOfType(t, signatureKind).length > 0);
                }
            }

            // Keep this up-to-date with the same logic within `getApparentTypeOfContextualType`, since they should behave similarly
            function findMatchingDiscriminantType(source: Type, target: UnionOrIntersectionType) {
                let match: Type | undefined;
                const sourceProperties = getPropertiesOfObjectType(source);
                if (sourceProperties) {
                    const sourcePropertiesFiltered = findDiscriminantProperties(sourceProperties, target);
                    if (sourcePropertiesFiltered) {
                        for (const sourceProperty of sourcePropertiesFiltered) {
                            const sourceType = getTypeOfSymbol(sourceProperty);
                            for (const type of target.types) {
                                const targetType = getTypeOfPropertyOfType(type, sourceProperty.escapedName);
                                if (targetType && isRelatedTo(sourceType, targetType)) {
                                    if (type === match) continue; // Finding multiple fields which discriminate to the same type is fine
                                    if (match) {
                                        return undefined;
                                    }
                                    match = type;
                                }
                            }
                        }
                    }
                }
                return match;
            }

            function typeRelatedToEachType(source: Type, target: IntersectionType, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                const targetTypes = target.types;
                for (const targetType of targetTypes) {
                    const related = isRelatedTo(source, targetType, reportErrors);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function someTypeRelatedToType(source: UnionOrIntersectionType, target: Type, reportErrors: boolean): Ternary {
                const sourceTypes = source.types;
                if (source.flags & TypeFlags.Union && containsType(sourceTypes, target)) {
                    return Ternary.True;
                }
                const len = sourceTypes.length;
                for (let i = 0; i < len; i++) {
                    const related = isRelatedTo(sourceTypes[i], target, reportErrors && i === len - 1);
                    if (related) {
                        return related;
                    }
                }
                return Ternary.False;
            }

            function eachTypeRelatedToType(source: UnionOrIntersectionType, target: Type, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                const sourceTypes = source.types;
                for (const sourceType of sourceTypes) {
                    const related = isRelatedTo(sourceType, target, reportErrors);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function typeArgumentsRelatedTo(source: TypeReference, target: TypeReference, variances: Variance[], reportErrors: boolean): Ternary {
                const sources = source.typeArguments || emptyArray;
                const targets = target.typeArguments || emptyArray;
                if (sources.length !== targets.length && relation === identityRelation) {
                    return Ternary.False;
                }
                const length = sources.length <= targets.length ? sources.length : targets.length;
                let result = Ternary.True;
                for (let i = 0; i < length; i++) {
                    // When variance information isn't available we default to covariance. This happens
                    // in the process of computing variance information for recursive types and when
                    // comparing 'this' type arguments.
                    const variance = i < variances.length ? variances[i] : Variance.Covariant;
                    // We ignore arguments for independent type parameters (because they're never witnessed).
                    if (variance !== Variance.Independent) {
                        const s = sources[i];
                        const t = targets[i];
                        let related = Ternary.True;
                        if (variance === Variance.Covariant) {
                            related = isRelatedTo(s, t, reportErrors);
                        }
                        else if (variance === Variance.Contravariant) {
                            related = isRelatedTo(t, s, reportErrors);
                        }
                        else if (variance === Variance.Bivariant) {
                            // In the bivariant case we first compare contravariantly without reporting
                            // errors. Then, if that doesn't succeed, we compare covariantly with error
                            // reporting. Thus, error elaboration will be based on the the covariant check,
                            // which is generally easier to reason about.
                            related = isRelatedTo(t, s, /*reportErrors*/ false);
                            if (!related) {
                                related = isRelatedTo(s, t, reportErrors);
                            }
                        }
                        else {
                            // In the invariant case we first compare covariantly, and only when that
                            // succeeds do we proceed to compare contravariantly. Thus, error elaboration
                            // will typically be based on the covariant check.
                            related = isRelatedTo(s, t, reportErrors);
                            if (related) {
                                related &= isRelatedTo(t, s, reportErrors);
                            }
                        }
                        if (!related) {
                            return Ternary.False;
                        }
                        result &= related;
                    }
                }
                return result;
            }

            // Determine if possibly recursive types are related. First, check if the result is already available in the global cache.
            // Second, check if we have already started a comparison of the given two types in which case we assume the result to be true.
            // Third, check if both types are part of deeply nested chains of generic type instantiations and if so assume the types are
            // equal and infinitely expanding. Fourth, if we have reached a depth of 100 nested comparisons, assume we have runaway recursion
            // and issue an error. Otherwise, actually compare the structure of the two types.
            function recursiveTypeRelatedTo(source: Type, target: Type, reportErrors: boolean): Ternary {
                if (overflow) {
                    return Ternary.False;
                }
                const id = getRelationKey(source, target, relation);
                const related = relation.get(id);
                if (related !== undefined) {
                    if (reportErrors && related === RelationComparisonResult.Failed) {
                        // We are elaborating errors and the cached result is an unreported failure. The result will be reported
                        // as a failure, and should be updated as a reported failure by the bottom of this function.
                    }
                    else {
                        return related === RelationComparisonResult.Succeeded ? Ternary.True : Ternary.False;
                    }
                }
                if (!maybeKeys) {
                    maybeKeys = [];
                    sourceStack = [];
                    targetStack = [];
                }
                else {
                    for (let i = 0; i < maybeCount; i++) {
                        // If source and target are already being compared, consider them related with assumptions
                        if (id === maybeKeys[i]) {
                            return Ternary.Maybe;
                        }
                    }
                    if (depth === 100) {
                        overflow = true;
                        return Ternary.False;
                    }
                }
                const maybeStart = maybeCount;
                maybeKeys[maybeCount] = id;
                maybeCount++;
                sourceStack[depth] = source;
                targetStack[depth] = target;
                depth++;
                const saveExpandingFlags = expandingFlags;
                if (!(expandingFlags & ExpandingFlags.Source) && isDeeplyNestedType(source, sourceStack, depth)) expandingFlags |= ExpandingFlags.Source;
                if (!(expandingFlags & ExpandingFlags.Target) && isDeeplyNestedType(target, targetStack, depth)) expandingFlags |= ExpandingFlags.Target;
                const result = expandingFlags !== ExpandingFlags.Both ? structuredTypeRelatedTo(source, target, reportErrors) : Ternary.Maybe;
                expandingFlags = saveExpandingFlags;
                depth--;
                if (result) {
                    if (result === Ternary.True || depth === 0) {
                        // If result is definitely true, record all maybe keys as having succeeded
                        for (let i = maybeStart; i < maybeCount; i++) {
                            relation.set(maybeKeys[i], RelationComparisonResult.Succeeded);
                        }
                        maybeCount = maybeStart;
                    }
                }
                else {
                    // A false result goes straight into global cache (when something is false under
                    // assumptions it will also be false without assumptions)
                    relation.set(id, reportErrors ? RelationComparisonResult.FailedAndReported : RelationComparisonResult.Failed);
                    maybeCount = maybeStart;
                }
                return result;
            }

            function getConstraintForRelation(type: Type) {
                return relation === definitelyAssignableRelation ? undefined : getConstraintOfType(type);
            }

            function structuredTypeRelatedTo(source: Type, target: Type, reportErrors: boolean): Ternary {
                const flags = source.flags & target.flags;
                if (relation === identityRelation && !(flags & TypeFlags.Object)) {
                    if (flags & TypeFlags.Index) {
                        return isRelatedTo((<IndexType>source).type, (<IndexType>target).type, /*reportErrors*/ false);
                    }
                    let result = Ternary.False;
                    if (flags & TypeFlags.IndexedAccess) {
                        if (result = isRelatedTo((<IndexedAccessType>source).objectType, (<IndexedAccessType>target).objectType, /*reportErrors*/ false)) {
                            if (result &= isRelatedTo((<IndexedAccessType>source).indexType, (<IndexedAccessType>target).indexType, /*reportErrors*/ false)) {
                                return result;
                            }
                        }
                    }
                    if (flags & TypeFlags.Conditional) {
                        if ((<ConditionalType>source).root.isDistributive === (<ConditionalType>target).root.isDistributive) {
                            if (result = isRelatedTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType, /*reportErrors*/ false)) {
                                if (result &= isRelatedTo((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType, /*reportErrors*/ false)) {
                                    if (result &= isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), /*reportErrors*/ false)) {
                                        if (result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), /*reportErrors*/ false)) {
                                            return result;
                                        }
                                    }
                                }
                            }
                        }
                    }
                    if (flags & TypeFlags.Substitution) {
                        return isRelatedTo((<SubstitutionType>source).substitute, (<SubstitutionType>target).substitute, /*reportErrors*/ false);
                    }
                    return Ternary.False;
                }
                let result: Ternary;
                let originalErrorInfo: DiagnosticMessageChain | undefined;
                const saveErrorInfo = errorInfo;
                if (target.flags & TypeFlags.TypeParameter) {
                    // A source type { [P in keyof T]: X } is related to a target type T if X is related to T[P].
                    if (getObjectFlags(source) & ObjectFlags.Mapped && getConstraintTypeFromMappedType(<MappedType>source) === getIndexType(target)) {
                        if (!(getMappedTypeModifiers(<MappedType>source) & MappedTypeModifiers.IncludeOptional)) {
                            const templateType = getTemplateTypeFromMappedType(<MappedType>source);
                            const indexedAccessType = getIndexedAccessType(target, getTypeParameterFromMappedType(<MappedType>source));
                            if (result = isRelatedTo(templateType, indexedAccessType, reportErrors)) {
                                return result;
                            }
                        }
                    }
                }
                else if (target.flags & TypeFlags.Index) {
                    // A keyof S is related to a keyof T if T is related to S.
                    if (source.flags & TypeFlags.Index) {
                        if (result = isRelatedTo((<IndexType>target).type, (<IndexType>source).type, /*reportErrors*/ false)) {
                            return result;
                        }
                    }
                    // A type S is assignable to keyof T if S is assignable to keyof C, where C is the
                    // simplified form of T or, if T doesn't simplify, the constraint of T.
                    if (relation !== definitelyAssignableRelation) {
                        const simplified = getSimplifiedType((<IndexType>target).type);
                        const constraint = simplified !== (<IndexType>target).type ? simplified : getConstraintOfType((<IndexType>target).type);
                        if (constraint) {
                            if (result = isRelatedTo(source, getIndexType(constraint, (target as IndexType).stringsOnly), reportErrors)) {
                                return result;
                            }
                        }
                    }
                }
                else if (target.flags & TypeFlags.IndexedAccess) {
                    // A type S is related to a type T[K] if S is related to C, where C is the base constraint of T[K]
                    if (relation !== identityRelation) {
                        const constraint = getBaseConstraintOfType(target);
                        if (constraint && constraint !== target) {
                            if (result = isRelatedTo(source, constraint, reportErrors)) {
                                return result;
                            }
                        }
                    }
                }
                else if (isGenericMappedType(target)) {
                    // A source type T is related to a target type { [P in X]: T[P] }
                    const template = getTemplateTypeFromMappedType(target);
                    const modifiers = getMappedTypeModifiers(target);
                    if (!(modifiers & MappedTypeModifiers.ExcludeOptional)) {
                        if (template.flags & TypeFlags.IndexedAccess && (<IndexedAccessType>template).objectType === source &&
                            (<IndexedAccessType>template).indexType === getTypeParameterFromMappedType(target)) {
                            return Ternary.True;
                        }
                        // A source type T is related to a target type { [P in keyof T]: X } if T[P] is related to X.
                        if (!isGenericMappedType(source) && getConstraintTypeFromMappedType(target) === getIndexType(source)) {
                            const indexedAccessType = getIndexedAccessType(source, getTypeParameterFromMappedType(target));
                            const templateType = getTemplateTypeFromMappedType(target);
                            if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
                                return result;
                            }
                        }
                    }
                }

                if (source.flags & TypeFlags.TypeVariable) {
                    if (source.flags & TypeFlags.IndexedAccess && target.flags & TypeFlags.IndexedAccess) {
                        // A type S[K] is related to a type T[J] if S is related to T and K is related to J.
                        if (result = isRelatedTo((<IndexedAccessType>source).objectType, (<IndexedAccessType>target).objectType, reportErrors)) {
                            result &= isRelatedTo((<IndexedAccessType>source).indexType, (<IndexedAccessType>target).indexType, reportErrors);
                        }
                        if (result) {
                            errorInfo = saveErrorInfo;
                            return result;
                        }
                    }
                    const constraint = getConstraintForRelation(<TypeParameter>source);
                    if (!constraint || (source.flags & TypeFlags.TypeParameter && constraint.flags & TypeFlags.AnyOrUnknown)) {
                        // A type variable with no constraint is not related to the non-primitive object type.
                        if (result = isRelatedTo(emptyObjectType, extractTypesOfKind(target, ~TypeFlags.NonPrimitive))) {
                            errorInfo = saveErrorInfo;
                            return result;
                        }
                    }
                    else {
                        const instantiated = getTypeWithThisArgument(constraint, source);
                        if (result = isRelatedTo(instantiated, target, reportErrors)) {
                            errorInfo = saveErrorInfo;
                            return result;
                        }
                    }
                }
                else if (source.flags & TypeFlags.Index) {
                    if (result = isRelatedTo(keyofConstraintType, target, reportErrors)) {
                        errorInfo = saveErrorInfo;
                        return result;
                    }
                }
                else if (source.flags & TypeFlags.Conditional) {
                    if (target.flags & TypeFlags.Conditional) {
                        // Two conditional types 'T1 extends U1 ? X1 : Y1' and 'T2 extends U2 ? X2 : Y2' are related if
                        // one of T1 and T2 is related to the other, U1 and U2 are identical types, X1 is related to X2,
                        // and Y1 is related to Y2.
                        if (isTypeIdenticalTo((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType) &&
                            (isRelatedTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType) || isRelatedTo((<ConditionalType>target).checkType, (<ConditionalType>source).checkType))) {
                            if (result = isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), reportErrors)) {
                                result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), reportErrors);
                            }
                            if (result) {
                                errorInfo = saveErrorInfo;
                                return result;
                            }
                        }
                    }
                    else if (relation !== definitelyAssignableRelation) {
                        const distributiveConstraint = getConstraintOfDistributiveConditionalType(<ConditionalType>source);
                        if (distributiveConstraint) {
                            if (result = isRelatedTo(distributiveConstraint, target, reportErrors)) {
                                errorInfo = saveErrorInfo;
                                return result;
                            }
                        }
                        const defaultConstraint = getDefaultConstraintOfConditionalType(<ConditionalType>source);
                        if (defaultConstraint) {
                            if (result = isRelatedTo(defaultConstraint, target, reportErrors)) {
                                errorInfo = saveErrorInfo;
                                return result;
                            }
                        }
                    }
                }
                else {
                    // An empty object type is related to any mapped type that includes a '?' modifier.
                    if (isPartialMappedType(target) && !isGenericMappedType(source) && isEmptyObjectType(source)) {
                        return Ternary.True;
                    }
                    if (isGenericMappedType(target)) {
                        if (isGenericMappedType(source)) {
                            if (result = mappedTypeRelatedTo(source, target, reportErrors)) {
                                errorInfo = saveErrorInfo;
                                return result;
                            }
                        }
                        return Ternary.False;
                    }
                    if (relation === definitelyAssignableRelation && isGenericMappedType(source)) {
                        return Ternary.False;
                    }
                    const sourceIsPrimitive = !!(source.flags & TypeFlags.Primitive);
                    if (relation !== identityRelation) {
                        source = getApparentType(source);
                    }
                    if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target &&
                        !(getObjectFlags(source) & ObjectFlags.MarkerType || getObjectFlags(target) & ObjectFlags.MarkerType)) {
                        // We have type references to the same generic type, and the type references are not marker
                        // type references (which are intended by be compared structurally). Obtain the variance
                        // information for the type parameters and relate the type arguments accordingly.
                        const variances = getVariances((<TypeReference>source).target);
                        if (result = typeArgumentsRelatedTo(<TypeReference>source, <TypeReference>target, variances, reportErrors)) {
                            return result;
                        }
                        // The type arguments did not relate appropriately, but it may be because we have no variance
                        // information (in which case typeArgumentsRelatedTo defaulted to covariance for all type
                        // arguments). It might also be the case that the target type has a 'void' type argument for
                        // a covariant type parameter that is only used in return positions within the generic type
                        // (in which case any type argument is permitted on the source side). In those cases we proceed
                        // with a structural comparison. Otherwise, we know for certain the instantiations aren't
                        // related and we can return here.
                        if (variances !== emptyArray && !hasCovariantVoidArgument(<TypeReference>target, variances)) {
                            // In some cases generic types that are covariant in regular type checking mode become
                            // invariant in --strictFunctionTypes mode because one or more type parameters are used in
                            // both co- and contravariant positions. In order to make it easier to diagnose *why* such
                            // types are invariant, if any of the type parameters are invariant we reset the reported
                            // errors and instead force a structural comparison (which will include elaborations that
                            // reveal the reason).
                            if (!(reportErrors && some(variances, v => v === Variance.Invariant))) {
                                return Ternary.False;
                            }
                            // We remember the original error information so we can restore it in case the structural
                            // comparison unexpectedly succeeds. This can happen when the structural comparison result
                            // is a Ternary.Maybe for example caused by the recursion depth limiter.
                            originalErrorInfo = errorInfo;
                            errorInfo = saveErrorInfo;
                        }
                    }
                    else if (isTupleType(source) && (isArrayType(target) || isReadonlyArrayType(target)) || isArrayType(source) && isReadonlyArrayType(target)) {
                        return isRelatedTo(getIndexTypeOfType(source, IndexKind.Number) || anyType, getIndexTypeOfType(target, IndexKind.Number) || anyType, reportErrors);
                    }
                    // Even if relationship doesn't hold for unions, intersections, or generic type references,
                    // it may hold in a structural comparison.
                    // In a check of the form X = A & B, we will have previously checked if A relates to X or B relates
                    // to X. Failing both of those we want to check if the aggregation of A and B's members structurally
                    // relates to X. Thus, we include intersection types on the source side here.
                    if (source.flags & (TypeFlags.Object | TypeFlags.Intersection) && target.flags & TypeFlags.Object) {
                        // Report structural errors only if we haven't reported any errors yet
                        const reportStructuralErrors = reportErrors && errorInfo === saveErrorInfo && !sourceIsPrimitive;
                        result = propertiesRelatedTo(source, target, reportStructuralErrors);
                        if (result) {
                            result &= signaturesRelatedTo(source, target, SignatureKind.Call, reportStructuralErrors);
                            if (result) {
                                result &= signaturesRelatedTo(source, target, SignatureKind.Construct, reportStructuralErrors);
                                if (result) {
                                    result &= indexTypesRelatedTo(source, target, IndexKind.String, sourceIsPrimitive, reportStructuralErrors);
                                    if (result) {
                                        result &= indexTypesRelatedTo(source, target, IndexKind.Number, sourceIsPrimitive, reportStructuralErrors);
                                    }
                                }
                            }
                        }
                        if (result) {
                            if (!originalErrorInfo) {
                                errorInfo = saveErrorInfo;
                                return result;
                            }
                            errorInfo = originalErrorInfo;
                        }
                    }
                }
                return Ternary.False;
            }

            // A type [P in S]: X is related to a type [Q in T]: Y if T is related to S and X' is
            // related to Y, where X' is an instantiation of X in which P is replaced with Q. Notice
            // that S and T are contra-variant whereas X and Y are co-variant.
            function mappedTypeRelatedTo(source: MappedType, target: MappedType, reportErrors: boolean): Ternary {
                const modifiersRelated = relation === comparableRelation || (relation === identityRelation ? getMappedTypeModifiers(source) === getMappedTypeModifiers(target) :
                        getCombinedMappedTypeOptionality(source) <= getCombinedMappedTypeOptionality(target));
                if (modifiersRelated) {
                    let result: Ternary;
                    if (result = isRelatedTo(getConstraintTypeFromMappedType(target), getConstraintTypeFromMappedType(source), reportErrors)) {
                        const mapper = createTypeMapper([getTypeParameterFromMappedType(source)], [getTypeParameterFromMappedType(target)]);
                        return result & isRelatedTo(instantiateType(getTemplateTypeFromMappedType(source), mapper), getTemplateTypeFromMappedType(target), reportErrors);
                    }
                }
                return Ternary.False;
            }

            function propertiesRelatedTo(source: Type, target: Type, reportErrors: boolean): Ternary {
                if (relation === identityRelation) {
                    return propertiesIdenticalTo(source, target);
                }
                const requireOptionalProperties = relation === subtypeRelation && !isObjectLiteralType(source) && !isEmptyArrayLiteralType(source) && !isTupleType(source);
                const unmatchedProperty = getUnmatchedProperty(source, target, requireOptionalProperties);
                if (unmatchedProperty) {
                    if (reportErrors) {
                        reportError(Diagnostics.Property_0_is_missing_in_type_1, symbolToString(unmatchedProperty), typeToString(source));
                    }
                    return Ternary.False;
                }
                if (isObjectLiteralType(target)) {
                    for (const sourceProp of getPropertiesOfType(source)) {
                        if (!getPropertyOfObjectType(target, sourceProp.escapedName)) {
                            const sourceType = getTypeOfSymbol(sourceProp);
                            if (!(sourceType === undefinedType || sourceType === undefinedWideningType)) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_does_not_exist_on_type_1, symbolToString(sourceProp), typeToString(target));
                                }
                                return Ternary.False;
                            }
                        }
                    }
                }
                let result = Ternary.True;
                if (isTupleType(target)) {
                    const targetRestType = getRestTypeOfTupleType(target);
                    if (targetRestType) {
                        if (!isTupleType(source)) {
                            return Ternary.False;
                        }
                        const sourceRestType = getRestTypeOfTupleType(source);
                        if (sourceRestType && !isRelatedTo(sourceRestType, targetRestType, reportErrors)) {
                            if (reportErrors) {
                                reportError(Diagnostics.Rest_signatures_are_incompatible);
                            }
                            return Ternary.False;
                        }
                        const targetCount = getTypeReferenceArity(target) - 1;
                        const sourceCount = getTypeReferenceArity(source) - (sourceRestType ? 1 : 0);
                        for (let i = targetCount; i < sourceCount; i++) {
                            const related = isRelatedTo((<TypeReference>source).typeArguments![i], targetRestType, reportErrors);
                            if (!related) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_is_incompatible_with_rest_element_type, "" + i);
                                }
                                return Ternary.False;
                            }
                            result &= related;
                        }
                    }
                }
                const properties = getPropertiesOfObjectType(target);
                for (const targetProp of properties) {
                    if (!(targetProp.flags & SymbolFlags.Prototype)) {
                        const sourceProp = getPropertyOfType(source, targetProp.escapedName);
                        if (sourceProp && sourceProp !== targetProp) {
                            if (isIgnoredJsxProperty(source, sourceProp, getTypeOfSymbol(targetProp))) {
                                continue;
                            }
                            const sourcePropFlags = getDeclarationModifierFlagsFromSymbol(sourceProp);
                            const targetPropFlags = getDeclarationModifierFlagsFromSymbol(targetProp);
                            if (sourcePropFlags & ModifierFlags.Private || targetPropFlags & ModifierFlags.Private) {
                                const hasDifferingDeclarations = sourceProp.valueDeclaration !== targetProp.valueDeclaration;
                                if (getCheckFlags(sourceProp) & CheckFlags.ContainsPrivate && hasDifferingDeclarations) {
                                    if (reportErrors) {
                                        reportError(Diagnostics.Property_0_has_conflicting_declarations_and_is_inaccessible_in_type_1, symbolToString(sourceProp), typeToString(source));
                                    }
                                    return Ternary.False;
                                }
                                if (hasDifferingDeclarations) {
                                    if (reportErrors) {
                                        if (sourcePropFlags & ModifierFlags.Private && targetPropFlags & ModifierFlags.Private) {
                                            reportError(Diagnostics.Types_have_separate_declarations_of_a_private_property_0, symbolToString(targetProp));
                                        }
                                        else {
                                            reportError(Diagnostics.Property_0_is_private_in_type_1_but_not_in_type_2, symbolToString(targetProp),
                                                typeToString(sourcePropFlags & ModifierFlags.Private ? source : target),
                                                typeToString(sourcePropFlags & ModifierFlags.Private ? target : source));
                                        }
                                    }
                                    return Ternary.False;
                                }
                            }
                            else if (targetPropFlags & ModifierFlags.Protected) {
                                if (!isValidOverrideOf(sourceProp, targetProp)) {
                                    if (reportErrors) {
                                        reportError(Diagnostics.Property_0_is_protected_but_type_1_is_not_a_class_derived_from_2, symbolToString(targetProp),
                                            typeToString(getDeclaringClass(sourceProp) || source), typeToString(getDeclaringClass(targetProp) || target));
                                    }
                                    return Ternary.False;
                                }
                            }
                            else if (sourcePropFlags & ModifierFlags.Protected) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_is_protected_in_type_1_but_public_in_type_2,
                                        symbolToString(targetProp), typeToString(source), typeToString(target));
                                }
                                return Ternary.False;
                            }
                            const related = isRelatedTo(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp), reportErrors);
                            if (!related) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Types_of_property_0_are_incompatible, symbolToString(targetProp));
                                }
                                return Ternary.False;
                            }
                            result &= related;
                            // When checking for comparability, be more lenient with optional properties.
                            if (relation !== comparableRelation && sourceProp.flags & SymbolFlags.Optional && !(targetProp.flags & SymbolFlags.Optional)) {
                                // TypeScript 1.0 spec (April 2014): 3.8.3
                                // S is a subtype of a type T, and T is a supertype of S if ...
                                // S' and T are object types and, for each member M in T..
                                // M is a property and S' contains a property N where
                                // if M is a required property, N is also a required property
                                // (M - property in T)
                                // (N - property in S)
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_is_optional_in_type_1_but_required_in_type_2,
                                        symbolToString(targetProp), typeToString(source), typeToString(target));
                                }
                                return Ternary.False;
                            }
                        }
                    }
                }
                return result;
            }

            function propertiesIdenticalTo(source: Type, target: Type): Ternary {
                if (!(source.flags & TypeFlags.Object && target.flags & TypeFlags.Object)) {
                    return Ternary.False;
                }
                const sourceProperties = getPropertiesOfObjectType(source);
                const targetProperties = getPropertiesOfObjectType(target);
                if (sourceProperties.length !== targetProperties.length) {
                    return Ternary.False;
                }
                let result = Ternary.True;
                for (const sourceProp of sourceProperties) {
                    const targetProp = getPropertyOfObjectType(target, sourceProp.escapedName);
                    if (!targetProp) {
                        return Ternary.False;
                    }
                    const related = compareProperties(sourceProp, targetProp, isRelatedTo);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function signaturesRelatedTo(source: Type, target: Type, kind: SignatureKind, reportErrors: boolean): Ternary {
                if (relation === identityRelation) {
                    return signaturesIdenticalTo(source, target, kind);
                }
                if (target === anyFunctionType || source === anyFunctionType) {
                    return Ternary.True;
                }

                const sourceIsJSConstructor = source.symbol && isJavascriptConstructor(source.symbol.valueDeclaration);
                const targetIsJSConstructor = target.symbol && isJavascriptConstructor(target.symbol.valueDeclaration);

                const sourceSignatures = getSignaturesOfType(source, (sourceIsJSConstructor && kind === SignatureKind.Construct) ?
                    SignatureKind.Call : kind);
                const targetSignatures = getSignaturesOfType(target, (targetIsJSConstructor && kind === SignatureKind.Construct) ?
                    SignatureKind.Call : kind);

                if (kind === SignatureKind.Construct && sourceSignatures.length && targetSignatures.length) {
                    if (isAbstractConstructorType(source) && !isAbstractConstructorType(target)) {
                        // An abstract constructor type is not assignable to a non-abstract constructor type
                        // as it would otherwise be possible to new an abstract class. Note that the assignability
                        // check we perform for an extends clause excludes construct signatures from the target,
                        // so this check never proceeds.
                        if (reportErrors) {
                            reportError(Diagnostics.Cannot_assign_an_abstract_constructor_type_to_a_non_abstract_constructor_type);
                        }
                        return Ternary.False;
                    }
                    if (!constructorVisibilitiesAreCompatible(sourceSignatures[0], targetSignatures[0], reportErrors)) {
                        return Ternary.False;
                    }
                }

                let result = Ternary.True;
                const saveErrorInfo = errorInfo;

                if (getObjectFlags(source) & ObjectFlags.Instantiated && getObjectFlags(target) & ObjectFlags.Instantiated && source.symbol === target.symbol) {
                    // We have instantiations of the same anonymous type (which typically will be the type of a
                    // method). Simply do a pairwise comparison of the signatures in the two signature lists instead
                    // of the much more expensive N * M comparison matrix we explore below. We erase type parameters
                    // as they are known to always be the same.
                    for (let i = 0; i < targetSignatures.length; i++) {
                        const related = signatureRelatedTo(sourceSignatures[i], targetSignatures[i], /*erase*/ true, reportErrors);
                        if (!related) {
                            return Ternary.False;
                        }
                        result &= related;
                    }
                }
                else if (sourceSignatures.length === 1 && targetSignatures.length === 1) {
                    // For simple functions (functions with a single signature) we only erase type parameters for
                    // the comparable relation. Otherwise, if the source signature is generic, we instantiate it
                    // in the context of the target signature before checking the relationship. Ideally we'd do
                    // this regardless of the number of signatures, but the potential costs are prohibitive due
                    // to the quadratic nature of the logic below.
                    const eraseGenerics = relation === comparableRelation || !!compilerOptions.noStrictGenericChecks;
                    result = signatureRelatedTo(sourceSignatures[0], targetSignatures[0], eraseGenerics, reportErrors);
                }
                else {
                    outer: for (const t of targetSignatures) {
                        // Only elaborate errors from the first failure
                        let shouldElaborateErrors = reportErrors;
                        for (const s of sourceSignatures) {
                            const related = signatureRelatedTo(s, t, /*erase*/ true, shouldElaborateErrors);
                            if (related) {
                                result &= related;
                                errorInfo = saveErrorInfo;
                                continue outer;
                            }
                            shouldElaborateErrors = false;
                        }

                        if (shouldElaborateErrors) {
                            reportError(Diagnostics.Type_0_provides_no_match_for_the_signature_1,
                                typeToString(source),
                                signatureToString(t, /*enclosingDeclaration*/ undefined, /*flags*/ undefined, kind));
                        }
                        return Ternary.False;
                    }
                }
                return result;
            }

            /**
             * See signatureAssignableTo, compareSignaturesIdentical
             */
            function signatureRelatedTo(source: Signature, target: Signature, erase: boolean, reportErrors: boolean): Ternary {
                return compareSignaturesRelated(erase ? getErasedSignature(source) : source, erase ? getErasedSignature(target) : target,
                    CallbackCheck.None, /*ignoreReturnTypes*/ false, reportErrors, reportError, isRelatedTo);
            }

            function signaturesIdenticalTo(source: Type, target: Type, kind: SignatureKind): Ternary {
                const sourceSignatures = getSignaturesOfType(source, kind);
                const targetSignatures = getSignaturesOfType(target, kind);
                if (sourceSignatures.length !== targetSignatures.length) {
                    return Ternary.False;
                }
                let result = Ternary.True;
                for (let i = 0; i < sourceSignatures.length; i++) {
                    const related = compareSignaturesIdentical(sourceSignatures[i], targetSignatures[i], /*partialMatch*/ false, /*ignoreThisTypes*/ false, /*ignoreReturnTypes*/ false, isRelatedTo);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function eachPropertyRelatedTo(source: Type, target: Type, kind: IndexKind, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                for (const prop of getPropertiesOfObjectType(source)) {
                    if (isIgnoredJsxProperty(source, prop, /*targetMemberType*/ undefined)) {
                        continue;
                    }
                    // Skip over symbol-named members
                    if (prop.nameType && prop.nameType.flags & TypeFlags.UniqueESSymbol) {
                        continue;
                    }
                    if (kind === IndexKind.String || isNumericLiteralName(prop.escapedName)) {
                        const related = isRelatedTo(getTypeOfSymbol(prop), target, reportErrors);
                        if (!related) {
                            if (reportErrors) {
                                reportError(Diagnostics.Property_0_is_incompatible_with_index_signature, symbolToString(prop));
                            }
                            return Ternary.False;
                        }
                        result &= related;
                    }
                }
                return result;
            }

            function indexInfoRelatedTo(sourceInfo: IndexInfo, targetInfo: IndexInfo, reportErrors: boolean) {
                const related = isRelatedTo(sourceInfo.type, targetInfo.type, reportErrors);
                if (!related && reportErrors) {
                    reportError(Diagnostics.Index_signatures_are_incompatible);
                }
                return related;
            }

            function indexTypesRelatedTo(source: Type, target: Type, kind: IndexKind, sourceIsPrimitive: boolean, reportErrors: boolean): Ternary {
                if (relation === identityRelation) {
                    return indexTypesIdenticalTo(source, target, kind);
                }
                const targetInfo = getIndexInfoOfType(target, kind);
                if (!targetInfo || targetInfo.type.flags & TypeFlags.AnyOrUnknown && !sourceIsPrimitive) {
                    // Index signature of type any permits assignment from everything but primitives
                    return Ternary.True;
                }
                const sourceInfo = getIndexInfoOfType(source, kind) ||
                    kind === IndexKind.Number && getIndexInfoOfType(source, IndexKind.String);
                if (sourceInfo) {
                    return indexInfoRelatedTo(sourceInfo, targetInfo, reportErrors);
                }
                if (isGenericMappedType(source)) {
                    // A generic mapped type { [P in K]: T } is related to an index signature { [x: string]: U }
                    // if T is related to U.
                    return (kind === IndexKind.String && isRelatedTo(getTemplateTypeFromMappedType(source), targetInfo.type, reportErrors)) as any as Ternary; // TODO: GH#18217
                }
                if (isObjectTypeWithInferableIndex(source)) {
                    let related = Ternary.True;
                    if (kind === IndexKind.String) {
                        const sourceNumberInfo = getIndexInfoOfType(source, IndexKind.Number);
                        if (sourceNumberInfo) {
                            related = indexInfoRelatedTo(sourceNumberInfo, targetInfo, reportErrors);
                        }
                    }
                    if (related) {
                        related &= eachPropertyRelatedTo(source, targetInfo.type, kind, reportErrors);
                    }
                    return related;
                }
                if (reportErrors) {
                    reportError(Diagnostics.Index_signature_is_missing_in_type_0, typeToString(source));
                }
                return Ternary.False;
            }

            function indexTypesIdenticalTo(source: Type, target: Type, indexKind: IndexKind): Ternary {
                const targetInfo = getIndexInfoOfType(target, indexKind);
                const sourceInfo = getIndexInfoOfType(source, indexKind);
                if (!sourceInfo && !targetInfo) {
                    return Ternary.True;
                }
                if (sourceInfo && targetInfo && sourceInfo.isReadonly === targetInfo.isReadonly) {
                    return isRelatedTo(sourceInfo.type, targetInfo.type);
                }
                return Ternary.False;
            }

            function constructorVisibilitiesAreCompatible(sourceSignature: Signature, targetSignature: Signature, reportErrors: boolean) {
                if (!sourceSignature.declaration || !targetSignature.declaration) {
                    return true;
                }

                const sourceAccessibility = getSelectedModifierFlags(sourceSignature.declaration, ModifierFlags.NonPublicAccessibilityModifier);
                const targetAccessibility = getSelectedModifierFlags(targetSignature.declaration, ModifierFlags.NonPublicAccessibilityModifier);

                // A public, protected and private signature is assignable to a private signature.
                if (targetAccessibility === ModifierFlags.Private) {
                    return true;
                }

                // A public and protected signature is assignable to a protected signature.
                if (targetAccessibility === ModifierFlags.Protected && sourceAccessibility !== ModifierFlags.Private) {
                    return true;
                }

                // Only a public signature is assignable to public signature.
                if (targetAccessibility !== ModifierFlags.Protected && !sourceAccessibility) {
                    return true;
                }

                if (reportErrors) {
                    reportError(Diagnostics.Cannot_assign_a_0_constructor_type_to_a_1_constructor_type, visibilityToString(sourceAccessibility), visibilityToString(targetAccessibility));
                }

                return false;
            }
        }

        /**
         * A type is 'weak' if it is an object type with at least one optional property
         * and no required properties, call/construct signatures or index signatures
         */
        function isWeakType(type: Type): boolean {
            if (type.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                return resolved.callSignatures.length === 0 && resolved.constructSignatures.length === 0 &&
                    !resolved.stringIndexInfo && !resolved.numberIndexInfo &&
                    resolved.properties.length > 0 &&
                    every(resolved.properties, p => !!(p.flags & SymbolFlags.Optional));
            }
            if (type.flags & TypeFlags.Intersection) {
                return every((<IntersectionType>type).types, isWeakType);
            }
            return false;
        }

        function hasCommonProperties(source: Type, target: Type) {
            const isComparingJsxAttributes = !!(getObjectFlags(source) & ObjectFlags.JsxAttributes);
            for (const prop of getPropertiesOfType(source)) {
                if (isKnownProperty(target, prop.escapedName, isComparingJsxAttributes)) {
                    return true;
                }
            }
            return false;
        }

        // Return a type reference where the source type parameter is replaced with the target marker
        // type, and flag the result as a marker type reference.
        function getMarkerTypeReference(type: GenericType, source: TypeParameter, target: Type) {
            const result = createTypeReference(type, map(type.typeParameters, t => t === source ? target : t));
            result.objectFlags |= ObjectFlags.MarkerType;
            return result;
        }

        // Return an array containing the variance of each type parameter. The variance is effectively
        // a digest of the type comparisons that occur for each type argument when instantiations of the
        // generic type are structurally compared. We infer the variance information by comparing
        // instantiations of the generic type for type arguments with known relations. The function
        // returns the emptyArray singleton if we're not in strictFunctionTypes mode or if the function
        // has been invoked recursively for the given generic type.
        function getVariances(type: GenericType): Variance[] {
            if (!strictFunctionTypes) {
                return emptyArray;
            }
            const typeParameters = type.typeParameters || emptyArray;
            let variances = type.variances;
            if (!variances) {
                if (type === globalArrayType || type === globalReadonlyArrayType) {
                    // Arrays are known to be covariant, no need to spend time computing this
                    variances = [Variance.Covariant];
                }
                else {
                    // The emptyArray singleton is used to signal a recursive invocation.
                    type.variances = emptyArray;
                    variances = [];
                    for (const tp of typeParameters) {
                        // We first compare instantiations where the type parameter is replaced with
                        // marker types that have a known subtype relationship. From this we can infer
                        // invariance, covariance, contravariance or bivariance.
                        const typeWithSuper = getMarkerTypeReference(type, tp, markerSuperType);
                        const typeWithSub = getMarkerTypeReference(type, tp, markerSubType);
                        let variance = (isTypeAssignableTo(typeWithSub, typeWithSuper) ? Variance.Covariant : 0) |
                            (isTypeAssignableTo(typeWithSuper, typeWithSub) ? Variance.Contravariant : 0);
                        // If the instantiations appear to be related bivariantly it may be because the
                        // type parameter is independent (i.e. it isn't witnessed anywhere in the generic
                        // type). To determine this we compare instantiations where the type parameter is
                        // replaced with marker types that are known to be unrelated.
                        if (variance === Variance.Bivariant && isTypeAssignableTo(getMarkerTypeReference(type, tp, markerOtherType), typeWithSuper)) {
                            variance = Variance.Independent;
                        }
                        variances.push(variance);
                    }
                }
                type.variances = variances;
            }
            return variances;
        }

        // Return true if the given type reference has a 'void' type argument for a covariant type parameter.
        // See comment at call in recursiveTypeRelatedTo for when this case matters.
        function hasCovariantVoidArgument(type: TypeReference, variances: Variance[]): boolean {
            for (let i = 0; i < variances.length; i++) {
                if (variances[i] === Variance.Covariant && type.typeArguments![i].flags & TypeFlags.Void) {
                    return true;
                }
            }
            return false;
        }

        function isUnconstrainedTypeParameter(type: Type) {
            return type.flags & TypeFlags.TypeParameter && !getConstraintOfTypeParameter(<TypeParameter>type);
        }

        function isTypeReferenceWithGenericArguments(type: Type): boolean {
            return !!(getObjectFlags(type) & ObjectFlags.Reference) && some((<TypeReference>type).typeArguments, t => isUnconstrainedTypeParameter(t) || isTypeReferenceWithGenericArguments(t));
        }

        /**
         * getTypeReferenceId(A<T, number, U>) returns "111=0-12=1"
         *   where A.id=111 and number.id=12
         */
        function getTypeReferenceId(type: TypeReference, typeParameters: Type[], depth = 0) {
            let result = "" + type.target.id;
            for (const t of type.typeArguments!) {
                if (isUnconstrainedTypeParameter(t)) {
                    let index = typeParameters.indexOf(t);
                    if (index < 0) {
                        index = typeParameters.length;
                        typeParameters.push(t);
                    }
                    result += "=" + index;
                }
                else if (depth < 4 && isTypeReferenceWithGenericArguments(t)) {
                    result += "<" + getTypeReferenceId(t as TypeReference, typeParameters, depth + 1) + ">";
                }
                else {
                    result += "-" + t.id;
                }
            }
            return result;
        }

        /**
         * To improve caching, the relation key for two generic types uses the target's id plus ids of the type parameters.
         * For other cases, the types ids are used.
         */
        function getRelationKey(source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            if (relation === identityRelation && source.id > target.id) {
                const temp = source;
                source = target;
                target = temp;
            }
            if (isTypeReferenceWithGenericArguments(source) && isTypeReferenceWithGenericArguments(target)) {
                const typeParameters: Type[] = [];
                return getTypeReferenceId(<TypeReference>source, typeParameters) + "," + getTypeReferenceId(<TypeReference>target, typeParameters);
            }
            return source.id + "," + target.id;
        }

        // Invoke the callback for each underlying property symbol of the given symbol and return the first
        // value that isn't undefined.
        function forEachProperty<T>(prop: Symbol, callback: (p: Symbol) => T): T | undefined {
            if (getCheckFlags(prop) & CheckFlags.Synthetic) {
                for (const t of (<TransientSymbol>prop).containingType!.types) {
                    const p = getPropertyOfType(t, prop.escapedName);
                    const result = p && forEachProperty(p, callback);
                    if (result) {
                        return result;
                    }
                }
                return undefined;
            }
            return callback(prop);
        }

        // Return the declaring class type of a property or undefined if property not declared in class
        function getDeclaringClass(prop: Symbol) {
            return prop.parent && prop.parent.flags & SymbolFlags.Class ? getDeclaredTypeOfSymbol(getParentOfSymbol(prop)!) : undefined;
        }

        // Return true if some underlying source property is declared in a class that derives
        // from the given base class.
        function isPropertyInClassDerivedFrom(prop: Symbol, baseClass: Type | undefined) {
            return forEachProperty(prop, sp => {
                const sourceClass = getDeclaringClass(sp);
                return sourceClass ? hasBaseType(sourceClass, baseClass) : false;
            });
        }

        // Return true if source property is a valid override of protected parts of target property.
        function isValidOverrideOf(sourceProp: Symbol, targetProp: Symbol) {
            return !forEachProperty(targetProp, tp => getDeclarationModifierFlagsFromSymbol(tp) & ModifierFlags.Protected ?
                !isPropertyInClassDerivedFrom(sourceProp, getDeclaringClass(tp)) : false);
        }

        // Return true if the given class derives from each of the declaring classes of the protected
        // constituents of the given property.
        function isClassDerivedFromDeclaringClasses(checkClass: Type, prop: Symbol) {
            return forEachProperty(prop, p => getDeclarationModifierFlagsFromSymbol(p) & ModifierFlags.Protected ?
                !hasBaseType(checkClass, getDeclaringClass(p)) : false) ? undefined : checkClass;
        }

        // Return true if the given type is deeply nested. We consider this to be the case when structural type comparisons
        // for 5 or more occurrences or instantiations of the type have been recorded on the given stack. It is possible,
        // though highly unlikely, for this test to be true in a situation where a chain of instantiations is not infinitely
        // expanding. Effectively, we will generate a false positive when two types are structurally equal to at least 5
        // levels, but unequal at some level beyond that.
        function isDeeplyNestedType(type: Type, stack: Type[], depth: number): boolean {
            // We track all object types that have an associated symbol (representing the origin of the type)
            if (depth >= 5 && type.flags & TypeFlags.Object) {
                const symbol = type.symbol;
                if (symbol) {
                    let count = 0;
                    for (let i = 0; i < depth; i++) {
                        const t = stack[i];
                        if (t.flags & TypeFlags.Object && t.symbol === symbol) {
                            count++;
                            if (count >= 5) return true;
                        }
                    }
                }
            }
            return false;
        }

        function isPropertyIdenticalTo(sourceProp: Symbol, targetProp: Symbol): boolean {
            return compareProperties(sourceProp, targetProp, compareTypesIdentical) !== Ternary.False;
        }

        function compareProperties(sourceProp: Symbol, targetProp: Symbol, compareTypes: (source: Type, target: Type) => Ternary): Ternary {
            // Two members are considered identical when
            // - they are public properties with identical names, optionality, and types,
            // - they are private or protected properties originating in the same declaration and having identical types
            if (sourceProp === targetProp) {
                return Ternary.True;
            }
            const sourcePropAccessibility = getDeclarationModifierFlagsFromSymbol(sourceProp) & ModifierFlags.NonPublicAccessibilityModifier;
            const targetPropAccessibility = getDeclarationModifierFlagsFromSymbol(targetProp) & ModifierFlags.NonPublicAccessibilityModifier;
            if (sourcePropAccessibility !== targetPropAccessibility) {
                return Ternary.False;
            }
            if (sourcePropAccessibility) {
                if (getTargetSymbol(sourceProp) !== getTargetSymbol(targetProp)) {
                    return Ternary.False;
                }
            }
            else {
                if ((sourceProp.flags & SymbolFlags.Optional) !== (targetProp.flags & SymbolFlags.Optional)) {
                    return Ternary.False;
                }
            }
            if (isReadonlySymbol(sourceProp) !== isReadonlySymbol(targetProp)) {
                return Ternary.False;
            }
            return compareTypes(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp));
        }

        function isMatchingSignature(source: Signature, target: Signature, partialMatch: boolean) {
            const sourceParameterCount = getParameterCount(source);
            const targetParameterCount = getParameterCount(target);
            const sourceMinArgumentCount = getMinArgumentCount(source);
            const targetMinArgumentCount = getMinArgumentCount(target);
            const sourceHasRestParameter = hasEffectiveRestParameter(source);
            const targetHasRestParameter = hasEffectiveRestParameter(target);
            // A source signature matches a target signature if the two signatures have the same number of required,
            // optional, and rest parameters.
            if (sourceParameterCount === targetParameterCount &&
                sourceMinArgumentCount === targetMinArgumentCount &&
                sourceHasRestParameter === targetHasRestParameter) {
                return true;
            }
            // A source signature partially matches a target signature if the target signature has no fewer required
            // parameters and no more overall parameters than the source signature (where a signature with a rest
            // parameter is always considered to have more overall parameters than one without).
            const sourceRestCount = sourceHasRestParameter ? 1 : 0;
            const targetRestCount = targetHasRestParameter ? 1 : 0;
            if (partialMatch && sourceMinArgumentCount <= targetMinArgumentCount && (
                sourceRestCount > targetRestCount ||
                sourceRestCount === targetRestCount && sourceParameterCount >= targetParameterCount)) {
                return true;
            }
            return false;
        }

        /**
         * See signatureRelatedTo, compareSignaturesIdentical
         */
        function compareSignaturesIdentical(source: Signature, target: Signature, partialMatch: boolean, ignoreThisTypes: boolean, ignoreReturnTypes: boolean, compareTypes: (s: Type, t: Type) => Ternary): Ternary {
            // TODO (drosen): De-duplicate code between related functions.
            if (source === target) {
                return Ternary.True;
            }
            if (!(isMatchingSignature(source, target, partialMatch))) {
                return Ternary.False;
            }
            // Check that the two signatures have the same number of type parameters. We might consider
            // also checking that any type parameter constraints match, but that would require instantiating
            // the constraints with a common set of type arguments to get relatable entities in places where
            // type parameters occur in the constraints. The complexity of doing that doesn't seem worthwhile,
            // particularly as we're comparing erased versions of the signatures below.
            if (length(source.typeParameters) !== length(target.typeParameters)) {
                return Ternary.False;
            }
            // Spec 1.0 Section 3.8.3 & 3.8.4:
            // M and N (the signatures) are instantiated using type Any as the type argument for all type parameters declared by M and N
            source = getErasedSignature(source);
            target = getErasedSignature(target);
            let result = Ternary.True;

            if (!ignoreThisTypes) {
                const sourceThisType = getThisTypeOfSignature(source);
                if (sourceThisType) {
                    const targetThisType = getThisTypeOfSignature(target);
                    if (targetThisType) {
                        const related = compareTypes(sourceThisType, targetThisType);
                        if (!related) {
                            return Ternary.False;
                        }
                        result &= related;
                    }
                }
            }

            const targetLen = getParameterCount(target);
            for (let i = 0; i < targetLen; i++) {
                const s = getTypeAtPosition(source, i);
                const t = getTypeAtPosition(target, i);
                const related = compareTypes(s, t);
                if (!related) {
                    return Ternary.False;
                }
                result &= related;
            }
            if (!ignoreReturnTypes) {
                const sourceTypePredicate = getTypePredicateOfSignature(source);
                const targetTypePredicate = getTypePredicateOfSignature(target);
                result &= sourceTypePredicate !== undefined || targetTypePredicate !== undefined
                    ? compareTypePredicatesIdentical(sourceTypePredicate, targetTypePredicate, compareTypes)
                    // If they're both type predicates their return types will both be `boolean`, so no need to compare those.
                    : compareTypes(getReturnTypeOfSignature(source), getReturnTypeOfSignature(target));
            }
            return result;
        }

        function compareTypePredicatesIdentical(source: TypePredicate | undefined, target: TypePredicate | undefined, compareTypes: (s: Type, t: Type) => Ternary): Ternary {
            return source === undefined || target === undefined || !typePredicateKindsMatch(source, target) ? Ternary.False : compareTypes(source.type, target.type);
        }

        function literalTypesWithSameBaseType(types: Type[]): boolean {
            let commonBaseType: Type | undefined;
            for (const t of types) {
                const baseType = getBaseTypeOfLiteralType(t);
                if (!commonBaseType) {
                    commonBaseType = baseType;
                }
                if (baseType === t || baseType !== commonBaseType) {
                    return false;
                }
            }
            return true;
        }

        // When the candidate types are all literal types with the same base type, return a union
        // of those literal types. Otherwise, return the leftmost type for which no type to the
        // right is a supertype.
        function getSupertypeOrUnion(types: Type[]): Type {
            return literalTypesWithSameBaseType(types) ?
                getUnionType(types) :
                reduceLeft(types, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;
        }

        function getCommonSupertype(types: Type[]): Type {
            if (!strictNullChecks) {
                return getSupertypeOrUnion(types);
            }
            const primaryTypes = filter(types, t => !(t.flags & TypeFlags.Nullable));
            return primaryTypes.length ?
                getNullableType(getSupertypeOrUnion(primaryTypes), getFalsyFlagsOfTypes(types) & TypeFlags.Nullable) :
                getUnionType(types, UnionReduction.Subtype);
        }

        // Return the leftmost type for which no type to the right is a subtype.
        function getCommonSubtype(types: Type[]) {
            return reduceLeft(types, (s, t) => isTypeSubtypeOf(t, s) ? t : s)!;
        }

        function isArrayType(type: Type): boolean {
            return !!(getObjectFlags(type) & ObjectFlags.Reference) && (<TypeReference>type).target === globalArrayType;
        }

        function isReadonlyArrayType(type: Type): boolean {
            return !!(getObjectFlags(type) & ObjectFlags.Reference) && (<TypeReference>type).target === globalReadonlyArrayType;
        }

        function isArrayLikeType(type: Type): boolean {
            // A type is array-like if it is a reference to the global Array or global ReadonlyArray type,
            // or if it is not the undefined or null type and if it is assignable to ReadonlyArray<any>
            return getObjectFlags(type) & ObjectFlags.Reference && ((<TypeReference>type).target === globalArrayType || (<TypeReference>type).target === globalReadonlyArrayType) ||
                !(type.flags & TypeFlags.Nullable) && isTypeAssignableTo(type, anyReadonlyArrayType);
        }

        function isEmptyArrayLiteralType(type: Type): boolean {
            const elementType = isArrayType(type) ? (<TypeReference>type).typeArguments![0] : undefined;
            return elementType === undefinedWideningType || elementType === implicitNeverType;
        }

        function isTupleLikeType(type: Type): boolean {
            return isTupleType(type) || !!getPropertyOfType(type, "0" as __String);
        }

        function getTupleElementType(type: Type, index: number) {
            return isTupleType(type) ?
                index < getLengthOfTupleType(type) ? type.typeArguments![index] : getRestTypeOfTupleType(type) :
                getTypeOfPropertyOfType(type, "" + index as __String);
        }

        function isNeitherUnitTypeNorNever(type: Type): boolean {
            return !(type.flags & (TypeFlags.Unit | TypeFlags.Never));
        }

        function isUnitType(type: Type): boolean {
            return !!(type.flags & TypeFlags.Unit);
        }

        function isLiteralType(type: Type): boolean {
            return type.flags & TypeFlags.Boolean ? true :
                type.flags & TypeFlags.Union ? type.flags & TypeFlags.EnumLiteral ? true : every((<UnionType>type).types, isUnitType) :
                isUnitType(type);
        }

        function getBaseTypeOfLiteralType(type: Type): Type {
            return type.flags & TypeFlags.EnumLiteral ? getBaseTypeOfEnumLiteralType(<LiteralType>type) :
                type.flags & TypeFlags.StringLiteral ? stringType :
                type.flags & TypeFlags.NumberLiteral ? numberType :
                type.flags & TypeFlags.BooleanLiteral ? booleanType :
                type.flags & TypeFlags.Union ? getUnionType(sameMap((<UnionType>type).types, getBaseTypeOfLiteralType)) :
                type;
        }

        function getWidenedLiteralType(type: Type): Type {
            return type.flags & TypeFlags.EnumLiteral && type.flags & TypeFlags.FreshLiteral ? getBaseTypeOfEnumLiteralType(<LiteralType>type) :
                type.flags & TypeFlags.StringLiteral && type.flags & TypeFlags.FreshLiteral ? stringType :
                type.flags & TypeFlags.NumberLiteral && type.flags & TypeFlags.FreshLiteral ? numberType :
                type.flags & TypeFlags.BooleanLiteral ? booleanType :
                type.flags & TypeFlags.Union ? getUnionType(sameMap((<UnionType>type).types, getWidenedLiteralType)) :
                type;
        }

        function getWidenedUniqueESSymbolType(type: Type): Type {
            return type.flags & TypeFlags.UniqueESSymbol ? esSymbolType :
                type.flags & TypeFlags.Union ? getUnionType(sameMap((<UnionType>type).types, getWidenedUniqueESSymbolType)) :
                type;
        }

        function getWidenedLiteralLikeTypeForContextualType(type: Type, contextualType: Type | undefined) {
            if (!isLiteralOfContextualType(type, contextualType)) {
                type = getWidenedUniqueESSymbolType(getWidenedLiteralType(type));
            }
            return type;
        }

        /**
         * Check if a Type was written as a tuple type literal.
         * Prefer using isTupleLikeType() unless the use of `elementTypes` is required.
         */
        function isTupleType(type: Type): type is TupleTypeReference {
            return !!(getObjectFlags(type) & ObjectFlags.Reference && (<TypeReference>type).target.objectFlags & ObjectFlags.Tuple);
        }

        function getRestTypeOfTupleType(type: TupleTypeReference) {
            return type.target.hasRestElement ? type.typeArguments![type.target.typeParameters!.length - 1] : undefined;
        }

        function getRestArrayTypeOfTupleType(type: TupleTypeReference) {
            const restType = getRestTypeOfTupleType(type);
            return restType && createArrayType(restType);
        }

        function getLengthOfTupleType(type: TupleTypeReference) {
            return getTypeReferenceArity(type) - (type.target.hasRestElement ? 1 : 0);
        }

        function getFalsyFlagsOfTypes(types: Type[]): TypeFlags {
            let result: TypeFlags = 0;
            for (const t of types) {
                result |= getFalsyFlags(t);
            }
            return result;
        }

        // Returns the String, Number, Boolean, StringLiteral, NumberLiteral, BooleanLiteral, Void, Undefined, or Null
        // flags for the string, number, boolean, "", 0, false, void, undefined, or null types respectively. Returns
        // no flags for all other types (including non-falsy literal types).
        function getFalsyFlags(type: Type): TypeFlags {
            return type.flags & TypeFlags.Union ? getFalsyFlagsOfTypes((<UnionType>type).types) :
                type.flags & TypeFlags.StringLiteral ? (<LiteralType>type).value === "" ? TypeFlags.StringLiteral : 0 :
                type.flags & TypeFlags.NumberLiteral ? (<LiteralType>type).value === 0 ? TypeFlags.NumberLiteral : 0 :
                type.flags & TypeFlags.BooleanLiteral ? type === falseType ? TypeFlags.BooleanLiteral : 0 :
                type.flags & TypeFlags.PossiblyFalsy;
        }

        function removeDefinitelyFalsyTypes(type: Type): Type {
            return getFalsyFlags(type) & TypeFlags.DefinitelyFalsy ?
                filterType(type, t => !(getFalsyFlags(t) & TypeFlags.DefinitelyFalsy)) :
                type;
        }

        function extractDefinitelyFalsyTypes(type: Type): Type {
            return mapType(type, getDefinitelyFalsyPartOfType);
        }

        function getDefinitelyFalsyPartOfType(type: Type): Type {
            return type.flags & TypeFlags.String ? emptyStringType :
                type.flags & TypeFlags.Number ? zeroType :
                type.flags & TypeFlags.Boolean || type === falseType ? falseType :
                type.flags & (TypeFlags.Void | TypeFlags.Undefined | TypeFlags.Null) ||
                type.flags & TypeFlags.StringLiteral && (<LiteralType>type).value === "" ||
                type.flags & TypeFlags.NumberLiteral && (<LiteralType>type).value === 0 ? type :
                neverType;
        }

        /**
         * Add undefined or null or both to a type if they are missing.
         * @param type - type to add undefined and/or null to if not present
         * @param flags - Either TypeFlags.Undefined or TypeFlags.Null, or both
         */
        function getNullableType(type: Type, flags: TypeFlags): Type {
            const missing = (flags & ~type.flags) & (TypeFlags.Undefined | TypeFlags.Null);
            return missing === 0 ? type :
                missing === TypeFlags.Undefined ? getUnionType([type, undefinedType]) :
                missing === TypeFlags.Null ? getUnionType([type, nullType]) :
                getUnionType([type, undefinedType, nullType]);
        }

        function getOptionalType(type: Type): Type {
            Debug.assert(strictNullChecks);
            return type.flags & TypeFlags.Undefined ? type : getUnionType([type, undefinedType]);
        }

        function getGlobalNonNullableTypeInstantiation(type: Type) {
            if (!deferredGlobalNonNullableTypeAlias) {
                deferredGlobalNonNullableTypeAlias = getGlobalSymbol("NonNullable" as __String, SymbolFlags.TypeAlias, /*diagnostic*/ undefined) || unknownSymbol;
            }
            // Use NonNullable global type alias if available to improve quick info/declaration emit
            if (deferredGlobalNonNullableTypeAlias !== unknownSymbol) {
                return getTypeAliasInstantiation(deferredGlobalNonNullableTypeAlias, [type]);
            }
            return getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull); // Type alias unavailable, fall back to non-higher-order behavior
        }

        function getNonNullableType(type: Type): Type {
            return strictNullChecks ? getGlobalNonNullableTypeInstantiation(type) : type;
        }

        /**
         * Return true if type was inferred from an object literal, written as an object type literal, or is the shape of a module
         * with no call or construct signatures.
         */
        function isObjectTypeWithInferableIndex(type: Type) {
            return type.symbol && (type.symbol.flags & (SymbolFlags.ObjectLiteral | SymbolFlags.TypeLiteral | SymbolFlags.ValueModule)) !== 0 &&
                !typeHasCallOrConstructSignatures(type);
        }

        function createSymbolWithType(source: Symbol, type: Type | undefined) {
            const symbol = createSymbol(source.flags, source.escapedName);
            symbol.declarations = source.declarations;
            symbol.parent = source.parent;
            symbol.type = type;
            symbol.target = source;
            if (source.valueDeclaration) {
                symbol.valueDeclaration = source.valueDeclaration;
            }
            if (source.nameType) {
                symbol.nameType = source.nameType;
            }
            return symbol;
        }

        function transformTypeOfMembers(type: Type, f: (propertyType: Type) => Type) {
            const members = createSymbolTable();
            for (const property of getPropertiesOfObjectType(type)) {
                const original = getTypeOfSymbol(property);
                const updated = f(original);
                members.set(property.escapedName, updated === original ? property : createSymbolWithType(property, updated));
            }
            return members;
        }

        /**
         * If the the provided object literal is subject to the excess properties check,
         * create a new that is exempt. Recursively mark object literal members as exempt.
         * Leave signatures alone since they are not subject to the check.
         */
        function getRegularTypeOfObjectLiteral(type: Type): Type {
            if (!(isObjectLiteralType(type) && type.flags & TypeFlags.FreshLiteral)) {
                return type;
            }
            const regularType = (<FreshObjectLiteralType>type).regularType;
            if (regularType) {
                return regularType;
            }

            const resolved = <ResolvedType>type;
            const members = transformTypeOfMembers(type, getRegularTypeOfObjectLiteral);
            const regularNew = createAnonymousType(resolved.symbol,
                members,
                resolved.callSignatures,
                resolved.constructSignatures,
                resolved.stringIndexInfo,
                resolved.numberIndexInfo);
            regularNew.flags = resolved.flags & ~TypeFlags.FreshLiteral;
            regularNew.objectFlags |= ObjectFlags.ObjectLiteral | (getObjectFlags(resolved) & ObjectFlags.JSLiteral);
            (<FreshObjectLiteralType>type).regularType = regularNew;
            return regularNew;
        }

        function createWideningContext(parent: WideningContext | undefined, propertyName: __String | undefined, siblings: Type[] | undefined): WideningContext {
            return { parent, propertyName, siblings, resolvedProperties: undefined };
        }

        function getSiblingsOfContext(context: WideningContext): Type[] {
            if (!context.siblings) {
                const siblings: Type[] = [];
                for (const type of getSiblingsOfContext(context.parent!)) {
                    if (isObjectLiteralType(type)) {
                        const prop = getPropertyOfObjectType(type, context.propertyName!);
                        if (prop) {
                            forEachType(getTypeOfSymbol(prop), t => {
                                siblings.push(t);
                            });
                        }
                    }
                }
                context.siblings = siblings;
            }
            return context.siblings;
        }

        function getPropertiesOfContext(context: WideningContext): Symbol[] {
            if (!context.resolvedProperties) {
                const names = createMap<Symbol>() as UnderscoreEscapedMap<Symbol>;
                for (const t of getSiblingsOfContext(context)) {
                    if (isObjectLiteralType(t) && !(getObjectFlags(t) & ObjectFlags.ContainsSpread)) {
                        for (const prop of getPropertiesOfType(t)) {
                            names.set(prop.escapedName, prop);
                        }
                    }
                }
                context.resolvedProperties = arrayFrom(names.values());
            }
            return context.resolvedProperties;
        }

        function getWidenedProperty(prop: Symbol, context: WideningContext | undefined): Symbol {
            if (!(prop.flags & SymbolFlags.Property)) {
                // Since get accessors already widen their return value there is no need to
                // widen accessor based properties here.
                return prop;
            }
            const original = getTypeOfSymbol(prop);
            const propContext = context && createWideningContext(context, prop.escapedName, /*siblings*/ undefined);
            const widened = getWidenedTypeWithContext(original, propContext);
            return widened === original ? prop : createSymbolWithType(prop, widened);
        }

        function getUndefinedProperty(prop: Symbol) {
            const cached = undefinedProperties.get(prop.escapedName);
            if (cached) {
                return cached;
            }
            const result = createSymbolWithType(prop, undefinedType);
            result.flags |= SymbolFlags.Optional;
            undefinedProperties.set(prop.escapedName, result);
            return result;
        }

        function getWidenedTypeOfObjectLiteral(type: Type, context: WideningContext | undefined): Type {
            const members = createSymbolTable();
            for (const prop of getPropertiesOfObjectType(type)) {
                members.set(prop.escapedName, getWidenedProperty(prop, context));
            }
            if (context) {
                for (const prop of getPropertiesOfContext(context)) {
                    if (!members.has(prop.escapedName)) {
                        members.set(prop.escapedName, getUndefinedProperty(prop));
                    }
                }
            }
            const stringIndexInfo = getIndexInfoOfType(type, IndexKind.String);
            const numberIndexInfo = getIndexInfoOfType(type, IndexKind.Number);
            const result = createAnonymousType(type.symbol, members, emptyArray, emptyArray,
                stringIndexInfo && createIndexInfo(getWidenedType(stringIndexInfo.type), stringIndexInfo.isReadonly),
                numberIndexInfo && createIndexInfo(getWidenedType(numberIndexInfo.type), numberIndexInfo.isReadonly));
            result.objectFlags |= (getObjectFlags(type) & ObjectFlags.JSLiteral); // Retain js literal flag through widening
            return result;
        }

        function getWidenedType(type: Type) {
            return getWidenedTypeWithContext(type, /*context*/ undefined);
        }

        function getWidenedTypeWithContext(type: Type, context: WideningContext | undefined): Type {
            if (type.flags & TypeFlags.RequiresWidening) {
                if (type.flags & TypeFlags.Nullable) {
                    return anyType;
                }
                if (isObjectLiteralType(type)) {
                    return getWidenedTypeOfObjectLiteral(type, context);
                }
                if (type.flags & TypeFlags.Union) {
                    const unionContext = context || createWideningContext(/*parent*/ undefined, /*propertyName*/ undefined, (<UnionType>type).types);
                    const widenedTypes = sameMap((<UnionType>type).types, t => t.flags & TypeFlags.Nullable ? t : getWidenedTypeWithContext(t, unionContext));
                    // Widening an empty object literal transitions from a highly restrictive type to
                    // a highly inclusive one. For that reason we perform subtype reduction here if the
                    // union includes empty object types (e.g. reducing {} | string to just {}).
                    return getUnionType(widenedTypes, some(widenedTypes, isEmptyObjectType) ? UnionReduction.Subtype : UnionReduction.Literal);
                }
                if (isArrayType(type) || isTupleType(type)) {
                    return createTypeReference((<TypeReference>type).target, sameMap((<TypeReference>type).typeArguments, getWidenedType));
                }
            }
            return type;
        }

        /**
         * Reports implicit any errors that occur as a result of widening 'null' and 'undefined'
         * to 'any'. A call to reportWideningErrorsInType is normally accompanied by a call to
         * getWidenedType. But in some cases getWidenedType is called without reporting errors
         * (type argument inference is an example).
         *
         * The return value indicates whether an error was in fact reported. The particular circumstances
         * are on a best effort basis. Currently, if the null or undefined that causes widening is inside
         * an object literal property (arbitrarily deeply), this function reports an error. If no error is
         * reported, reportImplicitAnyError is a suitable fallback to report a general error.
         */
        function reportWideningErrorsInType(type: Type): boolean {
            let errorReported = false;
            if (type.flags & TypeFlags.ContainsWideningType) {
                if (type.flags & TypeFlags.Union) {
                    if (some((<UnionType>type).types, isEmptyObjectType)) {
                        errorReported = true;
                    }
                    else {
                        for (const t of (<UnionType>type).types) {
                            if (reportWideningErrorsInType(t)) {
                                errorReported = true;
                            }
                        }
                    }
                }
                if (isArrayType(type) || isTupleType(type)) {
                    for (const t of (<TypeReference>type).typeArguments!) {
                        if (reportWideningErrorsInType(t)) {
                            errorReported = true;
                        }
                    }
                }
                if (isObjectLiteralType(type)) {
                    for (const p of getPropertiesOfObjectType(type)) {
                        const t = getTypeOfSymbol(p);
                        if (t.flags & TypeFlags.ContainsWideningType) {
                            if (!reportWideningErrorsInType(t)) {
                                error(p.valueDeclaration, Diagnostics.Object_literal_s_property_0_implicitly_has_an_1_type, symbolToString(p), typeToString(getWidenedType(t)));
                            }
                            errorReported = true;
                        }
                    }
                }
            }
            return errorReported;
        }

        function reportImplicitAnyError(declaration: Declaration, type: Type) {
            const typeAsString = typeToString(getWidenedType(type));
            let diagnostic: DiagnosticMessage;
            switch (declaration.kind) {
                case SyntaxKind.BinaryExpression:
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                    diagnostic = Diagnostics.Member_0_implicitly_has_an_1_type;
                    break;
                case SyntaxKind.Parameter:
                    diagnostic = (<ParameterDeclaration>declaration).dotDotDotToken ?
                        Diagnostics.Rest_parameter_0_implicitly_has_an_any_type :
                        Diagnostics.Parameter_0_implicitly_has_an_1_type;
                    break;
                case SyntaxKind.BindingElement:
                    diagnostic = Diagnostics.Binding_element_0_implicitly_has_an_1_type;
                    break;
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    if (!(declaration as NamedDeclaration).name) {
                        error(declaration, Diagnostics.Function_expression_which_lacks_return_type_annotation_implicitly_has_an_0_return_type, typeAsString);
                        return;
                    }
                    diagnostic = Diagnostics._0_which_lacks_return_type_annotation_implicitly_has_an_1_return_type;
                    break;
                case SyntaxKind.MappedType:
                    error(declaration, Diagnostics.Mapped_object_type_implicitly_has_an_any_template_type);
                    return;
                default:
                    diagnostic = Diagnostics.Variable_0_implicitly_has_an_1_type;
            }
            error(declaration, diagnostic, declarationNameToString(getNameOfDeclaration(declaration)), typeAsString);
        }

        function reportErrorsFromWidening(declaration: Declaration, type: Type) {
            if (produceDiagnostics && noImplicitAny && type.flags & TypeFlags.ContainsWideningType) {
                // Report implicit any error within type if possible, otherwise report error on declaration
                if (!reportWideningErrorsInType(type)) {
                    reportImplicitAnyError(declaration, type);
                }
            }
        }

        function forEachMatchingParameterType(source: Signature, target: Signature, callback: (s: Type, t: Type) => void) {
            const sourceCount = getParameterCount(source);
            const targetCount = getParameterCount(target);
            const sourceRestType = getEffectiveRestType(source);
            const targetRestType = getEffectiveRestType(target);
            const paramCount = targetRestType ? Math.min(targetCount - 1, sourceCount) :
                sourceRestType ? targetCount :
                Math.min(sourceCount, targetCount);

            const sourceThisType = getThisTypeOfSignature(source);
            if (sourceThisType) {
                const targetThisType = getThisTypeOfSignature(target);
                if (targetThisType) {
                    callback(sourceThisType, targetThisType);
                }
            }
            for (let i = 0; i < paramCount; i++) {
                callback(getTypeAtPosition(source, i), getTypeAtPosition(target, i));
            }
            if (targetRestType) {
                callback(getRestTypeAtPosition(source, paramCount), targetRestType);
            }
        }

        function createInferenceContext(typeParameters: ReadonlyArray<TypeParameter>, signature: Signature | undefined, flags: InferenceFlags, compareTypes?: TypeComparer, baseInferences?: InferenceInfo[]): InferenceContext {
            const inferences = baseInferences ? baseInferences.map(cloneInferenceInfo) : typeParameters.map(createInferenceInfo);
            const context = mapper as InferenceContext;
            context.typeParameters = typeParameters;
            context.signature = signature;
            context.inferences = inferences;
            context.flags = flags;
            context.compareTypes = compareTypes || compareTypesAssignable;
            return context;

            function mapper(t: Type): Type {
                for (let i = 0; i < inferences.length; i++) {
                    if (t === inferences[i].typeParameter) {
                        inferences[i].isFixed = true;
                        return getInferredType(context, i);
                    }
                }
                return t;
            }
        }

        function createInferenceInfo(typeParameter: TypeParameter): InferenceInfo {
            return {
                typeParameter,
                candidates: undefined,
                contraCandidates: undefined,
                inferredType: undefined,
                priority: undefined,
                topLevel: true,
                isFixed: false
            };
        }

        function cloneInferenceInfo(inference: InferenceInfo): InferenceInfo {
            return {
                typeParameter: inference.typeParameter,
                candidates: inference.candidates && inference.candidates.slice(),
                contraCandidates: inference.contraCandidates && inference.contraCandidates.slice(),
                inferredType: inference.inferredType,
                priority: inference.priority,
                topLevel: inference.topLevel,
                isFixed: inference.isFixed
            };
        }

        // Return true if the given type could possibly reference a type parameter for which
        // we perform type inference (i.e. a type parameter of a generic function). We cache
        // results for union and intersection types for performance reasons.
        function couldContainTypeVariables(type: Type): boolean {
            const objectFlags = getObjectFlags(type);
            return !!(type.flags & TypeFlags.Instantiable ||
                objectFlags & ObjectFlags.Reference && forEach((<TypeReference>type).typeArguments, couldContainTypeVariables) ||
                objectFlags & ObjectFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class) ||
                objectFlags & ObjectFlags.Mapped ||
                type.flags & TypeFlags.UnionOrIntersection && couldUnionOrIntersectionContainTypeVariables(<UnionOrIntersectionType>type));
        }

        function couldUnionOrIntersectionContainTypeVariables(type: UnionOrIntersectionType): boolean {
            if (type.couldContainTypeVariables === undefined) {
                type.couldContainTypeVariables = some(type.types, couldContainTypeVariables);
            }
            return type.couldContainTypeVariables;
        }

        function isTypeParameterAtTopLevel(type: Type, typeParameter: TypeParameter): boolean {
            return type === typeParameter || !!(type.flags & TypeFlags.UnionOrIntersection) && some((<UnionOrIntersectionType>type).types, t => isTypeParameterAtTopLevel(t, typeParameter));
        }

        /** Create an object with properties named in the string literal type. Every property has type `any` */
        function createEmptyObjectTypeFromStringLiteral(type: Type) {
            const members = createSymbolTable();
            forEachType(type, t => {
                if (!(t.flags & TypeFlags.StringLiteral)) {
                    return;
                }
                const name = escapeLeadingUnderscores((t as StringLiteralType).value);
                const literalProp = createSymbol(SymbolFlags.Property, name);
                literalProp.type = anyType;
                if (t.symbol) {
                    literalProp.declarations = t.symbol.declarations;
                    literalProp.valueDeclaration = t.symbol.valueDeclaration;
                }
                members.set(name, literalProp);
            });
            const indexInfo = type.flags & TypeFlags.String ? createIndexInfo(emptyObjectType, /*isReadonly*/ false) : undefined;
            return createAnonymousType(undefined, members, emptyArray, emptyArray, indexInfo, undefined);
        }

        /**
         * Infer a suitable input type for a homomorphic mapped type { [P in keyof T]: X }. We construct
         * an object type with the same set of properties as the source type, where the type of each
         * property is computed by inferring from the source property type to X for the type
         * variable T[P] (i.e. we treat the type T[P] as the type variable we're inferring for).
         */
        function inferTypeForHomomorphicMappedType(source: Type, target: MappedType): Type | undefined {
            const key = source.id + "," + target.id;
            if (reverseMappedCache.has(key)) {
                return reverseMappedCache.get(key);
            }
            reverseMappedCache.set(key, undefined);
            const type = createReverseMappedType(source, target);
            reverseMappedCache.set(key, type);
            return type;
        }

        function createReverseMappedType(source: Type, target: MappedType) {
            const properties = getPropertiesOfType(source);
            if (properties.length === 0 && !getIndexInfoOfType(source, IndexKind.String)) {
                return undefined;
            }
            // If any property contains context sensitive functions that have been skipped, the source type
            // is incomplete and we can't infer a meaningful input type.
            for (const prop of properties) {
                if (getTypeOfSymbol(prop).flags & TypeFlags.ContainsAnyFunctionType) {
                    return undefined;
                }
            }
            // For arrays and tuples we infer new arrays and tuples where the reverse mapping has been
            // applied to the element type(s).
            if (isArrayType(source)) {
                return createArrayType(inferReverseMappedType((<TypeReference>source).typeArguments![0], target));
            }
            if (isReadonlyArrayType(source)) {
                return createReadonlyArrayType(inferReverseMappedType((<TypeReference>source).typeArguments![0], target));
            }
            if (isTupleType(source)) {
                const elementTypes = map(source.typeArguments || emptyArray, t => inferReverseMappedType(t, target));
                const minLength = getMappedTypeModifiers(target) & MappedTypeModifiers.IncludeOptional ?
                    getTypeReferenceArity(source) - (source.target.hasRestElement ? 1 : 0) : source.target.minLength;
                return createTupleType(elementTypes, minLength, source.target.hasRestElement, source.target.associatedNames);
            }
            // For all other object types we infer a new object type where the reverse mapping has been
            // applied to the type of each property.
            const reversed = createObjectType(ObjectFlags.ReverseMapped | ObjectFlags.Anonymous, /*symbol*/ undefined) as ReverseMappedType;
            reversed.source = source;
            reversed.mappedType = target;
            return reversed;
        }

        function getTypeOfReverseMappedSymbol(symbol: ReverseMappedSymbol) {
            return inferReverseMappedType(symbol.propertyType, symbol.mappedType);
        }

        function inferReverseMappedType(sourceType: Type, target: MappedType): Type {
            const typeParameter = <TypeParameter>getIndexedAccessType((<IndexType>getConstraintTypeFromMappedType(target)).type, getTypeParameterFromMappedType(target));
            const templateType = getTemplateTypeFromMappedType(target);
            const inference = createInferenceInfo(typeParameter);
            inferTypes([inference], sourceType, templateType);
            return getTypeFromInference(inference);
        }

        function getUnmatchedProperty(source: Type, target: Type, requireOptionalProperties: boolean) {
            const properties = target.flags & TypeFlags.Intersection ? getPropertiesOfUnionOrIntersectionType(<IntersectionType>target) : getPropertiesOfObjectType(target);
            for (const targetProp of properties) {
                if (requireOptionalProperties || !(targetProp.flags & SymbolFlags.Optional)) {
                    const sourceProp = getPropertyOfType(source, targetProp.escapedName);
                    if (!sourceProp) {
                        return targetProp;
                    }
                }
            }
            return undefined;
        }

        function tupleTypesDefinitelyUnrelated(source: TupleTypeReference, target: TupleTypeReference) {
            return target.target.minLength > source.target.minLength ||
                !getRestTypeOfTupleType(target) && (!!getRestTypeOfTupleType(source) || getLengthOfTupleType(target) < getLengthOfTupleType(source));
        }

        function typesDefinitelyUnrelated(source: Type, target: Type) {
            // Two tuple types with incompatible arities are definitely unrelated.
            // Two object types that each have a property that is unmatched in the other are definitely unrelated.
            return isTupleType(source) && isTupleType(target) && tupleTypesDefinitelyUnrelated(source, target) ||
                !!getUnmatchedProperty(source, target, /*requireOptionalProperties*/ false) && !!getUnmatchedProperty(target, source, /*requireOptionalProperties*/ false);
        }

        function getTypeFromInference(inference: InferenceInfo) {
            return inference.candidates ? getUnionType(inference.candidates, UnionReduction.Subtype) :
                inference.contraCandidates ? getIntersectionType(inference.contraCandidates) :
                emptyObjectType;
        }

        function inferTypes(inferences: InferenceInfo[], originalSource: Type, originalTarget: Type, priority: InferencePriority = 0) {
            let symbolStack: Symbol[];
            let visited: Map<boolean>;
            let contravariant = false;
            let propagationType: Type;
            inferFromTypes(originalSource, originalTarget);

            function inferFromTypes(source: Type, target: Type) {
                if (!couldContainTypeVariables(target)) {
                    return;
                }
                if (source === wildcardType) {
                    // We are inferring from an 'any' type. We want to infer this type for every type parameter
                    // referenced in the target type, so we record it as the propagation type and infer from the
                    // target to itself. Then, as we find candidates we substitute the propagation type.
                    const savePropagationType = propagationType;
                    propagationType = source;
                    inferFromTypes(target, target);
                    propagationType = savePropagationType;
                    return;
                }
                if (source.aliasSymbol && source.aliasTypeArguments && source.aliasSymbol === target.aliasSymbol) {
                    // Source and target are types originating in the same generic type alias declaration.
                    // Simply infer from source type arguments to target type arguments.
                    const sourceTypes = source.aliasTypeArguments;
                    const targetTypes = target.aliasTypeArguments!;
                    for (let i = 0; i < sourceTypes.length; i++) {
                        inferFromTypes(sourceTypes[i], targetTypes[i]);
                    }
                    return;
                }
                if (source.flags & TypeFlags.Union && target.flags & TypeFlags.Union && !(source.flags & TypeFlags.EnumLiteral && target.flags & TypeFlags.EnumLiteral) ||
                    source.flags & TypeFlags.Intersection && target.flags & TypeFlags.Intersection) {
                    // Source and target are both unions or both intersections. If source and target
                    // are the same type, just relate each constituent type to itself.
                    if (source === target) {
                        for (const t of (<UnionOrIntersectionType>source).types) {
                            inferFromTypes(t, t);
                        }
                        return;
                    }
                    // Find each source constituent type that has an identically matching target constituent
                    // type, and for each such type infer from the type to itself. When inferring from a
                    // type to itself we effectively find all type parameter occurrences within that type
                    // and infer themselves as their type arguments. We have special handling for numeric
                    // and string literals because the number and string types are not represented as unions
                    // of all their possible values.
                    let matchingTypes: Type[] | undefined;
                    for (const t of (<UnionOrIntersectionType>source).types) {
                        if (typeIdenticalToSomeType(t, (<UnionOrIntersectionType>target).types)) {
                            (matchingTypes || (matchingTypes = [])).push(t);
                            inferFromTypes(t, t);
                        }
                        else if (t.flags & (TypeFlags.NumberLiteral | TypeFlags.StringLiteral)) {
                            const b = getBaseTypeOfLiteralType(t);
                            if (typeIdenticalToSomeType(b, (<UnionOrIntersectionType>target).types)) {
                                (matchingTypes || (matchingTypes = [])).push(t, b);
                            }
                        }
                    }
                    // Next, to improve the quality of inferences, reduce the source and target types by
                    // removing the identically matched constituents. For example, when inferring from
                    // 'string | string[]' to 'string | T' we reduce the types to 'string[]' and 'T'.
                    if (matchingTypes) {
                        source = removeTypesFromUnionOrIntersection(<UnionOrIntersectionType>source, matchingTypes);
                        target = removeTypesFromUnionOrIntersection(<UnionOrIntersectionType>target, matchingTypes);
                    }
                }
                if (target.flags & TypeFlags.TypeVariable) {
                    // If target is a type parameter, make an inference, unless the source type contains
                    // the anyFunctionType (the wildcard type that's used to avoid contextually typing functions).
                    // Because the anyFunctionType is internal, it should not be exposed to the user by adding
                    // it as an inference candidate. Hopefully, a better candidate will come along that does
                    // not contain anyFunctionType when we come back to this argument for its second round
                    // of inference. Also, we exclude inferences for silentNeverType (which is used as a wildcard
                    // when constructing types from type parameters that had no inference candidates).
                    if (source.flags & TypeFlags.ContainsAnyFunctionType || source === silentNeverType) {
                        return;
                    }
                    const inference = getInferenceInfoForType(target);
                    if (inference) {
                        if (!inference.isFixed) {
                            if (inference.priority === undefined || priority < inference.priority) {
                                inference.candidates = undefined;
                                inference.contraCandidates = undefined;
                                inference.priority = priority;
                            }
                            if (priority === inference.priority) {
                                const candidate = propagationType || source;
                                if (contravariant) {
                                    inference.contraCandidates = append(inference.contraCandidates, candidate);
                                }
                                else {
                                    inference.candidates = append(inference.candidates, candidate);
                                }
                            }
                            if (!(priority & InferencePriority.ReturnType) && target.flags & TypeFlags.TypeParameter && !isTypeParameterAtTopLevel(originalTarget, <TypeParameter>target)) {
                                inference.topLevel = false;
                            }
                        }
                        return;
                    }
                }
                if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target) {
                    // If source and target are references to the same generic type, infer from type arguments
                    const sourceTypes = (<TypeReference>source).typeArguments || emptyArray;
                    const targetTypes = (<TypeReference>target).typeArguments || emptyArray;
                    const count = sourceTypes.length < targetTypes.length ? sourceTypes.length : targetTypes.length;
                    const variances = getVariances((<TypeReference>source).target);
                    for (let i = 0; i < count; i++) {
                        if (i < variances.length && variances[i] === Variance.Contravariant) {
                            inferFromContravariantTypes(sourceTypes[i], targetTypes[i]);
                        }
                        else {
                            inferFromTypes(sourceTypes[i], targetTypes[i]);
                        }
                    }
                }
                else if (source.flags & TypeFlags.Index && target.flags & TypeFlags.Index) {
                    contravariant = !contravariant;
                    inferFromTypes((<IndexType>source).type, (<IndexType>target).type);
                    contravariant = !contravariant;
                }
                else if ((isLiteralType(source) || source.flags & TypeFlags.String) && target.flags & TypeFlags.Index) {
                    const empty = createEmptyObjectTypeFromStringLiteral(source);
                    contravariant = !contravariant;
                    const savePriority = priority;
                    priority |= InferencePriority.LiteralKeyof;
                    inferFromTypes(empty, (target as IndexType).type);
                    priority = savePriority;
                    contravariant = !contravariant;
                }
                else if (source.flags & TypeFlags.IndexedAccess && target.flags & TypeFlags.IndexedAccess) {
                    inferFromTypes((<IndexedAccessType>source).objectType, (<IndexedAccessType>target).objectType);
                    inferFromTypes((<IndexedAccessType>source).indexType, (<IndexedAccessType>target).indexType);
                }
                else if (source.flags & TypeFlags.Conditional && target.flags & TypeFlags.Conditional) {
                    inferFromTypes((<ConditionalType>source).checkType, (<ConditionalType>target).checkType);
                    inferFromTypes((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType);
                    inferFromTypes(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target));
                    inferFromTypes(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target));
                }
                else if (target.flags & TypeFlags.UnionOrIntersection) {
                    const targetTypes = (<UnionOrIntersectionType>target).types;
                    let typeVariableCount = 0;
                    let typeVariable: TypeParameter | IndexedAccessType | undefined;
                    // First infer to each type in union or intersection that isn't a type variable
                    for (const t of targetTypes) {
                        if (getInferenceInfoForType(t)) {
                            typeVariable = <InstantiableType>t;
                            typeVariableCount++;
                        }
                        else {
                            inferFromTypes(source, t);
                        }
                    }
                    // Next, if target containings a single naked type variable, make a secondary inference to that type
                    // variable. This gives meaningful results for union types in co-variant positions and intersection
                    // types in contra-variant positions (such as callback parameters).
                    if (typeVariableCount === 1) {
                        const savePriority = priority;
                        priority |= InferencePriority.NakedTypeVariable;
                        inferFromTypes(source, typeVariable!);
                        priority = savePriority;
                    }
                }
                else if (source.flags & TypeFlags.Union) {
                    // Source is a union or intersection type, infer from each constituent type
                    const sourceTypes = (<UnionOrIntersectionType>source).types;
                    for (const sourceType of sourceTypes) {
                        inferFromTypes(sourceType, target);
                    }
                }
                else {
                    if (!(priority & InferencePriority.NoConstraints && source.flags & (TypeFlags.Intersection | TypeFlags.Instantiable))) {
                        source = getApparentType(source);
                    }
                    if (source.flags & (TypeFlags.Object | TypeFlags.Intersection)) {
                        const key = source.id + "," + target.id;
                        if (visited && visited.get(key)) {
                            return;
                        }
                        (visited || (visited = createMap<boolean>())).set(key, true);
                        // If we are already processing another target type with the same associated symbol (such as
                        // an instantiation of the same generic type), we do not explore this target as it would yield
                        // no further inferences. We exclude the static side of classes from this check since it shares
                        // its symbol with the instance side which would lead to false positives.
                        const isNonConstructorObject = target.flags & TypeFlags.Object &&
                            !(getObjectFlags(target) & ObjectFlags.Anonymous && target.symbol && target.symbol.flags & SymbolFlags.Class);
                        const symbol = isNonConstructorObject ? target.symbol : undefined;
                        if (symbol) {
                            if (contains(symbolStack, symbol)) {
                                return;
                            }
                            (symbolStack || (symbolStack = [])).push(symbol);
                            inferFromObjectTypes(source, target);
                            symbolStack.pop();
                        }
                        else {
                            inferFromObjectTypes(source, target);
                        }
                    }
                }
            }

            function inferFromContravariantTypes(source: Type, target: Type) {
                if (strictFunctionTypes || priority & InferencePriority.AlwaysStrict) {
                    contravariant = !contravariant;
                    inferFromTypes(source, target);
                    contravariant = !contravariant;
                }
                else {
                    inferFromTypes(source, target);
                }
            }

            function getInferenceInfoForType(type: Type) {
                if (type.flags & TypeFlags.TypeVariable) {
                    for (const inference of inferences) {
                        if (type === inference.typeParameter) {
                            return inference;
                        }
                    }
                }
                return undefined;
            }

            function inferFromObjectTypes(source: Type, target: Type) {
                if (isGenericMappedType(source) && isGenericMappedType(target)) {
                    // The source and target types are generic types { [P in S]: X } and { [P in T]: Y }, so we infer
                    // from S to T and from X to Y.
                    inferFromTypes(getConstraintTypeFromMappedType(source), getConstraintTypeFromMappedType(target));
                    inferFromTypes(getTemplateTypeFromMappedType(source), getTemplateTypeFromMappedType(target));
                }
                if (getObjectFlags(target) & ObjectFlags.Mapped) {
                    const constraintType = getConstraintTypeFromMappedType(<MappedType>target);
                    if (constraintType.flags & TypeFlags.Index) {
                        // We're inferring from some source type S to a homomorphic mapped type { [P in keyof T]: X },
                        // where T is a type variable. Use inferTypeForHomomorphicMappedType to infer a suitable source
                        // type and then make a secondary inference from that type to T. We make a secondary inference
                        // such that direct inferences to T get priority over inferences to Partial<T>, for example.
                        const inference = getInferenceInfoForType((<IndexType>constraintType).type);
                        if (inference && !inference.isFixed) {
                            const inferredType = inferTypeForHomomorphicMappedType(source, <MappedType>target);
                            if (inferredType) {
                                const savePriority = priority;
                                priority |= InferencePriority.HomomorphicMappedType;
                                inferFromTypes(inferredType, inference.typeParameter);
                                priority = savePriority;
                            }
                        }
                        return;
                    }
                    if (constraintType.flags & TypeFlags.TypeParameter) {
                        // We're inferring from some source type S to a mapped type { [P in T]: X }, where T is a type
                        // parameter. Infer from 'keyof S' to T and infer from a union of each property type in S to X.
                        const savePriority = priority;
                        priority |= InferencePriority.MappedTypeConstraint;
                        inferFromTypes(getIndexType(source), constraintType);
                        priority = savePriority;
                        inferFromTypes(getUnionType(map(getPropertiesOfType(source), getTypeOfSymbol)), getTemplateTypeFromMappedType(<MappedType>target));
                        return;
                    }
                }
                // Infer from the members of source and target only if the two types are possibly related
                if (!typesDefinitelyUnrelated(source, target)) {
                    inferFromProperties(source, target);
                    inferFromSignatures(source, target, SignatureKind.Call);
                    inferFromSignatures(source, target, SignatureKind.Construct);
                    inferFromIndexTypes(source, target);
                }
            }

            function inferFromProperties(source: Type, target: Type) {
                if (isTupleType(source)) {
                    if (isTupleType(target)) {
                        const sourceLength = getLengthOfTupleType(source);
                        const targetLength = getLengthOfTupleType(target);
                        const sourceRestType = getRestTypeOfTupleType(source);
                        const targetRestType = getRestTypeOfTupleType(target);
                        const fixedLength = targetLength < sourceLength || sourceRestType ? targetLength : sourceLength;
                        for (let i = 0; i < fixedLength; i++) {
                            inferFromTypes(i < sourceLength ? source.typeArguments![i] : sourceRestType!, target.typeArguments![i]);
                        }
                        if (targetRestType) {
                            const types = fixedLength < sourceLength ? source.typeArguments!.slice(fixedLength, sourceLength) : [];
                            if (sourceRestType) {
                                types.push(sourceRestType);
                            }
                            if (types.length) {
                                inferFromTypes(getUnionType(types), targetRestType);
                            }
                        }
                        return;
                    }
                    if (isArrayType(target)) {
                        inferFromIndexTypes(source, target);
                        return;
                    }
                }
                const properties = getPropertiesOfObjectType(target);
                for (const targetProp of properties) {
                    const sourceProp = getPropertyOfType(source, targetProp.escapedName);
                    if (sourceProp) {
                        inferFromTypes(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp));
                    }
                }
            }

            function inferFromSignatures(source: Type, target: Type, kind: SignatureKind) {
                const sourceSignatures = getSignaturesOfType(source, kind);
                const targetSignatures = getSignaturesOfType(target, kind);
                const sourceLen = sourceSignatures.length;
                const targetLen = targetSignatures.length;
                const len = sourceLen < targetLen ? sourceLen : targetLen;
                const skipParameters = !!(source.flags & TypeFlags.ContainsAnyFunctionType);
                for (let i = 0; i < len; i++) {
                    inferFromSignature(getBaseSignature(sourceSignatures[sourceLen - len + i]), getBaseSignature(targetSignatures[targetLen - len + i]), skipParameters);
                }
            }

            function inferFromSignature(source: Signature, target: Signature, skipParameters: boolean) {
                if (!skipParameters) {
                    forEachMatchingParameterType(source, target, inferFromContravariantTypes);
                }
                const sourceTypePredicate = getTypePredicateOfSignature(source);
                const targetTypePredicate = getTypePredicateOfSignature(target);
                if (sourceTypePredicate && targetTypePredicate && sourceTypePredicate.kind === targetTypePredicate.kind) {
                    inferFromTypes(sourceTypePredicate.type, targetTypePredicate.type);
                }
                else {
                    inferFromTypes(getReturnTypeOfSignature(source), getReturnTypeOfSignature(target));
                }
            }

            function inferFromIndexTypes(source: Type, target: Type) {
                const targetStringIndexType = getIndexTypeOfType(target, IndexKind.String);
                if (targetStringIndexType) {
                    const sourceIndexType = getIndexTypeOfType(source, IndexKind.String) ||
                        getImplicitIndexTypeOfType(source, IndexKind.String);
                    if (sourceIndexType) {
                        inferFromTypes(sourceIndexType, targetStringIndexType);
                    }
                }
                const targetNumberIndexType = getIndexTypeOfType(target, IndexKind.Number);
                if (targetNumberIndexType) {
                    const sourceIndexType = getIndexTypeOfType(source, IndexKind.Number) ||
                        getIndexTypeOfType(source, IndexKind.String) ||
                        getImplicitIndexTypeOfType(source, IndexKind.Number);
                    if (sourceIndexType) {
                        inferFromTypes(sourceIndexType, targetNumberIndexType);
                    }
                }
            }
        }

        function typeIdenticalToSomeType(type: Type, types: Type[]): boolean {
            for (const t of types) {
                if (isTypeIdenticalTo(t, type)) {
                    return true;
                }
            }
            return false;
        }

        /**
         * Return a new union or intersection type computed by removing a given set of types
         * from a given union or intersection type.
         */
        function removeTypesFromUnionOrIntersection(type: UnionOrIntersectionType, typesToRemove: Type[]) {
            const reducedTypes: Type[] = [];
            for (const t of type.types) {
                if (!typeIdenticalToSomeType(t, typesToRemove)) {
                    reducedTypes.push(t);
                }
            }
            return type.flags & TypeFlags.Union ? getUnionType(reducedTypes) : getIntersectionType(reducedTypes);
        }

        function hasPrimitiveConstraint(type: TypeParameter): boolean {
            const constraint = getConstraintOfTypeParameter(type);
            return !!constraint && maybeTypeOfKind(constraint, TypeFlags.Primitive | TypeFlags.Index);
        }

        function isObjectLiteralType(type: Type) {
            return !!(getObjectFlags(type) & ObjectFlags.ObjectLiteral);
        }

        function widenObjectLiteralCandidates(candidates: Type[]): Type[] {
            if (candidates.length > 1) {
                const objectLiterals = filter(candidates, isObjectLiteralType);
                if (objectLiterals.length) {
                    const objectLiteralsType = getWidenedType(getUnionType(objectLiterals, UnionReduction.Subtype));
                    return concatenate(filter(candidates, t => !isObjectLiteralType(t)), [objectLiteralsType]);
                }
            }
            return candidates;
        }

        function getContravariantInference(inference: InferenceInfo) {
            return inference.priority! & InferencePriority.PriorityImpliesCombination ? getIntersectionType(inference.contraCandidates!) : getCommonSubtype(inference.contraCandidates!);
        }

        function getCovariantInference(inference: InferenceInfo, signature: Signature) {
            // Extract all object literal types and replace them with a single widened and normalized type.
            const candidates = widenObjectLiteralCandidates(inference.candidates!);
            // We widen inferred literal types if
            // all inferences were made to top-level occurrences of the type parameter, and
            // the type parameter has no constraint or its constraint includes no primitive or literal types, and
            // the type parameter was fixed during inference or does not occur at top-level in the return type.
            const primitiveConstraint = hasPrimitiveConstraint(inference.typeParameter);
            const widenLiteralTypes = !primitiveConstraint && inference.topLevel &&
                (inference.isFixed || !isTypeParameterAtTopLevel(getReturnTypeOfSignature(signature), inference.typeParameter));
            const baseCandidates = primitiveConstraint ? sameMap(candidates, getRegularTypeOfLiteralType) :
                widenLiteralTypes ? sameMap(candidates, getWidenedLiteralType) :
                candidates;
            // If all inferences were made from a position that implies a combined result, infer a union type.
            // Otherwise, infer a common supertype.
            const unwidenedType = inference.priority! & InferencePriority.PriorityImpliesCombination ?
                getUnionType(baseCandidates, UnionReduction.Subtype) :
                getCommonSupertype(baseCandidates);
            return getWidenedType(unwidenedType);
        }

        function getInferredType(context: InferenceContext, index: number): Type {
            const inference = context.inferences[index];
            let inferredType = inference.inferredType;
            if (!inferredType) {
                const signature = context.signature;
                if (signature) {
                    if (inference.contraCandidates && (!inference.candidates || inference.candidates.length === 1 && inference.candidates[0].flags & TypeFlags.Never)) {
                        // If we have contravariant inferences, but no covariant inferences or a single
                        // covariant inference of 'never', we find the best common subtype and treat that
                        // as a single covariant candidate.
                        inference.candidates = [getContravariantInference(inference)];
                        inference.contraCandidates = undefined;
                    }
                    if (inference.candidates) {
                        inferredType = getCovariantInference(inference, signature);
                    }
                    else if (context.flags & InferenceFlags.NoDefault) {
                        // We use silentNeverType as the wildcard that signals no inferences.
                        inferredType = silentNeverType;
                    }
                    else {
                        // Infer either the default or the empty object type when no inferences were
                        // made. It is important to remember that in this case, inference still
                        // succeeds, meaning there is no error for not having inference candidates. An
                        // inference error only occurs when there are *conflicting* candidates, i.e.
                        // candidates with no common supertype.
                        const defaultType = getDefaultFromTypeParameter(inference.typeParameter);
                        if (defaultType) {
                            // Instantiate the default type. Any forward reference to a type
                            // parameter should be instantiated to the empty object type.
                            inferredType = instantiateType(defaultType,
                                combineTypeMappers(
                                    createBackreferenceMapper(context.signature!.typeParameters!, index),
                                    context));
                        }
                        else {
                            inferredType = getDefaultTypeArgumentType(!!(context.flags & InferenceFlags.AnyDefault));
                        }
                    }
                }
                else {
                    inferredType = getTypeFromInference(inference);
                }

                inference.inferredType = inferredType;

                const constraint = getConstraintOfTypeParameter(inference.typeParameter);
                if (constraint) {
                    const instantiatedConstraint = instantiateType(constraint, context);
                    if (!context.compareTypes(inferredType, getTypeWithThisArgument(instantiatedConstraint, inferredType))) {
                        inference.inferredType = inferredType = instantiatedConstraint;
                    }
                }
            }

            return inferredType;
        }

        function getDefaultTypeArgumentType(isInJavaScriptFile: boolean): Type {
            return isInJavaScriptFile ? anyType : emptyObjectType;
        }

        function getInferredTypes(context: InferenceContext): Type[] {
            const result: Type[] = [];
            for (let i = 0; i < context.inferences.length; i++) {
                result.push(getInferredType(context, i));
            }
            return result;
        }

        // EXPRESSION TYPE CHECKING

        function getCannotFindNameDiagnosticForName(name: __String): DiagnosticMessage {
            switch (name) {
                case "document":
                case "console":
                    return Diagnostics.Cannot_find_name_0_Do_you_need_to_change_your_target_library_Try_changing_the_lib_compiler_option_to_include_dom;
                case "$":
                    return Diagnostics.Cannot_find_name_0_Do_you_need_to_install_type_definitions_for_jQuery_Try_npm_i_types_Slashjquery;
                case "describe":
                case "suite":
                case "it":
                case "test":
                    return Diagnostics.Cannot_find_name_0_Do_you_need_to_install_type_definitions_for_a_test_runner_Try_npm_i_types_Slashjest_or_npm_i_types_Slashmocha;
                case "process":
                case "require":
                case "Buffer":
                case "module":
                    return Diagnostics.Cannot_find_name_0_Do_you_need_to_install_type_definitions_for_node_Try_npm_i_types_Slashnode;
                case "Map":
                case "Set":
                case "Promise":
                case "Symbol":
                case "WeakMap":
                case "WeakSet":
                case "Iterator":
                case "AsyncIterator":
                    return Diagnostics.Cannot_find_name_0_Do_you_need_to_change_your_target_library_Try_changing_the_lib_compiler_option_to_es2015_or_later;
                default: return Diagnostics.Cannot_find_name_0;
            }
        }

        function getResolvedSymbol(node: Identifier): Symbol {
            const links = getNodeLinks(node);
            if (!links.resolvedSymbol) {
                links.resolvedSymbol = !nodeIsMissing(node) &&
                    resolveName(
                        node,
                        node.escapedText,
                        SymbolFlags.Value | SymbolFlags.ExportValue,
                        getCannotFindNameDiagnosticForName(node.escapedText),
                        node,
                        !isWriteOnlyAccess(node),
                        /*excludeGlobals*/ false,
                        Diagnostics.Cannot_find_name_0_Did_you_mean_1) || unknownSymbol;
            }
            return links.resolvedSymbol;
        }

        function isInTypeQuery(node: Node): boolean {
            // TypeScript 1.0 spec (April 2014): 3.6.3
            // A type query consists of the keyword typeof followed by an expression.
            // The expression is restricted to a single identifier or a sequence of identifiers separated by periods
            return !!findAncestor(
                node,
                n => n.kind === SyntaxKind.TypeQuery ? true : n.kind === SyntaxKind.Identifier || n.kind === SyntaxKind.QualifiedName ? false : "quit");
        }

        // Return the flow cache key for a "dotted name" (i.e. a sequence of identifiers
        // separated by dots). The key consists of the id of the symbol referenced by the
        // leftmost identifier followed by zero or more property names separated by dots.
        // The result is undefined if the reference isn't a dotted name. We prefix nodes
        // occurring in an apparent type position with '@' because the control flow type
        // of such nodes may be based on the apparent type instead of the declared type.
        function getFlowCacheKey(node: Node): string | undefined {
            if (node.kind === SyntaxKind.Identifier) {
                const symbol = getResolvedSymbol(<Identifier>node);
                return symbol !== unknownSymbol ? (isConstraintPosition(node) ? "@" : "") + getSymbolId(symbol) : undefined;
            }
            if (node.kind === SyntaxKind.ThisKeyword) {
                return "0";
            }
            if (node.kind === SyntaxKind.PropertyAccessExpression) {
                const key = getFlowCacheKey((<PropertyAccessExpression>node).expression);
                return key && key + "." + idText((<PropertyAccessExpression>node).name);
            }
            if (node.kind === SyntaxKind.BindingElement) {
                const container = (node as BindingElement).parent.parent;
                const key = container.kind === SyntaxKind.BindingElement ? getFlowCacheKey(container) : (container.initializer && getFlowCacheKey(container.initializer));
                const text = getBindingElementNameText(node as BindingElement);
                const result = key && text && (key + "." + text);
                return result;
            }
            return undefined;
        }

        function getBindingElementNameText(element: BindingElement): string | undefined {
            const parent = element.parent;
            if (parent.kind === SyntaxKind.ObjectBindingPattern) {
                const name = element.propertyName || element.name;
                switch (name.kind) {
                    case SyntaxKind.Identifier:
                        return idText(name);
                    case SyntaxKind.ComputedPropertyName:
                        return isStringOrNumericLiteralLike(name.expression) ? name.expression.text : undefined;
                    case SyntaxKind.StringLiteral:
                    case SyntaxKind.NumericLiteral:
                        return name.text;
                    default:
                        // Per types, array and object binding patterns remain, however they should never be present if propertyName is not defined
                        Debug.fail("Unexpected name kind for binding element name");
                }
            }
            else {
                 return "" + parent.elements.indexOf(element);
            }
        }

        function isMatchingReference(source: Node, target: Node): boolean {
            switch (source.kind) {
                case SyntaxKind.Identifier:
                    return target.kind === SyntaxKind.Identifier && getResolvedSymbol(<Identifier>source) === getResolvedSymbol(<Identifier>target) ||
                        (target.kind === SyntaxKind.VariableDeclaration || target.kind === SyntaxKind.BindingElement) &&
                        getExportSymbolOfValueSymbolIfExported(getResolvedSymbol(<Identifier>source)) === getSymbolOfNode(target);
                case SyntaxKind.ThisKeyword:
                    return target.kind === SyntaxKind.ThisKeyword;
                case SyntaxKind.SuperKeyword:
                    return target.kind === SyntaxKind.SuperKeyword;
                case SyntaxKind.PropertyAccessExpression:
                case SyntaxKind.ElementAccessExpression:
                    return (isPropertyAccessExpression(target) || isElementAccessExpression(target)) &&
                        getAccessedPropertyName(source as PropertyAccessExpression | ElementAccessExpression) === getAccessedPropertyName(target) &&
                        isMatchingReference((source as PropertyAccessExpression | ElementAccessExpression).expression, target.expression);
                case SyntaxKind.BindingElement:
                    if (target.kind !== SyntaxKind.PropertyAccessExpression) return false;
                    const t = target as PropertyAccessExpression;
                    if (t.name.escapedText !== getBindingElementNameText(source as BindingElement)) return false;
                    if (source.parent.parent.kind === SyntaxKind.BindingElement && isMatchingReference(source.parent.parent, t.expression)) {
                            return true;
                    }
                    if (source.parent.parent.kind === SyntaxKind.VariableDeclaration) {
                        const maybeId = (source.parent.parent as VariableDeclaration).initializer;
                        return !!maybeId && isMatchingReference(maybeId, t.expression);
                    }
            }
            return false;
        }

        function getAccessedPropertyName(access: PropertyAccessExpression | ElementAccessExpression): __String | undefined {
            return isPropertyAccessExpression(access) ? access.name.escapedText :
                isStringLiteral(access.argumentExpression) || isNumericLiteral(access.argumentExpression) ? escapeLeadingUnderscores(access.argumentExpression.text) :
                undefined;
        }

        function containsMatchingReference(source: Node, target: Node) {
            while (source.kind === SyntaxKind.PropertyAccessExpression) {
                source = (<PropertyAccessExpression>source).expression;
                if (isMatchingReference(source, target)) {
                    return true;
                }
            }
            return false;
        }

        // Return true if target is a property access xxx.yyy, source is a property access xxx.zzz, the declared
        // type of xxx is a union type, and yyy is a property that is possibly a discriminant. We consider a property
        // a possible discriminant if its type differs in the constituents of containing union type, and if every
        // choice is a unit type or a union of unit types.
        function containsMatchingReferenceDiscriminant(source: Node, target: Node) {
            return target.kind === SyntaxKind.PropertyAccessExpression &&
                containsMatchingReference(source, (<PropertyAccessExpression>target).expression) &&
                isDiscriminantProperty(getDeclaredTypeOfReference((<PropertyAccessExpression>target).expression), (<PropertyAccessExpression>target).name.escapedText);
        }

        function getDeclaredTypeOfReference(expr: Node): Type | undefined {
            if (expr.kind === SyntaxKind.Identifier) {
                return getTypeOfSymbol(getResolvedSymbol(<Identifier>expr));
            }
            if (expr.kind === SyntaxKind.PropertyAccessExpression) {
                const type = getDeclaredTypeOfReference((<PropertyAccessExpression>expr).expression);
                return type && getTypeOfPropertyOfType(type, (<PropertyAccessExpression>expr).name.escapedText);
            }
            return undefined;
        }

        function isDiscriminantProperty(type: Type | undefined, name: __String) {
            if (type && type.flags & TypeFlags.Union) {
                const prop = getUnionOrIntersectionProperty(<UnionType>type, name);
                if (prop && getCheckFlags(prop) & CheckFlags.SyntheticProperty) {
                    if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
                        (<TransientSymbol>prop).isDiscriminantProperty = !!((<TransientSymbol>prop).checkFlags & CheckFlags.HasNonUniformType) && isLiteralType(getTypeOfSymbol(prop));
                    }
                    return (<TransientSymbol>prop).isDiscriminantProperty;
                }
            }
            return false;
        }

        function findDiscriminantProperties(sourceProperties: Symbol[], target: Type): Symbol[] | undefined {
            let result: Symbol[] | undefined;
            for (const sourceProperty of sourceProperties) {
                if (isDiscriminantProperty(target, sourceProperty.escapedName)) {
                    if (result) {
                        result.push(sourceProperty);
                        continue;
                    }
                    result = [sourceProperty];
                }
            }
            return result;
        }

        function isOrContainsMatchingReference(source: Node, target: Node) {
            return isMatchingReference(source, target) || containsMatchingReference(source, target);
        }

        function hasMatchingArgument(callExpression: CallExpression, reference: Node) {
            if (callExpression.arguments) {
                for (const argument of callExpression.arguments) {
                    if (isOrContainsMatchingReference(reference, argument)) {
                        return true;
                    }
                }
            }
            if (callExpression.expression.kind === SyntaxKind.PropertyAccessExpression &&
                isOrContainsMatchingReference(reference, (<PropertyAccessExpression>callExpression.expression).expression)) {
                return true;
            }
            return false;
        }

        function getFlowNodeId(flow: FlowNode): number {
            if (!flow.id) {
                flow.id = nextFlowId;
                nextFlowId++;
            }
            return flow.id;
        }

        function typeMaybeAssignableTo(source: Type, target: Type) {
            if (!(source.flags & TypeFlags.Union)) {
                return isTypeAssignableTo(source, target);
            }
            for (const t of (<UnionType>source).types) {
                if (isTypeAssignableTo(t, target)) {
                    return true;
                }
            }
            return false;
        }

        // Remove those constituent types of declaredType to which no constituent type of assignedType is assignable.
        // For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
        // we remove type string.
        function getAssignmentReducedType(declaredType: UnionType, assignedType: Type) {
            if (declaredType !== assignedType) {
                if (assignedType.flags & TypeFlags.Never) {
                    return assignedType;
                }
                const reducedType = filterType(declaredType, t => typeMaybeAssignableTo(assignedType, t));
                // Our crude heuristic produces an invalid result in some cases: see GH#26130.
                // For now, when that happens, we give up and don't narrow at all.  (This also
                // means we'll never narrow for erroneous assignments where the assigned type
                // is not assignable to the declared type.)
                if (isTypeAssignableTo(assignedType, reducedType)) {
                    return reducedType;
                }
            }
            return declaredType;
        }

        function getTypeFactsOfTypes(types: Type[]): TypeFacts {
            let result: TypeFacts = TypeFacts.None;
            for (const t of types) {
                result |= getTypeFacts(t);
            }
            return result;
        }

        function isFunctionObjectType(type: ObjectType): boolean {
            // We do a quick check for a "bind" property before performing the more expensive subtype
            // check. This gives us a quicker out in the common case where an object type is not a function.
            const resolved = resolveStructuredTypeMembers(type);
            return !!(resolved.callSignatures.length || resolved.constructSignatures.length ||
                resolved.members.get("bind" as __String) && isTypeSubtypeOf(type, globalFunctionType));
        }

        function getTypeFacts(type: Type): TypeFacts {
            const flags = type.flags;
            if (flags & TypeFlags.String) {
                return strictNullChecks ? TypeFacts.StringStrictFacts : TypeFacts.StringFacts;
            }
            if (flags & TypeFlags.StringLiteral) {
                const isEmpty = (<LiteralType>type).value === "";
                return strictNullChecks ?
                    isEmpty ? TypeFacts.EmptyStringStrictFacts : TypeFacts.NonEmptyStringStrictFacts :
                    isEmpty ? TypeFacts.EmptyStringFacts : TypeFacts.NonEmptyStringFacts;
            }
            if (flags & (TypeFlags.Number | TypeFlags.Enum)) {
                return strictNullChecks ? TypeFacts.NumberStrictFacts : TypeFacts.NumberFacts;
            }
            if (flags & TypeFlags.NumberLiteral) {
                const isZero = (<LiteralType>type).value === 0;
                return strictNullChecks ?
                    isZero ? TypeFacts.ZeroStrictFacts : TypeFacts.NonZeroStrictFacts :
                    isZero ? TypeFacts.ZeroFacts : TypeFacts.NonZeroFacts;
            }
            if (flags & TypeFlags.Boolean) {
                return strictNullChecks ? TypeFacts.BooleanStrictFacts : TypeFacts.BooleanFacts;
            }
            if (flags & TypeFlags.BooleanLike) {
                return strictNullChecks ?
                    type === falseType ? TypeFacts.FalseStrictFacts : TypeFacts.TrueStrictFacts :
                    type === falseType ? TypeFacts.FalseFacts : TypeFacts.TrueFacts;
            }
            if (flags & TypeFlags.Object) {
                return isFunctionObjectType(<ObjectType>type) ?
                    strictNullChecks ? TypeFacts.FunctionStrictFacts : TypeFacts.FunctionFacts :
                    strictNullChecks ? TypeFacts.ObjectStrictFacts : TypeFacts.ObjectFacts;
            }
            if (flags & (TypeFlags.Void | TypeFlags.Undefined)) {
                return TypeFacts.UndefinedFacts;
            }
            if (flags & TypeFlags.Null) {
                return TypeFacts.NullFacts;
            }
            if (flags & TypeFlags.ESSymbolLike) {
                return strictNullChecks ? TypeFacts.SymbolStrictFacts : TypeFacts.SymbolFacts;
            }
            if (flags & TypeFlags.NonPrimitive) {
                return strictNullChecks ? TypeFacts.ObjectStrictFacts : TypeFacts.ObjectFacts;
            }
            if (flags & TypeFlags.Instantiable) {
                return getTypeFacts(getBaseConstraintOfType(type) || emptyObjectType);
            }
            if (flags & TypeFlags.UnionOrIntersection) {
                return getTypeFactsOfTypes((<UnionOrIntersectionType>type).types);
            }
            return TypeFacts.All;
        }

        function getTypeWithFacts(type: Type, include: TypeFacts) {
            return filterType(type, t => (getTypeFacts(t) & include) !== 0);
        }

        function getTypeWithDefault(type: Type, defaultExpression: Expression) {
            if (defaultExpression) {
                const defaultType = getTypeOfExpression(defaultExpression);
                return getUnionType([getTypeWithFacts(type, TypeFacts.NEUndefined), defaultType]);
            }
            return type;
        }

        function getTypeOfDestructuredProperty(type: Type, name: PropertyName) {
            const text = getTextOfPropertyName(name);
            return getConstraintForLocation(getTypeOfPropertyOfType(type, text), name) ||
                isNumericLiteralName(text) && getIndexTypeOfType(type, IndexKind.Number) ||
                getIndexTypeOfType(type, IndexKind.String) ||
                errorType;
        }

        function getTypeOfDestructuredArrayElement(type: Type, index: number) {
            return isTupleLikeType(type) && getTupleElementType(type, index) ||
                checkIteratedTypeOrElementType(type, /*errorNode*/ undefined, /*allowStringInput*/ false, /*allowAsyncIterables*/ false) ||
                errorType;
        }

        function getTypeOfDestructuredSpreadExpression(type: Type) {
            return createArrayType(checkIteratedTypeOrElementType(type, /*errorNode*/ undefined, /*allowStringInput*/ false, /*allowAsyncIterables*/ false) || errorType);
        }

        function getAssignedTypeOfBinaryExpression(node: BinaryExpression): Type {
            const isDestructuringDefaultAssignment =
                node.parent.kind === SyntaxKind.ArrayLiteralExpression && isDestructuringAssignmentTarget(node.parent) ||
                node.parent.kind === SyntaxKind.PropertyAssignment && isDestructuringAssignmentTarget(node.parent.parent);
            return isDestructuringDefaultAssignment ?
                getTypeWithDefault(getAssignedType(node), node.right) :
                getTypeOfExpression(node.right);
        }

        function isDestructuringAssignmentTarget(parent: Node) {
            return parent.parent.kind === SyntaxKind.BinaryExpression && (parent.parent as BinaryExpression).left === parent ||
                parent.parent.kind === SyntaxKind.ForOfStatement && (parent.parent as ForOfStatement).initializer === parent;
        }

        function getAssignedTypeOfArrayLiteralElement(node: ArrayLiteralExpression, element: Expression): Type {
            return getTypeOfDestructuredArrayElement(getAssignedType(node), node.elements.indexOf(element));
        }

        function getAssignedTypeOfSpreadExpression(node: SpreadElement): Type {
            return getTypeOfDestructuredSpreadExpression(getAssignedType(<ArrayLiteralExpression>node.parent));
        }

        function getAssignedTypeOfPropertyAssignment(node: PropertyAssignment | ShorthandPropertyAssignment): Type {
            return getTypeOfDestructuredProperty(getAssignedType(node.parent), node.name);
        }

        function getAssignedTypeOfShorthandPropertyAssignment(node: ShorthandPropertyAssignment): Type {
            return getTypeWithDefault(getAssignedTypeOfPropertyAssignment(node), node.objectAssignmentInitializer!);
        }

        function getAssignedType(node: Expression): Type {
            const { parent } = node;
            switch (parent.kind) {
                case SyntaxKind.ForInStatement:
                    return stringType;
                case SyntaxKind.ForOfStatement:
                    return checkRightHandSideOfForOf((<ForOfStatement>parent).expression, (<ForOfStatement>parent).awaitModifier) || errorType;
                case SyntaxKind.BinaryExpression:
                    return getAssignedTypeOfBinaryExpression(<BinaryExpression>parent);
                case SyntaxKind.DeleteExpression:
                    return undefinedType;
                case SyntaxKind.ArrayLiteralExpression:
                    return getAssignedTypeOfArrayLiteralElement(<ArrayLiteralExpression>parent, node);
                case SyntaxKind.SpreadElement:
                    return getAssignedTypeOfSpreadExpression(<SpreadElement>parent);
                case SyntaxKind.PropertyAssignment:
                    return getAssignedTypeOfPropertyAssignment(<PropertyAssignment>parent);
                case SyntaxKind.ShorthandPropertyAssignment:
                    return getAssignedTypeOfShorthandPropertyAssignment(<ShorthandPropertyAssignment>parent);
            }
            return errorType;
        }

        function getInitialTypeOfBindingElement(node: BindingElement): Type {
            const pattern = node.parent;
            const parentType = getInitialType(<VariableDeclaration | BindingElement>pattern.parent);
            const type = pattern.kind === SyntaxKind.ObjectBindingPattern ?
                getTypeOfDestructuredProperty(parentType, node.propertyName || <Identifier>node.name) :
                !node.dotDotDotToken ?
                    getTypeOfDestructuredArrayElement(parentType, pattern.elements.indexOf(node)) :
                    getTypeOfDestructuredSpreadExpression(parentType);
            return getTypeWithDefault(type, node.initializer!);
        }

        function getTypeOfInitializer(node: Expression) {
            // Return the cached type if one is available. If the type of the variable was inferred
            // from its initializer, we'll already have cached the type. Otherwise we compute it now
            // without caching such that transient types are reflected.
            const links = getNodeLinks(node);
            return links.resolvedType || getTypeOfExpression(node);
        }

        function getInitialTypeOfVariableDeclaration(node: VariableDeclaration) {
            if (node.initializer) {
                return getTypeOfInitializer(node.initializer);
            }
            if (node.parent.parent.kind === SyntaxKind.ForInStatement) {
                return stringType;
            }
            if (node.parent.parent.kind === SyntaxKind.ForOfStatement) {
                return checkRightHandSideOfForOf(node.parent.parent.expression, node.parent.parent.awaitModifier) || errorType;
            }
            return errorType;
        }

        function getInitialType(node: VariableDeclaration | BindingElement) {
            return node.kind === SyntaxKind.VariableDeclaration ?
                getInitialTypeOfVariableDeclaration(node) :
                getInitialTypeOfBindingElement(node);
        }

        function getInitialOrAssignedType(node: VariableDeclaration | BindingElement | Expression, reference: Node) {
            return getConstraintForLocation(node.kind === SyntaxKind.VariableDeclaration || node.kind === SyntaxKind.BindingElement ?
                getInitialType(<VariableDeclaration | BindingElement>node) :
                getAssignedType(node), reference);
        }

        function isEmptyArrayAssignment(node: VariableDeclaration | BindingElement | Expression) {
            return node.kind === SyntaxKind.VariableDeclaration && (<VariableDeclaration>node).initializer &&
                isEmptyArrayLiteral((<VariableDeclaration>node).initializer!) ||
                node.kind !== SyntaxKind.BindingElement && node.parent.kind === SyntaxKind.BinaryExpression &&
                isEmptyArrayLiteral((<BinaryExpression>node.parent).right);
        }

        function getReferenceCandidate(node: Expression): Expression {
            switch (node.kind) {
                case SyntaxKind.ParenthesizedExpression:
                    return getReferenceCandidate((<ParenthesizedExpression>node).expression);
                case SyntaxKind.BinaryExpression:
                    switch ((<BinaryExpression>node).operatorToken.kind) {
                        case SyntaxKind.EqualsToken:
                            return getReferenceCandidate((<BinaryExpression>node).left);
                        case SyntaxKind.CommaToken:
                            return getReferenceCandidate((<BinaryExpression>node).right);
                    }
            }
            return node;
        }

        function getReferenceRoot(node: Node): Node {
            const { parent } = node;
            return parent.kind === SyntaxKind.ParenthesizedExpression ||
                parent.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>parent).operatorToken.kind === SyntaxKind.EqualsToken && (<BinaryExpression>parent).left === node ||
                parent.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>parent).operatorToken.kind === SyntaxKind.CommaToken && (<BinaryExpression>parent).right === node ?
                getReferenceRoot(parent) : node;
        }

        function getTypeOfSwitchClause(clause: CaseClause | DefaultClause) {
            if (clause.kind === SyntaxKind.CaseClause) {
                return getRegularTypeOfLiteralType(getTypeOfExpression(clause.expression));
            }
            return neverType;
        }

        function getSwitchClauseTypes(switchStatement: SwitchStatement): Type[] {
            const links = getNodeLinks(switchStatement);
            if (!links.switchTypes) {
                links.switchTypes = [];
                for (const clause of switchStatement.caseBlock.clauses) {
                    links.switchTypes.push(getTypeOfSwitchClause(clause));
                }
            }
            return links.switchTypes;
        }

        // Get the types from all cases in a switch on `typeof`. An
        // `undefined` element denotes an explicit `default` clause.
        function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement): (string | undefined)[] {
            const witnesses: (string | undefined)[] = [];
            for (const clause of switchStatement.caseBlock.clauses) {
                if (clause.kind === SyntaxKind.CaseClause) {
                    if (clause.expression.kind === SyntaxKind.StringLiteral) {
                        witnesses.push((clause.expression as StringLiteral).text);
                        continue;
                    }
                    return emptyArray;
                }
                witnesses.push(/*explicitDefaultStatement*/ undefined);
            }
            return witnesses;
        }

        function eachTypeContainedIn(source: Type, types: Type[]) {
            return source.flags & TypeFlags.Union ? !forEach((<UnionType>source).types, t => !contains(types, t)) : contains(types, source);
        }

        function isTypeSubsetOf(source: Type, target: Type) {
            return source === target || target.flags & TypeFlags.Union && isTypeSubsetOfUnion(source, <UnionType>target);
        }

        function isTypeSubsetOfUnion(source: Type, target: UnionType) {
            if (source.flags & TypeFlags.Union) {
                for (const t of (<UnionType>source).types) {
                    if (!containsType(target.types, t)) {
                        return false;
                    }
                }
                return true;
            }
            if (source.flags & TypeFlags.EnumLiteral && getBaseTypeOfEnumLiteralType(<LiteralType>source) === target) {
                return true;
            }
            return containsType(target.types, source);
        }

        function forEachType<T>(type: Type, f: (t: Type) => T | undefined): T | undefined {
            return type.flags & TypeFlags.Union ? forEach((<UnionType>type).types, f) : f(type);
        }

        function filterType(type: Type, f: (t: Type) => boolean): Type {
            if (type.flags & TypeFlags.Union) {
                const types = (<UnionType>type).types;
                const filtered = filter(types, f);
                return filtered === types ? type : getUnionTypeFromSortedList(filtered, type.flags & TypeFlags.UnionOfPrimitiveTypes);
            }
            return f(type) ? type : neverType;
        }

        // Apply a mapping function to a type and return the resulting type. If the source type
        // is a union type, the mapping function is applied to each constituent type and a union
        // of the resulting types is returned.
        function mapType(type: Type, mapper: (t: Type) => Type, noReductions?: boolean): Type;
        function mapType(type: Type, mapper: (t: Type) => Type | undefined, noReductions?: boolean): Type | undefined;
        function mapType(type: Type, mapper: (t: Type) => Type | undefined, noReductions?: boolean): Type | undefined {
            if (type.flags & TypeFlags.Never) {
                return type;
            }
            if (!(type.flags & TypeFlags.Union)) {
                return mapper(type);
            }
            const types = (<UnionType>type).types;
            let mappedType: Type | undefined;
            let mappedTypes: Type[] | undefined;
            for (const current of types) {
                const t = mapper(current);
                if (t) {
                    if (!mappedType) {
                        mappedType = t;
                    }
                    else if (!mappedTypes) {
                        mappedTypes = [mappedType, t];
                    }
                    else {
                        mappedTypes.push(t);
                    }
                }
            }
            return mappedTypes ? getUnionType(mappedTypes, noReductions ? UnionReduction.None : UnionReduction.Literal) : mappedType;
        }

        function extractTypesOfKind(type: Type, kind: TypeFlags) {
            return filterType(type, t => (t.flags & kind) !== 0);
        }

        // Return a new type in which occurrences of the string and number primitive types in
        // typeWithPrimitives have been replaced with occurrences of string literals and numeric
        // literals in typeWithLiterals, respectively.
        function replacePrimitivesWithLiterals(typeWithPrimitives: Type, typeWithLiterals: Type) {
            if (isTypeSubsetOf(stringType, typeWithPrimitives) && maybeTypeOfKind(typeWithLiterals, TypeFlags.StringLiteral) ||
                isTypeSubsetOf(numberType, typeWithPrimitives) && maybeTypeOfKind(typeWithLiterals, TypeFlags.NumberLiteral)) {
                return mapType(typeWithPrimitives, t =>
                    t.flags & TypeFlags.String ? extractTypesOfKind(typeWithLiterals, TypeFlags.String | TypeFlags.StringLiteral) :
                        t.flags & TypeFlags.Number ? extractTypesOfKind(typeWithLiterals, TypeFlags.Number | TypeFlags.NumberLiteral) :
                            t);
            }
            return typeWithPrimitives;
        }

        function isIncomplete(flowType: FlowType) {
            return flowType.flags === 0;
        }

        function getTypeFromFlowType(flowType: FlowType) {
            return flowType.flags === 0 ? (<IncompleteType>flowType).type : <Type>flowType;
        }

        function createFlowType(type: Type, incomplete: boolean): FlowType {
            return incomplete ? { flags: 0, type } : type;
        }

        // An evolving array type tracks the element types that have so far been seen in an
        // 'x.push(value)' or 'x[n] = value' operation along the control flow graph. Evolving
        // array types are ultimately converted into manifest array types (using getFinalArrayType)
        // and never escape the getFlowTypeOfReference function.
        function createEvolvingArrayType(elementType: Type): EvolvingArrayType {
            const result = <EvolvingArrayType>createObjectType(ObjectFlags.EvolvingArray);
            result.elementType = elementType;
            return result;
        }

        function getEvolvingArrayType(elementType: Type): EvolvingArrayType {
            return evolvingArrayTypes[elementType.id] || (evolvingArrayTypes[elementType.id] = createEvolvingArrayType(elementType));
        }

        // When adding evolving array element types we do not perform subtype reduction. Instead,
        // we defer subtype reduction until the evolving array type is finalized into a manifest
        // array type.
        function addEvolvingArrayElementType(evolvingArrayType: EvolvingArrayType, node: Expression): EvolvingArrayType {
            const elementType = getBaseTypeOfLiteralType(getContextFreeTypeOfExpression(node));
            return isTypeSubsetOf(elementType, evolvingArrayType.elementType) ? evolvingArrayType : getEvolvingArrayType(getUnionType([evolvingArrayType.elementType, elementType]));
        }

        function createFinalArrayType(elementType: Type) {
            return elementType.flags & TypeFlags.Never ?
                autoArrayType :
                createArrayType(elementType.flags & TypeFlags.Union ?
                    getUnionType((<UnionType>elementType).types, UnionReduction.Subtype) :
                    elementType);
        }

        // We perform subtype reduction upon obtaining the final array type from an evolving array type.
        function getFinalArrayType(evolvingArrayType: EvolvingArrayType): Type {
            return evolvingArrayType.finalArrayType || (evolvingArrayType.finalArrayType = createFinalArrayType(evolvingArrayType.elementType));
        }

        function finalizeEvolvingArrayType(type: Type): Type {
            return getObjectFlags(type) & ObjectFlags.EvolvingArray ? getFinalArrayType(<EvolvingArrayType>type) : type;
        }

        function getElementTypeOfEvolvingArrayType(type: Type) {
            return getObjectFlags(type) & ObjectFlags.EvolvingArray ? (<EvolvingArrayType>type).elementType : neverType;
        }

        function isEvolvingArrayTypeList(types: Type[]) {
            let hasEvolvingArrayType = false;
            for (const t of types) {
                if (!(t.flags & TypeFlags.Never)) {
                    if (!(getObjectFlags(t) & ObjectFlags.EvolvingArray)) {
                        return false;
                    }
                    hasEvolvingArrayType = true;
                }
            }
            return hasEvolvingArrayType;
        }

        // At flow control branch or loop junctions, if the type along every antecedent code path
        // is an evolving array type, we construct a combined evolving array type. Otherwise we
        // finalize all evolving array types.
        function getUnionOrEvolvingArrayType(types: Type[], subtypeReduction: UnionReduction) {
            return isEvolvingArrayTypeList(types) ?
                getEvolvingArrayType(getUnionType(map(types, getElementTypeOfEvolvingArrayType))) :
                getUnionType(sameMap(types, finalizeEvolvingArrayType), subtypeReduction);
        }

        // Return true if the given node is 'x' in an 'x.length', x.push(value)', 'x.unshift(value)' or
        // 'x[n] = value' operation, where 'n' is an expression of type any, undefined, or a number-like type.
        function isEvolvingArrayOperationTarget(node: Node) {
            const root = getReferenceRoot(node);
            const parent = root.parent;
            const isLengthPushOrUnshift = parent.kind === SyntaxKind.PropertyAccessExpression && (
                (<PropertyAccessExpression>parent).name.escapedText === "length" ||
                parent.parent.kind === SyntaxKind.CallExpression && isPushOrUnshiftIdentifier((<PropertyAccessExpression>parent).name));
            const isElementAssignment = parent.kind === SyntaxKind.ElementAccessExpression &&
                (<ElementAccessExpression>parent).expression === root &&
                parent.parent.kind === SyntaxKind.BinaryExpression &&
                (<BinaryExpression>parent.parent).operatorToken.kind === SyntaxKind.EqualsToken &&
                (<BinaryExpression>parent.parent).left === parent &&
                !isAssignmentTarget(parent.parent) &&
                isTypeAssignableToKind(getTypeOfExpression((<ElementAccessExpression>parent).argumentExpression), TypeFlags.NumberLike);
            return isLengthPushOrUnshift || isElementAssignment;
        }

        function maybeTypePredicateCall(node: CallExpression) {
            const links = getNodeLinks(node);
            if (links.maybeTypePredicate === undefined) {
                links.maybeTypePredicate = getMaybeTypePredicate(node);
            }
            return links.maybeTypePredicate;
        }

        function getMaybeTypePredicate(node: CallExpression) {
            if (node.expression.kind !== SyntaxKind.SuperKeyword) {
                const funcType = checkNonNullExpression(node.expression);
                if (funcType !== silentNeverType) {
                    const apparentType = getApparentType(funcType);
                    return apparentType !== errorType && some(getSignaturesOfType(apparentType, SignatureKind.Call), signatureHasTypePredicate);
                }
            }
            return false;
        }

        function reportFlowControlError(node: Node) {
            const block = <Block | ModuleBlock | SourceFile>findAncestor(node, isFunctionOrModuleBlock);
            const sourceFile = getSourceFileOfNode(node);
            const span = getSpanOfTokenAtPosition(sourceFile, block.statements.pos);
            diagnostics.add(createFileDiagnostic(sourceFile, span.start, span.length, Diagnostics.The_containing_function_or_module_body_is_too_large_for_control_flow_analysis));
        }

        function getFlowTypeOfReference(reference: Node, declaredType: Type, initialType = declaredType, flowContainer?: Node, couldBeUninitialized?: boolean) {
            let key: string | undefined;
            let flowDepth = 0;
            if (flowAnalysisDisabled) {
                return errorType;
            }
            if (!reference.flowNode || !couldBeUninitialized && !(declaredType.flags & TypeFlags.Narrowable)) {
                return declaredType;
            }
            const sharedFlowStart = sharedFlowCount;
            const evolvedType = getTypeFromFlowType(getTypeAtFlowNode(reference.flowNode));
            sharedFlowCount = sharedFlowStart;
            // When the reference is 'x' in an 'x.length', 'x.push(value)', 'x.unshift(value)' or x[n] = value' operation,
            // we give type 'any[]' to 'x' instead of using the type determined by control flow analysis such that operations
            // on empty arrays are possible without implicit any errors and new element types can be inferred without
            // type mismatch errors.
            const resultType = getObjectFlags(evolvedType) & ObjectFlags.EvolvingArray && isEvolvingArrayOperationTarget(reference) ? anyArrayType : finalizeEvolvingArrayType(evolvedType);
            if (reference.parent && reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(resultType, TypeFacts.NEUndefinedOrNull).flags & TypeFlags.Never) {
                return declaredType;
            }
            return resultType;

            function getTypeAtFlowNode(flow: FlowNode): FlowType {
                if (flowDepth === 2000) {
                    // We have made 2000 recursive invocations. To avoid overflowing the call stack we report an error
                    // and disable further control flow analysis in the containing function or module body.
                    flowAnalysisDisabled = true;
                    reportFlowControlError(reference);
                    return errorType;
                }
                flowDepth++;
                while (true) {
                    const flags = flow.flags;
                    if (flags & FlowFlags.Shared) {
                        // We cache results of flow type resolution for shared nodes that were previously visited in
                        // the same getFlowTypeOfReference invocation. A node is considered shared when it is the
                        // antecedent of more than one node.
                        for (let i = sharedFlowStart; i < sharedFlowCount; i++) {
                            if (sharedFlowNodes[i] === flow) {
                                flowDepth--;
                                return sharedFlowTypes[i];
                            }
                        }
                    }
                    let type: FlowType | undefined;
                    if (flags & FlowFlags.AfterFinally) {
                        // block flow edge: finally -> pre-try (for larger explanation check comment in binder.ts - bindTryStatement
                        (<AfterFinallyFlow>flow).locked = true;
                        type = getTypeAtFlowNode((<AfterFinallyFlow>flow).antecedent);
                        (<AfterFinallyFlow>flow).locked = false;
                    }
                    else if (flags & FlowFlags.PreFinally) {
                        // locked pre-finally flows are filtered out in getTypeAtFlowBranchLabel
                        // so here just redirect to antecedent
                        flow = (<PreFinallyFlow>flow).antecedent;
                        continue;
                    }
                    else if (flags & FlowFlags.Assignment) {
                        type = getTypeAtFlowAssignment(<FlowAssignment>flow);
                        if (!type) {
                            flow = (<FlowAssignment>flow).antecedent;
                            continue;
                        }
                    }
                    else if (flags & FlowFlags.Condition) {
                        type = getTypeAtFlowCondition(<FlowCondition>flow);
                    }
                    else if (flags & FlowFlags.SwitchClause) {
                        type = getTypeAtSwitchClause(<FlowSwitchClause>flow);
                    }
                    else if (flags & FlowFlags.Label) {
                        if ((<FlowLabel>flow).antecedents!.length === 1) {
                            flow = (<FlowLabel>flow).antecedents![0];
                            continue;
                        }
                        type = flags & FlowFlags.BranchLabel ?
                            getTypeAtFlowBranchLabel(<FlowLabel>flow) :
                            getTypeAtFlowLoopLabel(<FlowLabel>flow);
                    }
                    else if (flags & FlowFlags.ArrayMutation) {
                        type = getTypeAtFlowArrayMutation(<FlowArrayMutation>flow);
                        if (!type) {
                            flow = (<FlowArrayMutation>flow).antecedent;
                            continue;
                        }
                    }
                    else if (flags & FlowFlags.Start) {
                        // Check if we should continue with the control flow of the containing function.
                        const container = (<FlowStart>flow).container;
                        if (container && container !== flowContainer &&
                            reference.kind !== SyntaxKind.PropertyAccessExpression &&
                            reference.kind !== SyntaxKind.ElementAccessExpression &&
                            reference.kind !== SyntaxKind.ThisKeyword) {
                            flow = container.flowNode!;
                            continue;
                        }
                        // At the top of the flow we have the initial type.
                        type = initialType;
                    }
                    else {
                        // Unreachable code errors are reported in the binding phase. Here we
                        // simply return the non-auto declared type to reduce follow-on errors.
                        type = convertAutoToAny(declaredType);
                    }
                    if (flags & FlowFlags.Shared) {
                        // Record visited node and the associated type in the cache.
                        sharedFlowNodes[sharedFlowCount] = flow;
                        sharedFlowTypes[sharedFlowCount] = type;
                        sharedFlowCount++;
                    }
                    flowDepth--;
                    return type;
                }
            }

            function getTypeAtFlowAssignment(flow: FlowAssignment) {
                const node = flow.node;
                // Assignments only narrow the computed type if the declared type is a union type. Thus, we
                // only need to evaluate the assigned type if the declared type is a union type.
                if (isMatchingReference(reference, node)) {
                    if (getAssignmentTargetKind(node) === AssignmentKind.Compound) {
                        const flowType = getTypeAtFlowNode(flow.antecedent);
                        return createFlowType(getBaseTypeOfLiteralType(getTypeFromFlowType(flowType)), isIncomplete(flowType));
                    }
                    if (declaredType === autoType || declaredType === autoArrayType) {
                        if (isEmptyArrayAssignment(node)) {
                            return getEvolvingArrayType(neverType);
                        }
                        const assignedType = getBaseTypeOfLiteralType(getInitialOrAssignedType(node, reference));
                        return isTypeAssignableTo(assignedType, declaredType) ? assignedType : anyArrayType;
                    }
                    if (declaredType.flags & TypeFlags.Union) {
                        return getAssignmentReducedType(<UnionType>declaredType, getInitialOrAssignedType(node, reference));
                    }
                    return declaredType;
                }
                // We didn't have a direct match. However, if the reference is a dotted name, this
                // may be an assignment to a left hand part of the reference. For example, for a
                // reference 'x.y.z', we may be at an assignment to 'x.y' or 'x'. In that case,
                // return the declared type.
                if (containsMatchingReference(reference, node)) {
                    return declaredType;
                }
                // Assignment doesn't affect reference
                return undefined;
            }

            function getTypeAtFlowArrayMutation(flow: FlowArrayMutation): FlowType | undefined {
                if (declaredType === autoType || declaredType === autoArrayType) {
                    const node = flow.node;
                    const expr = node.kind === SyntaxKind.CallExpression ?
                        (<PropertyAccessExpression>node.expression).expression :
                        (<ElementAccessExpression>node.left).expression;
                    if (isMatchingReference(reference, getReferenceCandidate(expr))) {
                        const flowType = getTypeAtFlowNode(flow.antecedent);
                        const type = getTypeFromFlowType(flowType);
                        if (getObjectFlags(type) & ObjectFlags.EvolvingArray) {
                            let evolvedType = <EvolvingArrayType>type;
                            if (node.kind === SyntaxKind.CallExpression) {
                                for (const arg of node.arguments) {
                                    evolvedType = addEvolvingArrayElementType(evolvedType, arg);
                                }
                            }
                            else {
                                // We must get the context free expression type so as to not recur in an uncached fashion on the LHS (which causes exponential blowup in compile time)
                                const indexType = getContextFreeTypeOfExpression((<ElementAccessExpression>node.left).argumentExpression);
                                if (isTypeAssignableToKind(indexType, TypeFlags.NumberLike)) {
                                    evolvedType = addEvolvingArrayElementType(evolvedType, node.right);
                                }
                            }
                            return evolvedType === type ? flowType : createFlowType(evolvedType, isIncomplete(flowType));
                        }
                        return flowType;
                    }
                }
                return undefined;
            }

            function getTypeAtFlowCondition(flow: FlowCondition): FlowType {
                const flowType = getTypeAtFlowNode(flow.antecedent);
                const type = getTypeFromFlowType(flowType);
                if (type.flags & TypeFlags.Never) {
                    return flowType;
                }
                // If we have an antecedent type (meaning we're reachable in some way), we first
                // attempt to narrow the antecedent type. If that produces the never type, and if
                // the antecedent type is incomplete (i.e. a transient type in a loop), then we
                // take the type guard as an indication that control *could* reach here once we
                // have the complete type. We proceed by switching to the silent never type which
                // doesn't report errors when operators are applied to it. Note that this is the
                // *only* place a silent never type is ever generated.
                const assumeTrue = (flow.flags & FlowFlags.TrueCondition) !== 0;
                const nonEvolvingType = finalizeEvolvingArrayType(type);
                const narrowedType = narrowType(nonEvolvingType, flow.expression, assumeTrue);
                if (narrowedType === nonEvolvingType) {
                    return flowType;
                }
                const incomplete = isIncomplete(flowType);
                const resultType = incomplete && narrowedType.flags & TypeFlags.Never ? silentNeverType : narrowedType;
                return createFlowType(resultType, incomplete);
            }

            function getTypeAtSwitchClause(flow: FlowSwitchClause): FlowType {
                const flowType = getTypeAtFlowNode(flow.antecedent);
                let type = getTypeFromFlowType(flowType);
                const expr = flow.switchStatement.expression;
                if (isMatchingReference(reference, expr)) {
                    type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                }
                else if (isMatchingReferenceDiscriminant(expr, type)) {
                    type = narrowTypeByDiscriminant(
                        type,
                        expr as PropertyAccessExpression | ElementAccessExpression,
                        t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                }
                else if (expr.kind === SyntaxKind.TypeOfExpression && isMatchingReference(reference, (expr as TypeOfExpression).expression)) {
                    type = narrowBySwitchOnTypeOf(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                }
                return createFlowType(type, isIncomplete(flowType));
            }

            function getTypeAtFlowBranchLabel(flow: FlowLabel): FlowType {
                const antecedentTypes: Type[] = [];
                let subtypeReduction = false;
                let seenIncomplete = false;
                for (const antecedent of flow.antecedents!) {
                    if (antecedent.flags & FlowFlags.PreFinally && (<PreFinallyFlow>antecedent).lock.locked) {
                        // if flow correspond to branch from pre-try to finally and this branch is locked - this means that
                        // we initially have started following the flow outside the finally block.
                        // in this case we should ignore this branch.
                        continue;
                    }
                    const flowType = getTypeAtFlowNode(antecedent);
                    const type = getTypeFromFlowType(flowType);
                    // If the type at a particular antecedent path is the declared type and the
                    // reference is known to always be assigned (i.e. when declared and initial types
                    // are the same), there is no reason to process more antecedents since the only
                    // possible outcome is subtypes that will be removed in the final union type anyway.
                    if (type === declaredType && declaredType === initialType) {
                        return type;
                    }
                    pushIfUnique(antecedentTypes, type);
                    // If an antecedent type is not a subset of the declared type, we need to perform
                    // subtype reduction. This happens when a "foreign" type is injected into the control
                    // flow using the instanceof operator or a user defined type predicate.
                    if (!isTypeSubsetOf(type, declaredType)) {
                        subtypeReduction = true;
                    }
                    if (isIncomplete(flowType)) {
                        seenIncomplete = true;
                    }
                }
                return createFlowType(getUnionOrEvolvingArrayType(antecedentTypes, subtypeReduction ? UnionReduction.Subtype : UnionReduction.Literal), seenIncomplete);
            }

            function getTypeAtFlowLoopLabel(flow: FlowLabel): FlowType {
                // If we have previously computed the control flow type for the reference at
                // this flow loop junction, return the cached type.
                const id = getFlowNodeId(flow);
                const cache = flowLoopCaches[id] || (flowLoopCaches[id] = createMap<Type>());
                if (!key) {
                    key = getFlowCacheKey(reference);
                    // No cache key is generated when binding patterns are in unnarrowable situations
                    if (!key) {
                        return declaredType;
                    }
                }
                const cached = cache.get(key);
                if (cached) {
                    return cached;
                }
                // If this flow loop junction and reference are already being processed, return
                // the union of the types computed for each branch so far, marked as incomplete.
                // It is possible to see an empty array in cases where loops are nested and the
                // back edge of the outer loop reaches an inner loop that is already being analyzed.
                // In such cases we restart the analysis of the inner loop, which will then see
                // a non-empty in-process array for the outer loop and eventually terminate because
                // the first antecedent of a loop junction is always the non-looping control flow
                // path that leads to the top.
                for (let i = flowLoopStart; i < flowLoopCount; i++) {
                    if (flowLoopNodes[i] === flow && flowLoopKeys[i] === key && flowLoopTypes[i].length) {
                        return createFlowType(getUnionOrEvolvingArrayType(flowLoopTypes[i], UnionReduction.Literal), /*incomplete*/ true);
                    }
                }
                // Add the flow loop junction and reference to the in-process stack and analyze
                // each antecedent code path.
                const antecedentTypes: Type[] = [];
                let subtypeReduction = false;
                let firstAntecedentType: FlowType | undefined;
                flowLoopNodes[flowLoopCount] = flow;
                flowLoopKeys[flowLoopCount] = key;
                flowLoopTypes[flowLoopCount] = antecedentTypes;
                for (const antecedent of flow.antecedents!) {
                    flowLoopCount++;
                    const flowType = getTypeAtFlowNode(antecedent);
                    flowLoopCount--;
                    if (!firstAntecedentType) {
                        firstAntecedentType = flowType;
                    }
                    const type = getTypeFromFlowType(flowType);
                    // If we see a value appear in the cache it is a sign that control flow analysis
                    // was restarted and completed by checkExpressionCached. We can simply pick up
                    // the resulting type and bail out.
                    const cached = cache.get(key);
                    if (cached) {
                        return cached;
                    }
                    pushIfUnique(antecedentTypes, type);
                    // If an antecedent type is not a subset of the declared type, we need to perform
                    // subtype reduction. This happens when a "foreign" type is injected into the control
                    // flow using the instanceof operator or a user defined type predicate.
                    if (!isTypeSubsetOf(type, declaredType)) {
                        subtypeReduction = true;
                    }
                    // If the type at a particular antecedent path is the declared type there is no
                    // reason to process more antecedents since the only possible outcome is subtypes
                    // that will be removed in the final union type anyway.
                    if (type === declaredType) {
                        break;
                    }
                }
                // The result is incomplete if the first antecedent (the non-looping control flow path)
                // is incomplete.
                const result = getUnionOrEvolvingArrayType(antecedentTypes, subtypeReduction ? UnionReduction.Subtype : UnionReduction.Literal);
                if (isIncomplete(firstAntecedentType!)) {
                    return createFlowType(result, /*incomplete*/ true);
                }
                cache.set(key, result);
                return result;
            }

            function isMatchingReferenceDiscriminant(expr: Expression, computedType: Type) {
                if (!(computedType.flags & TypeFlags.Union) ||
                    expr.kind !== SyntaxKind.PropertyAccessExpression && expr.kind !== SyntaxKind.ElementAccessExpression) {
                    return false;
                }
                const access = expr as PropertyAccessExpression | ElementAccessExpression;
                const name = getAccessedPropertyName(access);
                if (!name) {
                    return false;
                }
                return isMatchingReference(reference, access.expression) && isDiscriminantProperty(computedType, name);
            }

            function narrowTypeByDiscriminant(type: Type, access: PropertyAccessExpression | ElementAccessExpression, narrowType: (t: Type) => Type): Type {
                const propName = getAccessedPropertyName(access);
                if (!propName) {
                    return type;
                }
                const propType = getTypeOfPropertyOfType(type, propName);
                const narrowedPropType = propType && narrowType(propType);
                return propType === narrowedPropType ? type : filterType(type, t => isTypeComparableTo(getTypeOfPropertyOfType(t, propName)!, narrowedPropType!));
            }

            function narrowTypeByTruthiness(type: Type, expr: Expression, assumeTrue: boolean): Type {
                if (isMatchingReference(reference, expr)) {
                    return getTypeWithFacts(type, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy);
                }
                if (isMatchingReferenceDiscriminant(expr, declaredType)) {
                    return narrowTypeByDiscriminant(type, <PropertyAccessExpression | ElementAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
                }
                if (containsMatchingReferenceDiscriminant(reference, expr)) {
                    return declaredType;
                }
                return type;
            }

            function isTypePresencePossible(type: Type, propName: __String, assumeTrue: boolean) {
                if (getIndexInfoOfType(type, IndexKind.String)) {
                    return true;
                }
                const prop = getPropertyOfType(type, propName);
                if (prop) {
                    return prop.flags & SymbolFlags.Optional ? true : assumeTrue;
                }
                return !assumeTrue;
            }

            function narrowByInKeyword(type: Type, literal: LiteralExpression, assumeTrue: boolean) {
                if ((type.flags & (TypeFlags.Union | TypeFlags.Object)) || (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType)) {
                    const propName = escapeLeadingUnderscores(literal.text);
                    return filterType(type, t => isTypePresencePossible(t, propName, assumeTrue));
                }
                return type;
            }

            function narrowTypeByBinaryExpression(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                switch (expr.operatorToken.kind) {
                    case SyntaxKind.EqualsToken:
                        return narrowTypeByTruthiness(type, expr.left, assumeTrue);
                    case SyntaxKind.EqualsEqualsToken:
                    case SyntaxKind.ExclamationEqualsToken:
                    case SyntaxKind.EqualsEqualsEqualsToken:
                    case SyntaxKind.ExclamationEqualsEqualsToken:
                        const operator = expr.operatorToken.kind;
                        const left = getReferenceCandidate(expr.left);
                        const right = getReferenceCandidate(expr.right);
                        if (left.kind === SyntaxKind.TypeOfExpression && isStringLiteralLike(right)) {
                            return narrowTypeByTypeof(type, <TypeOfExpression>left, operator, right, assumeTrue);
                        }
                        if (right.kind === SyntaxKind.TypeOfExpression && isStringLiteralLike(left)) {
                            return narrowTypeByTypeof(type, <TypeOfExpression>right, operator, left, assumeTrue);
                        }
                        if (isMatchingReference(reference, left)) {
                            return narrowTypeByEquality(type, operator, right, assumeTrue);
                        }
                        if (isMatchingReference(reference, right)) {
                            return narrowTypeByEquality(type, operator, left, assumeTrue);
                        }
                        if (isMatchingReferenceDiscriminant(left, declaredType)) {
                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression | ElementAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
                        }
                        if (isMatchingReferenceDiscriminant(right, declaredType)) {
                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression | ElementAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
                        }
                        if (containsMatchingReferenceDiscriminant(reference, left) || containsMatchingReferenceDiscriminant(reference, right)) {
                            return declaredType;
                        }
                        break;
                    case SyntaxKind.InstanceOfKeyword:
                        return narrowTypeByInstanceof(type, expr, assumeTrue);
                    case SyntaxKind.InKeyword:
                        const target = getReferenceCandidate(expr.right);
                        if (isStringLiteralLike(expr.left) && isMatchingReference(reference, target)) {
                            return narrowByInKeyword(type, expr.left, assumeTrue);
                        }
                        break;
                    case SyntaxKind.CommaToken:
                        return narrowType(type, expr.right, assumeTrue);
                }
                return type;
            }

            function narrowTypeByEquality(type: Type, operator: SyntaxKind, value: Expression, assumeTrue: boolean): Type {
                if (type.flags & TypeFlags.Any) {
                    return type;
                }
                if (operator === SyntaxKind.ExclamationEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken) {
                    assumeTrue = !assumeTrue;
                }
                const valueType = getTypeOfExpression(value);
                if (valueType.flags & TypeFlags.Nullable) {
                    if (!strictNullChecks) {
                        return type;
                    }
                    const doubleEquals = operator === SyntaxKind.EqualsEqualsToken || operator === SyntaxKind.ExclamationEqualsToken;
                    const facts = doubleEquals ?
                        assumeTrue ? TypeFacts.EQUndefinedOrNull : TypeFacts.NEUndefinedOrNull :
                        valueType.flags & TypeFlags.Null ?
                            assumeTrue ? TypeFacts.EQNull : TypeFacts.NENull :
                            assumeTrue ? TypeFacts.EQUndefined : TypeFacts.NEUndefined;
                    return getTypeWithFacts(type, facts);
                }
                if (type.flags & TypeFlags.NotUnionOrUnit) {
                    return type;
                }
                if (assumeTrue) {
                    const narrowedType = filterType(type, t => areTypesComparable(t, valueType));
                    return narrowedType.flags & TypeFlags.Never ? type : replacePrimitivesWithLiterals(narrowedType, valueType);
                }
                if (isUnitType(valueType)) {
                    const regularType = getRegularTypeOfLiteralType(valueType);
                    return filterType(type, t => getRegularTypeOfLiteralType(t) !== regularType);
                }
                return type;
            }

            function narrowTypeByTypeof(type: Type, typeOfExpr: TypeOfExpression, operator: SyntaxKind, literal: LiteralExpression, assumeTrue: boolean): Type {
                // We have '==', '!=', '====', or !==' operator with 'typeof xxx' and string literal operands
                const target = getReferenceCandidate(typeOfExpr.expression);
                if (!isMatchingReference(reference, target)) {
                    // For a reference of the form 'x.y', a 'typeof x === ...' type guard resets the
                    // narrowed type of 'y' to its declared type.
                    if (containsMatchingReference(reference, target)) {
                        return declaredType;
                    }
                    return type;
                }
                if (operator === SyntaxKind.ExclamationEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken) {
                    assumeTrue = !assumeTrue;
                }
                if (type.flags & TypeFlags.Any && literal.text === "function") {
                    return type;
                }
                const facts = assumeTrue ?
                    typeofEQFacts.get(literal.text) || TypeFacts.TypeofEQHostObject :
                    typeofNEFacts.get(literal.text) || TypeFacts.TypeofNEHostObject;
                return getTypeWithFacts(assumeTrue ? mapType(type, narrowTypeForTypeof) : type, facts);

                function narrowTypeForTypeof(type: Type) {
                    if (assumeTrue && !(type.flags & TypeFlags.Union)) {
                        if (type.flags & TypeFlags.Unknown && literal.text === "object") {
                            return getUnionType([nonPrimitiveType, nullType]);
                        }
                        // We narrow a non-union type to an exact primitive type if the non-union type
                        // is a supertype of that primitive type. For example, type 'any' can be narrowed
                        // to one of the primitive types.
                        const targetType = literal.text === "function" ? globalFunctionType : typeofTypesByName.get(literal.text);
                        if (targetType) {
                            if (isTypeSubtypeOf(targetType, type)) {
                                return isTypeAny(type) ? targetType : getIntersectionType([type, targetType]); // Intersection to handle `string` being a subtype of `keyof T`
                            }
                            if (type.flags & TypeFlags.Instantiable) {
                                const constraint = getBaseConstraintOfType(type) || anyType;
                                if (isTypeSubtypeOf(targetType, constraint)) {
                                    return getIntersectionType([type, targetType]);
                                }
                            }
                        }
                    }
                    return type;
                }
            }

            function narrowTypeBySwitchOnDiscriminant(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number) {
                // We only narrow if all case expressions specify values with unit types
                const switchTypes = getSwitchClauseTypes(switchStatement);
                if (!switchTypes.length) {
                    return type;
                }
                const clauseTypes = switchTypes.slice(clauseStart, clauseEnd);
                const hasDefaultClause = clauseStart === clauseEnd || contains(clauseTypes, neverType);
                const discriminantType = getUnionType(clauseTypes);
                const caseType =
                    discriminantType.flags & TypeFlags.Never ? neverType :
                        replacePrimitivesWithLiterals(filterType(type, t => areTypesComparable(discriminantType, t)), discriminantType);
                if (!hasDefaultClause) {
                    return caseType;
                }
                const defaultType = filterType(type, t => !(isUnitType(t) && contains(switchTypes, getRegularTypeOfLiteralType(t))));
                return caseType.flags & TypeFlags.Never ? defaultType : getUnionType([caseType, defaultType]);
            }

            function narrowBySwitchOnTypeOf(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number): Type {
                const switchWitnesses = getSwitchClauseTypeOfWitnesses(switchStatement);
                if (!switchWitnesses.length) {
                    return type;
                }
                //  Equal start and end denotes implicit fallthrough; undefined marks explicit default clause
                const defaultCaseLocation = findIndex(switchWitnesses, elem => elem === undefined);
                const hasDefaultClause = clauseStart === clauseEnd || (defaultCaseLocation >= clauseStart && defaultCaseLocation < clauseEnd);
                let clauseWitnesses: string[];
                let switchFacts: TypeFacts;
                if (defaultCaseLocation > -1) {
                    // We no longer need the undefined denoting an
                    // explicit default case. Remove the undefined and
                    // fix-up clauseStart and clauseEnd.  This means
                    // that we don't have to worry about undefined
                    // in the witness array.
                    const witnesses = <string[]>switchWitnesses.filter(witness => witness !== undefined);
                    // The adjust clause start and end after removing the `default` statement.
                    const fixedClauseStart = defaultCaseLocation < clauseStart ? clauseStart - 1 : clauseStart;
                    const fixedClauseEnd = defaultCaseLocation < clauseEnd ? clauseEnd - 1 : clauseEnd;
                    clauseWitnesses = witnesses.slice(fixedClauseStart, fixedClauseEnd);
                    switchFacts = getFactsFromTypeofSwitch(fixedClauseStart, fixedClauseEnd, witnesses, hasDefaultClause);
                }
                else {
                    clauseWitnesses = <string[]>switchWitnesses.slice(clauseStart, clauseEnd);
                    switchFacts = getFactsFromTypeofSwitch(clauseStart, clauseEnd, <string[]>switchWitnesses, hasDefaultClause);
                }
                /*
                  The implied type is the raw type suggested by a
                  value being caught in this clause.

                  When the clause contains a default case we ignore
                  the implied type and try to narrow using any facts
                  we can learn: see `switchFacts`.

                  Example:
                  switch (typeof x) {
                      case 'number':
                      case 'string': break;
                      default: break;
                      case 'number':
                      case 'boolean': break
                  }

                  In the first clause (case `number` and `string`) the
                  implied type is number | string.

                  In the default clause we de not compute an implied type.

                  In the third clause (case `number` and `boolean`)
                  the naive implied type is number | boolean, however
                  we use the type facts to narrow the implied type to
                  boolean. We know that number cannot be selected
                  because it is caught in the first clause.
                */
                if (!(hasDefaultClause || (type.flags & TypeFlags.Union))) {
                    let impliedType = getTypeWithFacts(getUnionType(clauseWitnesses.map(text => typeofTypesByName.get(text) || neverType)), switchFacts);
                    if (impliedType.flags & TypeFlags.Union) {
                        impliedType = getAssignmentReducedType(impliedType as UnionType, getBaseConstraintOfType(type) || type);
                    }
                    if (!(impliedType.flags & TypeFlags.Never)) {
                        if (isTypeSubtypeOf(impliedType, type)) {
                            return impliedType;
                        }
                        if (type.flags & TypeFlags.Instantiable) {
                            const constraint = getBaseConstraintOfType(type) || anyType;
                            if (isTypeSubtypeOf(impliedType, constraint)) {
                                return getIntersectionType([type, impliedType]);
                            }
                        }
                    }
                }
                return hasDefaultClause ?
                    filterType(type, t => (getTypeFacts(t) & switchFacts) === switchFacts) :
                    getTypeWithFacts(type, switchFacts);
            }

            function narrowTypeByInstanceof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                const left = getReferenceCandidate(expr.left);
                if (!isMatchingReference(reference, left)) {
                    // For a reference of the form 'x.y', an 'x instanceof T' type guard resets the
                    // narrowed type of 'y' to its declared type.
                    if (containsMatchingReference(reference, left)) {
                        return declaredType;
                    }
                    return type;
                }

                // Check that right operand is a function type with a prototype property
                const rightType = getTypeOfExpression(expr.right);
                if (!isTypeSubtypeOf(rightType, globalFunctionType)) {
                    return type;
                }

                let targetType: Type | undefined;
                const prototypeProperty = getPropertyOfType(rightType, "prototype" as __String);
                if (prototypeProperty) {
                    // Target type is type of the prototype property
                    const prototypePropertyType = getTypeOfSymbol(prototypeProperty);
                    if (!isTypeAny(prototypePropertyType)) {
                        targetType = prototypePropertyType;
                    }
                }

                // Don't narrow from 'any' if the target type is exactly 'Object' or 'Function'
                if (isTypeAny(type) && (targetType === globalObjectType || targetType === globalFunctionType)) {
                    return type;
                }

                if (!targetType) {
                    // Target type is type of construct signature
                    let constructSignatures: ReadonlyArray<Signature> | undefined;
                    if (getObjectFlags(rightType) & ObjectFlags.Interface) {
                        constructSignatures = resolveDeclaredMembers(<InterfaceType>rightType).declaredConstructSignatures;
                    }
                    else if (getObjectFlags(rightType) & ObjectFlags.Anonymous) {
                        constructSignatures = getSignaturesOfType(rightType, SignatureKind.Construct);
                    }
                    if (constructSignatures && constructSignatures.length) {
                        targetType = getUnionType(map(constructSignatures, signature => getReturnTypeOfSignature(getErasedSignature(signature))));
                    }
                }

                if (targetType) {
                    return getNarrowedType(type, targetType, assumeTrue, isTypeDerivedFrom);
                }

                return type;
            }

            function getNarrowedType(type: Type, candidate: Type, assumeTrue: boolean, isRelated: (source: Type, target: Type) => boolean) {
                if (!assumeTrue) {
                    return filterType(type, t => !isRelated(t, candidate));
                }
                // If the current type is a union type, remove all constituents that couldn't be instances of
                // the candidate type. If one or more constituents remain, return a union of those.
                if (type.flags & TypeFlags.Union) {
                    const assignableType = filterType(type, t => isRelated(t, candidate));
                    if (!(assignableType.flags & TypeFlags.Never)) {
                        return assignableType;
                    }
                }
                // If the candidate type is a subtype of the target type, narrow to the candidate type.
                // Otherwise, if the target type is assignable to the candidate type, keep the target type.
                // Otherwise, if the candidate type is assignable to the target type, narrow to the candidate
                // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
                // two types.
                return isTypeSubtypeOf(candidate, type) ? candidate :
                    isTypeAssignableTo(type, candidate) ? type :
                        isTypeAssignableTo(candidate, type) ? candidate :
                            getIntersectionType([type, candidate]);
            }

            function narrowTypeByTypePredicate(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
                if (!hasMatchingArgument(callExpression, reference) || !maybeTypePredicateCall(callExpression)) {
                    return type;
                }
                const signature = getResolvedSignature(callExpression);
                const predicate = getTypePredicateOfSignature(signature);
                if (!predicate) {
                    return type;
                }

                // Don't narrow from 'any' if the predicate type is exactly 'Object' or 'Function'
                if (isTypeAny(type) && (predicate.type === globalObjectType || predicate.type === globalFunctionType)) {
                    return type;
                }

                if (isIdentifierTypePredicate(predicate)) {
                    const predicateArgument = callExpression.arguments[predicate.parameterIndex - (signature.thisParameter ? 1 : 0)];
                    if (predicateArgument) {
                        if (isMatchingReference(reference, predicateArgument)) {
                            return getNarrowedType(type, predicate.type, assumeTrue, isTypeSubtypeOf);
                        }
                        if (containsMatchingReference(reference, predicateArgument)) {
                            return declaredType;
                        }
                    }
                }
                else {
                    const invokedExpression = skipParentheses(callExpression.expression);
                    if (invokedExpression.kind === SyntaxKind.ElementAccessExpression || invokedExpression.kind === SyntaxKind.PropertyAccessExpression) {
                        const accessExpression = invokedExpression as ElementAccessExpression | PropertyAccessExpression;
                        const possibleReference = skipParentheses(accessExpression.expression);
                        if (isMatchingReference(reference, possibleReference)) {
                            return getNarrowedType(type, predicate.type, assumeTrue, isTypeSubtypeOf);
                        }
                        if (containsMatchingReference(reference, possibleReference)) {
                            return declaredType;
                        }
                    }
                }
                return type;
            }

            // Narrow the given type based on the given expression having the assumed boolean value. The returned type
            // will be a subtype or the same type as the argument.
            function narrowType(type: Type, expr: Expression, assumeTrue: boolean): Type {
                switch (expr.kind) {
                    case SyntaxKind.Identifier:
                    case SyntaxKind.ThisKeyword:
                    case SyntaxKind.SuperKeyword:
                    case SyntaxKind.PropertyAccessExpression:
                    case SyntaxKind.ElementAccessExpression:
                        return narrowTypeByTruthiness(type, expr, assumeTrue);
                    case SyntaxKind.CallExpression:
                        return narrowTypeByTypePredicate(type, <CallExpression>expr, assumeTrue);
                    case SyntaxKind.ParenthesizedExpression:
                        return narrowType(type, (<ParenthesizedExpression>expr).expression, assumeTrue);
                    case SyntaxKind.BinaryExpression:
                        return narrowTypeByBinaryExpression(type, <BinaryExpression>expr, assumeTrue);
                    case SyntaxKind.PrefixUnaryExpression:
                        if ((<PrefixUnaryExpression>expr).operator === SyntaxKind.ExclamationToken) {
                            return narrowType(type, (<PrefixUnaryExpression>expr).operand, !assumeTrue);
                        }
                        break;
                }
                return type;
            }
        }

        function getTypeOfSymbolAtLocation(symbol: Symbol, location: Node) {
            symbol = symbol.exportSymbol || symbol;

            // If we have an identifier or a property access at the given location, if the location is
            // an dotted name expression, and if the location is not an assignment target, obtain the type
            // of the expression (which will reflect control flow analysis). If the expression indeed
            // resolved to the given symbol, return the narrowed type.
            if (location.kind === SyntaxKind.Identifier) {
                if (isRightSideOfQualifiedNameOrPropertyAccess(location)) {
                    location = location.parent;
                }
                if (isExpressionNode(location) && !isAssignmentTarget(location)) {
                    const type = getTypeOfExpression(<Expression>location);
                    if (getExportSymbolOfValueSymbolIfExported(getNodeLinks(location).resolvedSymbol) === symbol) {
                        return type;
                    }
                }
            }
            // The location isn't a reference to the given symbol, meaning we're being asked
            // a hypothetical question of what type the symbol would have if there was a reference
            // to it at the given location. Since we have no control flow information for the
            // hypothetical reference (control flow information is created and attached by the
            // binder), we simply return the declared type of the symbol.
            return getTypeOfSymbol(symbol);
        }

        function getControlFlowContainer(node: Node): Node {
            return findAncestor(node.parent, node =>
                isFunctionLike(node) && !getImmediatelyInvokedFunctionExpression(node) ||
                node.kind === SyntaxKind.ModuleBlock ||
                node.kind === SyntaxKind.SourceFile ||
                node.kind === SyntaxKind.PropertyDeclaration)!;
        }

        // Check if a parameter is assigned anywhere within its declaring function.
        function isParameterAssigned(symbol: Symbol) {
            const func = <FunctionLikeDeclaration>getRootDeclaration(symbol.valueDeclaration).parent;
            const links = getNodeLinks(func);
            if (!(links.flags & NodeCheckFlags.AssignmentsMarked)) {
                links.flags |= NodeCheckFlags.AssignmentsMarked;
                if (!hasParentWithAssignmentsMarked(func)) {
                    markParameterAssignments(func);
                }
            }
            return symbol.isAssigned || false;
        }

        function hasParentWithAssignmentsMarked(node: Node) {
            return !!findAncestor(node.parent, node => isFunctionLike(node) && !!(getNodeLinks(node).flags & NodeCheckFlags.AssignmentsMarked));
        }

        function markParameterAssignments(node: Node) {
            if (node.kind === SyntaxKind.Identifier) {
                if (isAssignmentTarget(node)) {
                    const symbol = getResolvedSymbol(<Identifier>node);
                    if (symbol.valueDeclaration && getRootDeclaration(symbol.valueDeclaration).kind === SyntaxKind.Parameter) {
                        symbol.isAssigned = true;
                    }
                }
            }
            else {
                forEachChild(node, markParameterAssignments);
            }
        }

        function isConstVariable(symbol: Symbol) {
            return symbol.flags & SymbolFlags.Variable && (getDeclarationNodeFlagsFromSymbol(symbol) & NodeFlags.Const) !== 0 && getTypeOfSymbol(symbol) !== autoArrayType;
        }

        /** remove undefined from the annotated type of a parameter when there is an initializer (that doesn't include undefined) */
        function removeOptionalityFromDeclaredType(declaredType: Type, declaration: VariableLikeDeclaration): Type {
            const annotationIncludesUndefined = strictNullChecks &&
                declaration.kind === SyntaxKind.Parameter &&
                declaration.initializer &&
                getFalsyFlags(declaredType) & TypeFlags.Undefined &&
                !(getFalsyFlags(checkExpression(declaration.initializer)) & TypeFlags.Undefined);
            return annotationIncludesUndefined ? getTypeWithFacts(declaredType, TypeFacts.NEUndefined) : declaredType;
        }

        function isConstraintPosition(node: Node) {
            const parent = node.parent;
            return parent.kind === SyntaxKind.PropertyAccessExpression ||
                parent.kind === SyntaxKind.CallExpression && (<CallExpression>parent).expression === node ||
                parent.kind === SyntaxKind.ElementAccessExpression && (<ElementAccessExpression>parent).expression === node ||
                parent.kind === SyntaxKind.BindingElement && (<BindingElement>parent).name === node && !!(<BindingElement>parent).initializer;
        }

        function typeHasNullableConstraint(type: Type) {
            return type.flags & TypeFlags.InstantiableNonPrimitive && maybeTypeOfKind(getBaseConstraintOfType(type) || emptyObjectType, TypeFlags.Nullable);
        }

        function getConstraintForLocation(type: Type, node: Node): Type;
        function getConstraintForLocation(type: Type | undefined, node: Node): Type | undefined;
        function getConstraintForLocation(type: Type, node: Node): Type | undefined {
            // When a node is the left hand expression of a property access, element access, or call expression,
            // and the type of the node includes type variables with constraints that are nullable, we fetch the
            // apparent type of the node *before* performing control flow analysis such that narrowings apply to
            // the constraint type.
            if (type && isConstraintPosition(node) && forEachType(type, typeHasNullableConstraint)) {
                return mapType(getWidenedType(type), getBaseConstraintOrType);
            }
            return type;
        }

        function markAliasReferenced(symbol: Symbol, location: Node) {
            if (isNonLocalAlias(symbol, /*excludes*/ SymbolFlags.Value) && !isInTypeQuery(location) && !isConstEnumOrConstEnumOnlyModule(resolveAlias(symbol))) {
                markAliasSymbolAsReferenced(symbol);
            }
        }

        function checkIdentifier(node: Identifier): Type {
            const symbol = getResolvedSymbol(node);
            if (symbol === unknownSymbol) {
                return errorType;
            }

            // As noted in ECMAScript 6 language spec, arrow functions never have an arguments objects.
            // Although in down-level emit of arrow function, we emit it using function expression which means that
            // arguments objects will be bound to the inner object; emitting arrow function natively in ES6, arguments objects
            // will be bound to non-arrow function that contain this arrow function. This results in inconsistent behavior.
            // To avoid that we will give an error to users if they use arguments objects in arrow function so that they
            // can explicitly bound arguments objects
            if (symbol === argumentsSymbol) {
                const container = getContainingFunction(node)!;
                if (languageVersion < ScriptTarget.ES2015) {
                    if (container.kind === SyntaxKind.ArrowFunction) {
                        error(node, Diagnostics.The_arguments_object_cannot_be_referenced_in_an_arrow_function_in_ES3_and_ES5_Consider_using_a_standard_function_expression);
                    }
                    else if (hasModifier(container, ModifierFlags.Async)) {
                        error(node, Diagnostics.The_arguments_object_cannot_be_referenced_in_an_async_function_or_method_in_ES3_and_ES5_Consider_using_a_standard_function_or_method);
                    }
                }

                getNodeLinks(container).flags |= NodeCheckFlags.CaptureArguments;
                return getTypeOfSymbol(symbol);
            }

            // We should only mark aliases as referenced if there isn't a local value declaration
            // for the symbol. Also, don't mark any property access expression LHS - checkPropertyAccessExpression will handle that
            if (!(node.parent && isPropertyAccessExpression(node.parent) && node.parent.expression === node)) {
                markAliasReferenced(symbol, node);
            }

            const localOrExportSymbol = getExportSymbolOfValueSymbolIfExported(symbol);
            let declaration: Declaration | undefined = localOrExportSymbol.valueDeclaration;

            if (localOrExportSymbol.flags & SymbolFlags.Class) {
                // Due to the emit for class decorators, any reference to the class from inside of the class body
                // must instead be rewritten to point to a temporary variable to avoid issues with the double-bind
                // behavior of class names in ES6.
                if (declaration.kind === SyntaxKind.ClassDeclaration
                    && nodeIsDecorated(declaration as ClassDeclaration)) {
                    let container = getContainingClass(node);
                    while (container !== undefined) {
                        if (container === declaration && container.name !== node) {
                            getNodeLinks(declaration).flags |= NodeCheckFlags.ClassWithConstructorReference;
                            getNodeLinks(node).flags |= NodeCheckFlags.ConstructorReferenceInClass;
                            break;
                        }

                        container = getContainingClass(container);
                    }
                }
                else if (declaration.kind === SyntaxKind.ClassExpression) {
                    // When we emit a class expression with static members that contain a reference
                    // to the constructor in the initializer, we will need to substitute that
                    // binding with an alias as the class name is not in scope.
                    let container = getThisContainer(node, /*includeArrowFunctions*/ false);
                    while (container.kind !== SyntaxKind.SourceFile) {
                        if (container.parent === declaration) {
                            if (container.kind === SyntaxKind.PropertyDeclaration && hasModifier(container, ModifierFlags.Static)) {
                                getNodeLinks(declaration).flags |= NodeCheckFlags.ClassWithConstructorReference;
                                getNodeLinks(node).flags |= NodeCheckFlags.ConstructorReferenceInClass;
                            }
                            break;
                        }

                        container = getThisContainer(container, /*includeArrowFunctions*/ false);
                    }
                }
            }

            checkNestedBlockScopedBinding(node, symbol);

            const type = getConstraintForLocation(getTypeOfSymbol(localOrExportSymbol), node);
            const assignmentKind = getAssignmentTargetKind(node);

            if (assignmentKind) {
                if (!(localOrExportSymbol.flags & SymbolFlags.Variable) &&
                    !(isInJSFile(node) && localOrExportSymbol.flags & SymbolFlags.ValueModule)) {
                    error(node, Diagnostics.Cannot_assign_to_0_because_it_is_not_a_variable, symbolToString(symbol));
                    return errorType;
                }
                if (isReadonlySymbol(localOrExportSymbol)) {
                    error(node, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, symbolToString(symbol));
                    return errorType;
                }
            }

            const isAlias = localOrExportSymbol.flags & SymbolFlags.Alias;

            // We only narrow variables and parameters occurring in a non-assignment position. For all other
            // entities we simply return the declared type.
            if (localOrExportSymbol.flags & SymbolFlags.Variable) {
                if (assignmentKind === AssignmentKind.Definite) {
                    return type;
                }
            }
            else if (isAlias) {
                declaration = find<Declaration>(symbol.declarations, isSomeImportDeclaration);
            }
            else {
                return type;
            }

            if (!declaration) {
                return type;
            }

            // The declaration container is the innermost function that encloses the declaration of the variable
            // or parameter. The flow container is the innermost function starting with which we analyze the control
            // flow graph to determine the control flow based type.
            const isParameter = getRootDeclaration(declaration).kind === SyntaxKind.Parameter;
            const declarationContainer = getControlFlowContainer(declaration);
            let flowContainer = getControlFlowContainer(node);
            const isOuterVariable = flowContainer !== declarationContainer;
            const isSpreadDestructuringAssignmentTarget = node.parent && node.parent.parent && isSpreadAssignment(node.parent) && isDestructuringAssignmentTarget(node.parent.parent);
            const isModuleExports = symbol.flags & SymbolFlags.ModuleExports;
            // When the control flow originates in a function expression or arrow function and we are referencing
            // a const variable or parameter from an outer function, we extend the origin of the control flow
            // analysis to include the immediately enclosing function.
            while (flowContainer !== declarationContainer && (flowContainer.kind === SyntaxKind.FunctionExpression ||
                flowContainer.kind === SyntaxKind.ArrowFunction || isObjectLiteralOrClassExpressionMethod(flowContainer)) &&
                (isConstVariable(localOrExportSymbol) || isParameter && !isParameterAssigned(localOrExportSymbol))) {
                flowContainer = getControlFlowContainer(flowContainer);
            }
            // We only look for uninitialized variables in strict null checking mode, and only when we can analyze
            // the entire control flow graph from the variable's declaration (i.e. when the flow container and
            // declaration container are the same).
            const assumeInitialized = isParameter || isAlias || isOuterVariable || isSpreadDestructuringAssignmentTarget || isModuleExports ||
                type !== autoType && type !== autoArrayType && (!strictNullChecks || (type.flags & TypeFlags.AnyOrUnknown) !== 0 ||
                isInTypeQuery(node) || node.parent.kind === SyntaxKind.ExportSpecifier) ||
                node.parent.kind === SyntaxKind.NonNullExpression ||
                declaration.kind === SyntaxKind.VariableDeclaration && (<VariableDeclaration>declaration).exclamationToken ||
                declaration.flags & NodeFlags.Ambient;
            const initialType = assumeInitialized ? (isParameter ? removeOptionalityFromDeclaredType(type, declaration as VariableLikeDeclaration) : type) :
                type === autoType || type === autoArrayType ? undefinedType :
                    getOptionalType(type);
            const flowType = getFlowTypeOfReference(node, type, initialType, flowContainer, !assumeInitialized);
            // A variable is considered uninitialized when it is possible to analyze the entire control flow graph
            // from declaration to use, and when the variable's declared type doesn't include undefined but the
            // control flow based type does include undefined.
            if (type === autoType || type === autoArrayType) {
                if (flowType === autoType || flowType === autoArrayType) {
                    if (noImplicitAny) {
                        error(getNameOfDeclaration(declaration), Diagnostics.Variable_0_implicitly_has_type_1_in_some_locations_where_its_type_cannot_be_determined, symbolToString(symbol), typeToString(flowType));
                        error(node, Diagnostics.Variable_0_implicitly_has_an_1_type, symbolToString(symbol), typeToString(flowType));
                    }
                    return convertAutoToAny(flowType);
                }
            }
            else if (!assumeInitialized && !(getFalsyFlags(type) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
                error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                // Return the declared type to reduce follow-on errors
                return type;
            }
            return assignmentKind ? getBaseTypeOfLiteralType(flowType) : flowType;
        }

        function isInsideFunction(node: Node, threshold: Node): boolean {
            return !!findAncestor(node, n => n === threshold ? "quit" : isFunctionLike(n));
        }

        function checkNestedBlockScopedBinding(node: Identifier, symbol: Symbol): void {
            if (languageVersion >= ScriptTarget.ES2015 ||
                (symbol.flags & (SymbolFlags.BlockScopedVariable | SymbolFlags.Class)) === 0 ||
                symbol.valueDeclaration.parent.kind === SyntaxKind.CatchClause) {
                return;
            }

            // 1. walk from the use site up to the declaration and check
            // if there is anything function like between declaration and use-site (is binding/class is captured in function).
            // 2. walk from the declaration up to the boundary of lexical environment and check
            // if there is an iteration statement in between declaration and boundary (is binding/class declared inside iteration statement)

            const container = getEnclosingBlockScopeContainer(symbol.valueDeclaration);
            const usedInFunction = isInsideFunction(node.parent, container);
            let current = container;

            let containedInIterationStatement = false;
            while (current && !nodeStartsNewLexicalEnvironment(current)) {
                if (isIterationStatement(current, /*lookInLabeledStatements*/ false)) {
                    containedInIterationStatement = true;
                    break;
                }
                current = current.parent;
            }

            if (containedInIterationStatement) {
                if (usedInFunction) {
                    // mark iteration statement as containing block-scoped binding captured in some function
                    getNodeLinks(current).flags |= NodeCheckFlags.LoopWithCapturedBlockScopedBinding;
                }

                // mark variables that are declared in loop initializer and reassigned inside the body of ForStatement.
                // if body of ForStatement will be converted to function then we'll need a extra machinery to propagate reassigned values back.
                if (container.kind === SyntaxKind.ForStatement &&
                    getAncestor(symbol.valueDeclaration, SyntaxKind.VariableDeclarationList)!.parent === container &&
                    isAssignedInBodyOfForStatement(node, <ForStatement>container)) {
                    getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.NeedsLoopOutParameter;
                }

                // set 'declared inside loop' bit on the block-scoped binding
                getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.BlockScopedBindingInLoop;
            }

            if (usedInFunction) {
                getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.CapturedBlockScopedBinding;
            }
        }

        function isAssignedInBodyOfForStatement(node: Identifier, container: ForStatement): boolean {
            // skip parenthesized nodes
            let current: Node = node;
            while (current.parent.kind === SyntaxKind.ParenthesizedExpression) {
                current = current.parent;
            }

            // check if node is used as LHS in some assignment expression
            let isAssigned = false;
            if (isAssignmentTarget(current)) {
                isAssigned = true;
            }
            else if ((current.parent.kind === SyntaxKind.PrefixUnaryExpression || current.parent.kind === SyntaxKind.PostfixUnaryExpression)) {
                const expr = <PrefixUnaryExpression | PostfixUnaryExpression>current.parent;
                isAssigned = expr.operator === SyntaxKind.PlusPlusToken || expr.operator === SyntaxKind.MinusMinusToken;
            }

            if (!isAssigned) {
                return false;
            }

            // at this point we know that node is the target of assignment
            // now check that modification happens inside the statement part of the ForStatement
            return !!findAncestor(current, n => n === container ? "quit" : n === container.statement);
        }

        function captureLexicalThis(node: Node, container: Node): void {
            getNodeLinks(node).flags |= NodeCheckFlags.LexicalThis;
            if (container.kind === SyntaxKind.PropertyDeclaration || container.kind === SyntaxKind.Constructor) {
                const classNode = container.parent;
                getNodeLinks(classNode).flags |= NodeCheckFlags.CaptureThis;
            }
            else {
                getNodeLinks(container).flags |= NodeCheckFlags.CaptureThis;
            }
        }

        function findFirstSuperCall(n: Node): SuperCall | undefined {
            if (isSuperCall(n)) {
                return n;
            }
            else if (isFunctionLike(n)) {
                return undefined;
            }
            return forEachChild(n, findFirstSuperCall);
        }

        /**
         * Return a cached result if super-statement is already found.
         * Otherwise, find a super statement in a given constructor function and cache the result in the node-links of the constructor
         *
         * @param constructor constructor-function to look for super statement
         */
        function getSuperCallInConstructor(constructor: ConstructorDeclaration): SuperCall | undefined {
            const links = getNodeLinks(constructor);

            // Only trying to find super-call if we haven't yet tried to find one.  Once we try, we will record the result
            if (links.hasSuperCall === undefined) {
                links.superCall = findFirstSuperCall(constructor.body!);
                links.hasSuperCall = links.superCall ? true : false;
            }
            return links.superCall!;
        }

        /**
         * Check if the given class-declaration extends null then return true.
         * Otherwise, return false
         * @param classDecl a class declaration to check if it extends null
         */
        function classDeclarationExtendsNull(classDecl: ClassDeclaration): boolean {
            const classSymbol = getSymbolOfNode(classDecl);
            const classInstanceType = <InterfaceType>getDeclaredTypeOfSymbol(classSymbol);
            const baseConstructorType = getBaseConstructorTypeOfClass(classInstanceType);

            return baseConstructorType === nullWideningType;
        }

        function checkThisBeforeSuper(node: Node, container: Node, diagnosticMessage: DiagnosticMessage) {
            const containingClassDecl = <ClassDeclaration>container.parent;
            const baseTypeNode = getEffectiveBaseTypeNode(containingClassDecl);

            // If a containing class does not have extends clause or the class extends null
            // skip checking whether super statement is called before "this" accessing.
            if (baseTypeNode && !classDeclarationExtendsNull(containingClassDecl)) {
                const superCall = getSuperCallInConstructor(<ConstructorDeclaration>container);

                // We should give an error in the following cases:
                //      - No super-call
                //      - "this" is accessing before super-call.
                //          i.e super(this)
                //              this.x; super();
                // We want to make sure that super-call is done before accessing "this" so that
                // "this" is not accessed as a parameter of the super-call.
                if (!superCall || superCall.end > node.pos) {
                    // In ES6, super inside constructor of class-declaration has to precede "this" accessing
                    error(node, diagnosticMessage);
                }
            }
        }

        function checkThisExpression(node: Node): Type {
            // Stop at the first arrow function so that we can
            // tell whether 'this' needs to be captured.
            let container = getThisContainer(node, /* includeArrowFunctions */ true);
            let capturedByArrowFunction = false;

            if (container.kind === SyntaxKind.Constructor) {
                checkThisBeforeSuper(node, container, Diagnostics.super_must_be_called_before_accessing_this_in_the_constructor_of_a_derived_class);
            }

            // Now skip arrow functions to get the "real" owner of 'this'.
            if (container.kind === SyntaxKind.ArrowFunction) {
                container = getThisContainer(container, /* includeArrowFunctions */ false);
                capturedByArrowFunction = true;
            }

            switch (container.kind) {
                case SyntaxKind.ModuleDeclaration:
                    error(node, Diagnostics.this_cannot_be_referenced_in_a_module_or_namespace_body);
                    // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    break;
                case SyntaxKind.EnumDeclaration:
                    error(node, Diagnostics.this_cannot_be_referenced_in_current_location);
                    // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    break;
                case SyntaxKind.Constructor:
                    if (isInConstructorArgumentInitializer(node, container)) {
                        error(node, Diagnostics.this_cannot_be_referenced_in_constructor_arguments);
                        // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    }
                    break;
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                    if (hasModifier(container, ModifierFlags.Static)) {
                        error(node, Diagnostics.this_cannot_be_referenced_in_a_static_property_initializer);
                        // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    }
                    break;
                case SyntaxKind.ComputedPropertyName:
                    error(node, Diagnostics.this_cannot_be_referenced_in_a_computed_property_name);
                    break;
            }

            // When targeting es6, mark that we'll need to capture `this` in its lexically bound scope.
            if (capturedByArrowFunction && languageVersion < ScriptTarget.ES2015) {
                captureLexicalThis(node, container);
            }

            const type = tryGetThisTypeAt(node, container);
            if (!type && noImplicitThis) {
                // With noImplicitThis, functions may not reference 'this' if it has type 'any'
                error(
                    node,
                    capturedByArrowFunction && container.kind === SyntaxKind.SourceFile ?
                        Diagnostics.The_containing_arrow_function_captures_the_global_value_of_this_which_implicitly_has_type_any :
                        Diagnostics.this_implicitly_has_type_any_because_it_does_not_have_a_type_annotation);
            }
            return type || anyType;
        }

        function tryGetThisTypeAt(node: Node, container = getThisContainer(node, /*includeArrowFunctions*/ false)): Type | undefined {
            const isInJS = isInJSFile(node);
            if (isFunctionLike(container) &&
                (!isInParameterInitializerBeforeContainingFunction(node) || getThisParameter(container))) {
                // Note: a parameter initializer should refer to class-this unless function-this is explicitly annotated.
                // If this is a function in a JS file, it might be a class method.
                const className = getClassNameFromPrototypeMethod(container);
                if (isInJS && className) {
                    const classSymbol = checkExpression(className).symbol;
                    if (classSymbol && classSymbol.members && (classSymbol.flags & SymbolFlags.Function)) {
                        const classType = getJavascriptClassType(classSymbol);
                        if (classType) {
                            return getFlowTypeOfReference(node, classType);
                        }
                    }
                }
                // Check if it's a constructor definition, can be either a variable decl or function decl
                // i.e.
                //   * /** @constructor */ function [name]() { ... }
                //   * /** @constructor */ var x = function() { ... }
                else if (isInJS &&
                         (container.kind === SyntaxKind.FunctionExpression || container.kind === SyntaxKind.FunctionDeclaration) &&
                         getJSDocClassTag(container)) {
                    const classType = getJavascriptClassType(container.symbol);
                    if (classType) {
                        return getFlowTypeOfReference(node, classType);
                    }
                }

                const thisType = getThisTypeOfDeclaration(container) || getContextualThisParameterType(container);
                if (thisType) {
                    return getFlowTypeOfReference(node, thisType);
                }
            }

            if (isClassLike(container.parent)) {
                const symbol = getSymbolOfNode(container.parent);
                const type = hasModifier(container, ModifierFlags.Static) ? getTypeOfSymbol(symbol) : (<InterfaceType>getDeclaredTypeOfSymbol(symbol)).thisType!;
                return getFlowTypeOfReference(node, type);
            }

            if (isInJS) {
                const type = getTypeForThisExpressionFromJSDoc(container);
                if (type && type !== errorType) {
                    return getFlowTypeOfReference(node, type);
                }
            }
        }

        function getClassNameFromPrototypeMethod(container: Node) {
            // Check if it's the RHS of a x.prototype.y = function [name]() { .... }
            if (container.kind === SyntaxKind.FunctionExpression &&
                isBinaryExpression(container.parent) &&
                getAssignmentDeclarationKind(container.parent) === AssignmentDeclarationKind.PrototypeProperty) {
                // Get the 'x' of 'x.prototype.y = container'
                return ((container.parent   // x.prototype.y = container
                    .left as PropertyAccessExpression)       // x.prototype.y
                    .expression as PropertyAccessExpression) // x.prototype
                    .expression;                             // x
            }
            // x.prototype = { method() { } }
            else if (container.kind === SyntaxKind.MethodDeclaration &&
                container.parent.kind === SyntaxKind.ObjectLiteralExpression &&
                isBinaryExpression(container.parent.parent) &&
                getAssignmentDeclarationKind(container.parent.parent) === AssignmentDeclarationKind.Prototype) {
                return (container.parent.parent.left as PropertyAccessExpression).expression;
            }
            // x.prototype = { method: function() { } }
            else if (container.kind === SyntaxKind.FunctionExpression &&
                container.parent.kind === SyntaxKind.PropertyAssignment &&
                container.parent.parent.kind === SyntaxKind.ObjectLiteralExpression &&
                isBinaryExpression(container.parent.parent.parent) &&
                getAssignmentDeclarationKind(container.parent.parent.parent) === AssignmentDeclarationKind.Prototype) {
                return (container.parent.parent.parent.left as PropertyAccessExpression).expression;
            }
        }

        function getTypeForThisExpressionFromJSDoc(node: Node) {
            const jsdocType = getJSDocType(node);
            if (jsdocType && jsdocType.kind === SyntaxKind.JSDocFunctionType) {
                const jsDocFunctionType = <JSDocFunctionType>jsdocType;
                if (jsDocFunctionType.parameters.length > 0 &&
                    jsDocFunctionType.parameters[0].name &&
                    (jsDocFunctionType.parameters[0].name as Identifier).escapedText === InternalSymbolName.This) {
                    return getTypeFromTypeNode(jsDocFunctionType.parameters[0].type!);
                }
            }
            const thisTag = getJSDocThisTag(node);
            if (thisTag && thisTag.typeExpression) {
                return getTypeFromTypeNode(thisTag.typeExpression);
            }
        }

        function isInConstructorArgumentInitializer(node: Node, constructorDecl: Node): boolean {
            return !!findAncestor(node, n => n === constructorDecl ? "quit" : n.kind === SyntaxKind.Parameter);
        }

        function checkSuperExpression(node: Node): Type {
            const isCallExpression = node.parent.kind === SyntaxKind.CallExpression && (<CallExpression>node.parent).expression === node;

            let container = getSuperContainer(node, /*stopOnFunctions*/ true);
            let needToCaptureLexicalThis = false;

            // adjust the container reference in case if super is used inside arrow functions with arbitrarily deep nesting
            if (!isCallExpression) {
                while (container && container.kind === SyntaxKind.ArrowFunction) {
                    container = getSuperContainer(container, /*stopOnFunctions*/ true);
                    needToCaptureLexicalThis = languageVersion < ScriptTarget.ES2015;
                }
            }

            const canUseSuperExpression = isLegalUsageOfSuperExpression(container);
            let nodeCheckFlag: NodeCheckFlags = 0;

            if (!canUseSuperExpression) {
                // issue more specific error if super is used in computed property name
                // class A { foo() { return "1" }}
                // class B {
                //     [super.foo()]() {}
                // }
                const current = findAncestor(node, n => n === container ? "quit" : n.kind === SyntaxKind.ComputedPropertyName);
                if (current && current.kind === SyntaxKind.ComputedPropertyName) {
                    error(node, Diagnostics.super_cannot_be_referenced_in_a_computed_property_name);
                }
                else if (isCallExpression) {
                    error(node, Diagnostics.Super_calls_are_not_permitted_outside_constructors_or_in_nested_functions_inside_constructors);
                }
                else if (!container || !container.parent || !(isClassLike(container.parent) || container.parent.kind === SyntaxKind.ObjectLiteralExpression)) {
                    error(node, Diagnostics.super_can_only_be_referenced_in_members_of_derived_classes_or_object_literal_expressions);
                }
                else {
                    error(node, Diagnostics.super_property_access_is_permitted_only_in_a_constructor_member_function_or_member_accessor_of_a_derived_class);
                }
                return errorType;
            }

            if (!isCallExpression && container.kind === SyntaxKind.Constructor) {
                checkThisBeforeSuper(node, container, Diagnostics.super_must_be_called_before_accessing_a_property_of_super_in_the_constructor_of_a_derived_class);
            }

            if (hasModifier(container, ModifierFlags.Static) || isCallExpression) {
                nodeCheckFlag = NodeCheckFlags.SuperStatic;
            }
            else {
                nodeCheckFlag = NodeCheckFlags.SuperInstance;
            }

            getNodeLinks(node).flags |= nodeCheckFlag;

            // Due to how we emit async functions, we need to specialize the emit for an async method that contains a `super` reference.
            // This is due to the fact that we emit the body of an async function inside of a generator function. As generator
            // functions cannot reference `super`, we emit a helper inside of the method body, but outside of the generator. This helper
            // uses an arrow function, which is permitted to reference `super`.
            //
            // There are two primary ways we can access `super` from within an async method. The first is getting the value of a property
            // or indexed access on super, either as part of a right-hand-side expression or call expression. The second is when setting the value
            // of a property or indexed access, either as part of an assignment expression or destructuring assignment.
            //
            // The simplest case is reading a value, in which case we will emit something like the following:
            //
            //  // ts
            //  ...
            //  async asyncMethod() {
            //    let x = await super.asyncMethod();
            //    return x;
            //  }
            //  ...
            //
            //  // js
            //  ...
            //  asyncMethod() {
            //      const _super = name => super[name];
            //      return __awaiter(this, arguments, Promise, function *() {
            //          let x = yield _super("asyncMethod").call(this);
            //          return x;
            //      });
            //  }
            //  ...
            //
            // The more complex case is when we wish to assign a value, especially as part of a destructuring assignment. As both cases
            // are legal in ES6, but also likely less frequent, we emit the same more complex helper for both scenarios:
            //
            //  // ts
            //  ...
            //  async asyncMethod(ar: Promise<any[]>) {
            //      [super.a, super.b] = await ar;
            //  }
            //  ...
            //
            //  // js
            //  ...
            //  asyncMethod(ar) {
            //      const _super = (function (geti, seti) {
            //          const cache = Object.create(null);
            //          return name => cache[name] || (cache[name] = { get value() { return geti(name); }, set value(v) { seti(name, v); } });
            //      })(name => super[name], (name, value) => super[name] = value);
            //      return __awaiter(this, arguments, Promise, function *() {
            //          [_super("a").value, _super("b").value] = yield ar;
            //      });
            //  }
            //  ...
            //
            // This helper creates an object with a "value" property that wraps the `super` property or indexed access for both get and set.
            // This is required for destructuring assignments, as a call expression cannot be used as the target of a destructuring assignment
            // while a property access can.
            if (container.kind === SyntaxKind.MethodDeclaration && hasModifier(container, ModifierFlags.Async)) {
                if (isSuperProperty(node.parent) && isAssignmentTarget(node.parent)) {
                    getNodeLinks(container).flags |= NodeCheckFlags.AsyncMethodWithSuperBinding;
                }
                else {
                    getNodeLinks(container).flags |= NodeCheckFlags.AsyncMethodWithSuper;
                }
            }

            if (needToCaptureLexicalThis) {
                // call expressions are allowed only in constructors so they should always capture correct 'this'
                // super property access expressions can also appear in arrow functions -
                // in this case they should also use correct lexical this
                captureLexicalThis(node.parent, container);
            }

            if (container.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                if (languageVersion < ScriptTarget.ES2015) {
                    error(node, Diagnostics.super_is_only_allowed_in_members_of_object_literal_expressions_when_option_target_is_ES2015_or_higher);
                    return errorType;
                }
                else {
                    // for object literal assume that type of 'super' is 'any'
                    return anyType;
                }
            }

            // at this point the only legal case for parent is ClassLikeDeclaration
            const classLikeDeclaration = <ClassLikeDeclaration>container.parent;
            if (!getEffectiveBaseTypeNode(classLikeDeclaration)) {
                error(node, Diagnostics.super_can_only_be_referenced_in_a_derived_class);
                return errorType;
            }

            const classType = <InterfaceType>getDeclaredTypeOfSymbol(getSymbolOfNode(classLikeDeclaration));
            const baseClassType = classType && getBaseTypes(classType)[0];
            if (!baseClassType) {
                return errorType;
            }

            if (container.kind === SyntaxKind.Constructor && isInConstructorArgumentInitializer(node, container)) {
                // issue custom error message for super property access in constructor arguments (to be aligned with old compiler)
                error(node, Diagnostics.super_cannot_be_referenced_in_constructor_arguments);
                return errorType;
            }

            return nodeCheckFlag === NodeCheckFlags.SuperStatic
                ? getBaseConstructorTypeOfClass(classType)
                : getTypeWithThisArgument(baseClassType, classType.thisType);

            function isLegalUsageOfSuperExpression(container: Node): boolean {
                if (!container) {
                    return false;
                }

                if (isCallExpression) {
                    // TS 1.0 SPEC (April 2014): 4.8.1
                    // Super calls are only permitted in constructors of derived classes
                    return container.kind === SyntaxKind.Constructor;
                }
                else {
                    // TS 1.0 SPEC (April 2014)
                    // 'super' property access is allowed
                    // - In a constructor, instance member function, instance member accessor, or instance member variable initializer where this references a derived class instance
                    // - In a static member function or static member accessor

                    // topmost container must be something that is directly nested in the class declaration\object literal expression
                    if (isClassLike(container.parent) || container.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                        if (hasModifier(container, ModifierFlags.Static)) {
                            return container.kind === SyntaxKind.MethodDeclaration ||
                                container.kind === SyntaxKind.MethodSignature ||
                                container.kind === SyntaxKind.GetAccessor ||
                                container.kind === SyntaxKind.SetAccessor;
                        }
                        else {
                            return container.kind === SyntaxKind.MethodDeclaration ||
                                container.kind === SyntaxKind.MethodSignature ||
                                container.kind === SyntaxKind.GetAccessor ||
                                container.kind === SyntaxKind.SetAccessor ||
                                container.kind === SyntaxKind.PropertyDeclaration ||
                                container.kind === SyntaxKind.PropertySignature ||
                                container.kind === SyntaxKind.Constructor;
                        }
                    }
                }

                return false;
            }
        }

        function getContainingObjectLiteral(func: SignatureDeclaration): ObjectLiteralExpression | undefined {
            return (func.kind === SyntaxKind.MethodDeclaration ||
                func.kind === SyntaxKind.GetAccessor ||
                func.kind === SyntaxKind.SetAccessor) && func.parent.kind === SyntaxKind.ObjectLiteralExpression ? func.parent :
                func.kind === SyntaxKind.FunctionExpression && func.parent.kind === SyntaxKind.PropertyAssignment ? <ObjectLiteralExpression>func.parent.parent :
                    undefined;
        }

        function getThisTypeArgument(type: Type): Type | undefined {
            return getObjectFlags(type) & ObjectFlags.Reference && (<TypeReference>type).target === globalThisType ? (<TypeReference>type).typeArguments![0] : undefined;
        }

        function getThisTypeFromContextualType(type: Type): Type | undefined {
            return mapType(type, t => {
                return t.flags & TypeFlags.Intersection ? forEach((<IntersectionType>t).types, getThisTypeArgument) : getThisTypeArgument(t);
            });
        }

        function getContextualThisParameterType(func: SignatureDeclaration): Type | undefined {
            if (func.kind === SyntaxKind.ArrowFunction) {
                return undefined;
            }
            if (isContextSensitiveFunctionOrObjectLiteralMethod(func)) {
                const contextualSignature = getContextualSignature(func);
                if (contextualSignature) {
                    const thisParameter = contextualSignature.thisParameter;
                    if (thisParameter) {
                        return getTypeOfSymbol(thisParameter);
                    }
                }
            }
            const inJs = isInJSFile(func);
            if (noImplicitThis || inJs) {
                const containingLiteral = getContainingObjectLiteral(func);
                if (containingLiteral) {
                    // We have an object literal method. Check if the containing object literal has a contextual type
                    // that includes a ThisType<T>. If so, T is the contextual type for 'this'. We continue looking in
                    // any directly enclosing object literals.
                    const contextualType = getApparentTypeOfContextualType(containingLiteral);
                    let literal = containingLiteral;
                    let type = contextualType;
                    while (type) {
                        const thisType = getThisTypeFromContextualType(type);
                        if (thisType) {
                            return instantiateType(thisType, getContextualMapper(containingLiteral));
                        }
                        if (literal.parent.kind !== SyntaxKind.PropertyAssignment) {
                            break;
                        }
                        literal = <ObjectLiteralExpression>literal.parent.parent;
                        type = getApparentTypeOfContextualType(literal);
                    }
                    // There was no contextual ThisType<T> for the containing object literal, so the contextual type
                    // for 'this' is the non-null form of the contextual type for the containing object literal or
                    // the type of the object literal itself.
                    return contextualType ? getNonNullableType(contextualType) : checkExpressionCached(containingLiteral);
                }
                // In an assignment of the form 'obj.xxx = function(...)' or 'obj[xxx] = function(...)', the
                // contextual type for 'this' is 'obj'.
                const parent = func.parent;
                if (parent.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>parent).operatorToken.kind === SyntaxKind.EqualsToken) {
                    const target = (<BinaryExpression>parent).left;
                    if (target.kind === SyntaxKind.PropertyAccessExpression || target.kind === SyntaxKind.ElementAccessExpression) {
                        const { expression } = target as PropertyAccessExpression | ElementAccessExpression;
                        // Don't contextually type `this` as `exports` in `exports.Point = function(x, y) { this.x = x; this.y = y; }`
                        if (inJs && isIdentifier(expression)) {
                            const sourceFile = getSourceFileOfNode(parent);
                            if (sourceFile.commonJsModuleIndicator && getResolvedSymbol(expression) === sourceFile.symbol) {
                                return undefined;
                            }
                        }

                        return checkExpressionCached(expression);
                    }
                }
            }
            return undefined;
        }

        // Return contextual type of parameter or undefined if no contextual type is available
        function getContextuallyTypedParameterType(parameter: ParameterDeclaration): Type | undefined {
            const func = parameter.parent;
            if (!isContextSensitiveFunctionOrObjectLiteralMethod(func)) {
                return undefined;
            }
            const iife = getImmediatelyInvokedFunctionExpression(func);
            if (iife && iife.arguments) {
                const args = getEffectiveCallArguments(iife);
                const indexOfParameter = func.parameters.indexOf(parameter);
                if (parameter.dotDotDotToken) {
                    return getSpreadArgumentType(args, indexOfParameter, args.length, anyType, /*context*/ undefined);
                }
                const links = getNodeLinks(iife);
                const cached = links.resolvedSignature;
                links.resolvedSignature = anySignature;
                const type = indexOfParameter < args.length ?
                    getWidenedLiteralType(checkExpression(args[indexOfParameter])) :
                    parameter.initializer ? undefined : undefinedWideningType;
                links.resolvedSignature = cached;
                return type;
            }
            const contextualSignature = getContextualSignature(func);
            if (contextualSignature) {
                const index = func.parameters.indexOf(parameter) - (getThisParameter(func) ? 1 : 0);
                return parameter.dotDotDotToken && lastOrUndefined(func.parameters) === parameter ?
                    getRestTypeAtPosition(contextualSignature, index) :
                    tryGetTypeAtPosition(contextualSignature, index);
            }
        }

        // In a variable, parameter or property declaration with a type annotation,
        //   the contextual type of an initializer expression is the type of the variable, parameter or property.
        // Otherwise, in a parameter declaration of a contextually typed function expression,
        //   the contextual type of an initializer expression is the contextual type of the parameter.
        // Otherwise, in a variable or parameter declaration with a binding pattern name,
        //   the contextual type of an initializer expression is the type implied by the binding pattern.
        // Otherwise, in a binding pattern inside a variable or parameter declaration,
        //   the contextual type of an initializer expression is the type annotation of the containing declaration, if present.
        function getContextualTypeForInitializerExpression(node: Expression): Type | undefined {
            const declaration = <VariableLikeDeclaration>node.parent;
            if (hasInitializer(declaration) && node === declaration.initializer) {
                const typeNode = getEffectiveTypeAnnotationNode(declaration);
                if (typeNode) {
                    return getTypeFromTypeNode(typeNode);
                }
                if (declaration.kind === SyntaxKind.Parameter) {
                    const type = getContextuallyTypedParameterType(declaration);
                    if (type) {
                        return type;
                    }
                }
                if (isBindingPattern(declaration.name)) {
                    return getTypeFromBindingPattern(declaration.name, /*includePatternInType*/ true, /*reportErrors*/ false);
                }
                if (isBindingPattern(declaration.parent)) {
                    const parentDeclaration = declaration.parent.parent;
                    const name = (declaration as BindingElement).propertyName || declaration.name;
                    if (parentDeclaration.kind !== SyntaxKind.BindingElement) {
                        const parentTypeNode = getEffectiveTypeAnnotationNode(parentDeclaration);
                        if (parentTypeNode && !isBindingPattern(name)) {
                            const text = getTextOfPropertyName(name);
                            if (text) {
                                return getTypeOfPropertyOfType(getTypeFromTypeNode(parentTypeNode), text);
                            }
                        }
                    }
                }
            }
            return undefined;
        }

        function getContextualTypeForReturnExpression(node: Expression): Type | undefined {
            const func = getContainingFunction(node);
            if (func) {
                const functionFlags = getFunctionFlags(func);
                if (functionFlags & FunctionFlags.Generator) { // AsyncGenerator function or Generator function
                    return undefined;
                }

                const contextualReturnType = getContextualReturnType(func);
                return functionFlags & FunctionFlags.Async
                    ? contextualReturnType && getAwaitedTypeOfPromise(contextualReturnType) // Async function
                    : contextualReturnType; // Regular function
            }
            return undefined;
        }

        function getContextualTypeForYieldOperand(node: YieldExpression): Type | undefined {
            const func = getContainingFunction(node);
            if (func) {
                const functionFlags = getFunctionFlags(func);
                const contextualReturnType = getContextualReturnType(func);
                if (contextualReturnType) {
                    return node.asteriskToken
                        ? contextualReturnType
                        : getIteratedTypeOfGenerator(contextualReturnType, (functionFlags & FunctionFlags.Async) !== 0);
                }
            }

            return undefined;
        }

        function isInParameterInitializerBeforeContainingFunction(node: Node) {
            let inBindingInitializer = false;
            while (node.parent && !isFunctionLike(node.parent)) {
                if (isParameter(node.parent) && (inBindingInitializer || node.parent.initializer === node)) {
                    return true;
                }
                if (isBindingElement(node.parent) && node.parent.initializer === node) {
                    inBindingInitializer = true;
                }

                node = node.parent;
            }

            return false;
        }

        function getContextualReturnType(functionDecl: SignatureDeclaration): Type | undefined {
            // If the containing function has a return type annotation, is a constructor, or is a get accessor whose
            // corresponding set accessor has a type annotation, return statements in the function are contextually typed
            const returnType = getReturnTypeFromAnnotation(functionDecl);
            if (returnType) {
                return returnType;
            }
            // Otherwise, if the containing function is contextually typed by a function type with exactly one call signature
            // and that call signature is non-generic, return statements are contextually typed by the return type of the signature
            const signature = getContextualSignatureForFunctionLikeDeclaration(<FunctionExpression>functionDecl);
            if (signature && !isResolvingReturnTypeOfSignature(signature)) {
                return getReturnTypeOfSignature(signature);
            }
            return undefined;
        }

        // In a typed function call, an argument or substitution expression is contextually typed by the type of the corresponding parameter.
        function getContextualTypeForArgument(callTarget: CallLikeExpression, arg: Expression): Type | undefined {
            const args = getEffectiveCallArguments(callTarget);
            const argIndex = args.indexOf(arg); // -1 for e.g. the expression of a CallExpression, or the tag of a TaggedTemplateExpression
            return argIndex === -1 ? undefined : getContextualTypeForArgumentAtIndex(callTarget, argIndex);
        }

        function getContextualTypeForArgumentAtIndex(callTarget: CallLikeExpression, argIndex: number): Type {
            // If we're already in the process of resolving the given signature, don't resolve again as
            // that could cause infinite recursion. Instead, return anySignature.
            const signature = getNodeLinks(callTarget).resolvedSignature === resolvingSignature ? resolvingSignature : getResolvedSignature(callTarget);
            return getTypeAtPosition(signature, argIndex);
        }

        function getContextualTypeForSubstitutionExpression(template: TemplateExpression, substitutionExpression: Expression) {
            if (template.parent.kind === SyntaxKind.TaggedTemplateExpression) {
                return getContextualTypeForArgument(<TaggedTemplateExpression>template.parent, substitutionExpression);
            }

            return undefined;
        }

        function getContextualTypeForBinaryOperand(node: Expression): Type | undefined {
            const binaryExpression = <BinaryExpression>node.parent;
            const { left, operatorToken, right } = binaryExpression;
            switch (operatorToken.kind) {
                case SyntaxKind.EqualsToken:
                    if (node !== right) {
                        return undefined;
                    }
                    const contextSensitive = getIsContextSensitiveAssignmentOrContextType(binaryExpression);
                    if (!contextSensitive) {
                        return undefined;
                    }
                    return contextSensitive === true ? getTypeOfExpression(left) : contextSensitive;
                case SyntaxKind.BarBarToken:
                    // When an || expression has a contextual type, the operands are contextually typed by that type. When an ||
                    // expression has no contextual type, the right operand is contextually typed by the type of the left operand,
                    // except for the special case of Javascript declarations of the form `namespace.prop = namespace.prop || {}`
                    const type = getContextualType(binaryExpression);
                    return !type && node === right && !isDefaultedExpandoInitializer(binaryExpression) ?
                        getTypeOfExpression(left) : type;
                case SyntaxKind.AmpersandAmpersandToken:
                case SyntaxKind.CommaToken:
                    return node === right ? getContextualType(binaryExpression) : undefined;
                default:
                    return undefined;
            }
        }

        // In an assignment expression, the right operand is contextually typed by the type of the left operand.
        // Don't do this for assignment declarations unless there is a type tag on the assignment, to avoid circularity from checking the right operand.
        function getIsContextSensitiveAssignmentOrContextType(binaryExpression: BinaryExpression): boolean | Type {
            const kind = getAssignmentDeclarationKind(binaryExpression);
            switch (kind) {
                case AssignmentDeclarationKind.None:
                    return true;
                case AssignmentDeclarationKind.Property:
                case AssignmentDeclarationKind.ExportsProperty:
                case AssignmentDeclarationKind.Prototype:
                case AssignmentDeclarationKind.PrototypeProperty:
                    // If `binaryExpression.left` was assigned a symbol, then this is a new declaration; otherwise it is an assignment to an existing declaration.
                    // See `bindStaticPropertyAssignment` in `binder.ts`.
                    if (!binaryExpression.left.symbol) {
                        return true;
                    }
                    else {
                        const decl = binaryExpression.left.symbol.valueDeclaration;
                        if (!decl) {
                            return false;
                        }
                        const lhs = binaryExpression.left as PropertyAccessExpression;
                        const overallAnnotation = getEffectiveTypeAnnotationNode(decl);
                        if (overallAnnotation) {
                            return getTypeFromTypeNode(overallAnnotation);
                        }
                        else if (isIdentifier(lhs.expression)) {
                            const id = lhs.expression;
                            const parentSymbol = resolveName(id, id.escapedText, SymbolFlags.Value, undefined, id.escapedText, /*isUse*/ true);
                            if (parentSymbol) {
                                const annotated = getEffectiveTypeAnnotationNode(parentSymbol.valueDeclaration);
                                if (annotated) {
                                    const type = getTypeOfPropertyOfContextualType(getTypeFromTypeNode(annotated), lhs.name.escapedText);
                                    return type || false;
                                }
                                return false;
                            }
                        }
                        return !isInJSFile(decl);
                    }
                case AssignmentDeclarationKind.ModuleExports:
                case AssignmentDeclarationKind.ThisProperty:
                    if (!binaryExpression.symbol) return true;
                    if (binaryExpression.symbol.valueDeclaration) {
                        const annotated = getEffectiveTypeAnnotationNode(binaryExpression.symbol.valueDeclaration);
                        if (annotated) {
                            const type = getTypeFromTypeNode(annotated);
                            if (type) {
                                return type;
                            }
                        }
                    }
                    if (kind === AssignmentDeclarationKind.ModuleExports) return false;
                    const thisAccess = binaryExpression.left as PropertyAccessExpression;
                    if (!isObjectLiteralMethod(getThisContainer(thisAccess.expression, /*includeArrowFunctions*/ false))) {
                        return false;
                    }
                    const thisType = checkThisExpression(thisAccess.expression);
                    return thisType && getTypeOfPropertyOfContextualType(thisType, thisAccess.name.escapedText) || false;
                default:
                    return Debug.assertNever(kind);
            }
        }

        function getTypeOfPropertyOfContextualType(type: Type, name: __String) {
            return mapType(type, t => {
                if (t.flags & TypeFlags.StructuredType) {
                    const prop = getPropertyOfType(t, name);
                    if (prop) {
                        return getTypeOfSymbol(prop);
                    }
                    if (isTupleType(t)) {
                        const restType = getRestTypeOfTupleType(t);
                        if (restType && isNumericLiteralName(name) && +name >= 0) {
                            return restType;
                        }
                    }
                }
                return undefined;
            }, /*noReductions*/ true);
        }

        function getIndexTypeOfContextualType(type: Type, kind: IndexKind) {
            return mapType(type, t => getIndexTypeOfStructuredType(t, kind), /*noReductions*/ true);
        }

        // Return true if the given contextual type is a tuple-like type
        function contextualTypeIsTupleLikeType(type: Type): boolean {
            return !!(type.flags & TypeFlags.Union ? forEach((<UnionType>type).types, isTupleLikeType) : isTupleLikeType(type));
        }

        // In an object literal contextually typed by a type T, the contextual type of a property assignment is the type of
        // the matching property in T, if one exists. Otherwise, it is the type of the numeric index signature in T, if one
        // exists. Otherwise, it is the type of the string index signature in T, if one exists.
        function getContextualTypeForObjectLiteralMethod(node: MethodDeclaration): Type | undefined {
            Debug.assert(isObjectLiteralMethod(node));
            if (node.flags & NodeFlags.InWithStatement) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return undefined;
            }

            return getContextualTypeForObjectLiteralElement(node);
        }

        function getContextualTypeForObjectLiteralElement(element: ObjectLiteralElementLike) {
            const objectLiteral = <ObjectLiteralExpression>element.parent;
            const type = getApparentTypeOfContextualType(objectLiteral);
            if (type) {
                if (!hasNonBindableDynamicName(element)) {
                    // For a (non-symbol) computed property, there is no reason to look up the name
                    // in the type. It will just be "__computed", which does not appear in any
                    // SymbolTable.
                    const symbolName = getSymbolOfNode(element).escapedName;
                    const propertyType = getTypeOfPropertyOfContextualType(type, symbolName);
                    if (propertyType) {
                        return propertyType;
                    }
                }

                return isNumericName(element.name!) && getIndexTypeOfContextualType(type, IndexKind.Number) ||
                    getIndexTypeOfContextualType(type, IndexKind.String);
            }

            return undefined;
        }

        // In an array literal contextually typed by a type T, the contextual type of an element expression at index N is
        // the type of the property with the numeric name N in T, if one exists. Otherwise, if T has a numeric index signature,
        // it is the type of the numeric index signature in T. Otherwise, in ES6 and higher, the contextual type is the iterated
        // type of T.
        function getContextualTypeForElementExpression(arrayContextualType: Type | undefined, index: number): Type | undefined {
            return arrayContextualType && (
                getTypeOfPropertyOfContextualType(arrayContextualType, "" + index as __String)
                || getIndexTypeOfContextualType(arrayContextualType, IndexKind.Number)
                || getIteratedTypeOrElementType(arrayContextualType, /*errorNode*/ undefined, /*allowStringInput*/ false, /*allowAsyncIterables*/ false, /*checkAssignability*/ false));
        }

        // In a contextually typed conditional expression, the true/false expressions are contextually typed by the same type.
        function getContextualTypeForConditionalOperand(node: Expression): Type | undefined {
            const conditional = <ConditionalExpression>node.parent;
            return node === conditional.whenTrue || node === conditional.whenFalse ? getContextualType(conditional) : undefined;
        }

        function getContextualTypeForChildJsxExpression(node: JsxElement) {
            const attributesType = getApparentTypeOfContextualType(node.openingElement.tagName);
            // JSX expression is in children of JSX Element, we will look for an "children" atttribute (we get the name from JSX.ElementAttributesProperty)
            const jsxChildrenPropertyName = getJsxElementChildrenPropertyName(getJsxNamespaceAt(node));
            return attributesType && !isTypeAny(attributesType) && jsxChildrenPropertyName && jsxChildrenPropertyName !== "" ? getTypeOfPropertyOfContextualType(attributesType, jsxChildrenPropertyName) : undefined;
        }

        function getContextualTypeForJsxExpression(node: JsxExpression): Type | undefined {
            const exprParent = node.parent;
            return isJsxAttributeLike(exprParent)
                ? getContextualType(node)
                : isJsxElement(exprParent)
                    ? getContextualTypeForChildJsxExpression(exprParent)
                    : undefined;
        }

        function getContextualTypeForJsxAttribute(attribute: JsxAttribute | JsxSpreadAttribute): Type | undefined {
            // When we trying to resolve JsxOpeningLikeElement as a stateless function element, we will already give its attributes a contextual type
            // which is a type of the parameter of the signature we are trying out.
            // If there is no contextual type (e.g. we are trying to resolve stateful component), get attributes type from resolving element's tagName
            if (isJsxAttribute(attribute)) {
                const attributesType = getApparentTypeOfContextualType(attribute.parent);
                if (!attributesType || isTypeAny(attributesType)) {
                    return undefined;
                }
                return getTypeOfPropertyOfContextualType(attributesType, attribute.name.escapedText);
            }
            else {
                return getContextualType(attribute.parent);
            }
        }

        // Return true if the given expression is possibly a discriminant value. We limit the kinds of
        // expressions we check to those that don't depend on their contextual type in order not to cause
        // recursive (and possibly infinite) invocations of getContextualType.
        function isPossiblyDiscriminantValue(node: Expression): boolean {
            switch (node.kind) {
                case SyntaxKind.StringLiteral:
                case SyntaxKind.NumericLiteral:
                case SyntaxKind.NoSubstitutionTemplateLiteral:
                case SyntaxKind.TrueKeyword:
                case SyntaxKind.FalseKeyword:
                case SyntaxKind.NullKeyword:
                case SyntaxKind.Identifier:
                    return true;
                case SyntaxKind.PropertyAccessExpression:
                case SyntaxKind.ParenthesizedExpression:
                    return isPossiblyDiscriminantValue((<PropertyAccessExpression | ParenthesizedExpression>node).expression);
            }
            return false;
        }

        // Return the contextual type for a given expression node. During overload resolution, a contextual type may temporarily
        // be "pushed" onto a node using the contextualType property.
        function getApparentTypeOfContextualType(node: Expression): Type | undefined {
            let contextualType = getContextualType(node);
            contextualType = contextualType && mapType(contextualType, getApparentType);
            if (!(contextualType && contextualType.flags & TypeFlags.Union && isObjectLiteralExpression(node))) {
                return contextualType;
            }
            // Keep the below up-to-date with the work done within `isRelatedTo` by `findMatchingDiscriminantType`
            let match: Type | undefined;
            propLoop: for (const prop of node.properties) {
                if (!prop.symbol) continue;
                if (prop.kind !== SyntaxKind.PropertyAssignment) continue;
                if (isPossiblyDiscriminantValue(prop.initializer) && isDiscriminantProperty(contextualType, prop.symbol.escapedName)) {
                    const discriminatingType = checkExpression(prop.initializer);
                    for (const type of (contextualType as UnionType).types) {
                        const targetType = getTypeOfPropertyOfType(type, prop.symbol.escapedName);
                        if (targetType && isTypeAssignableTo(discriminatingType, targetType)) {
                            if (match) {
                                if (type === match) continue; // Finding multiple fields which discriminate to the same type is fine
                                match = undefined;
                                break propLoop;
                            }
                            match = type;
                        }
                    }
                }
            }
            return match || contextualType;
        }

        /**
         * Woah! Do you really want to use this function?
         *
         * Unless you're trying to get the *non-apparent* type for a
         * value-literal type or you're authoring relevant portions of this algorithm,
         * you probably meant to use 'getApparentTypeOfContextualType'.
         * Otherwise this may not be very useful.
         *
         * In cases where you *are* working on this function, you should understand
         * when it is appropriate to use 'getContextualType' and 'getApparentTypeOfContextualType'.
         *
         *   - Use 'getContextualType' when you are simply going to propagate the result to the expression.
         *   - Use 'getApparentTypeOfContextualType' when you're going to need the members of the type.
         *
         * @param node the expression whose contextual type will be returned.
         * @returns the contextual type of an expression.
         */
        function getContextualType(node: Expression): Type | undefined {
            if (node.flags & NodeFlags.InWithStatement) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return undefined;
            }
            if (node.contextualType) {
                return node.contextualType;
            }
            const { parent } = node;
            switch (parent.kind) {
                case SyntaxKind.VariableDeclaration:
                case SyntaxKind.Parameter:
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                case SyntaxKind.BindingElement:
                    return getContextualTypeForInitializerExpression(node);
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.ReturnStatement:
                    return getContextualTypeForReturnExpression(node);
                case SyntaxKind.YieldExpression:
                    return getContextualTypeForYieldOperand(<YieldExpression>parent);
                    case SyntaxKind.CallExpression:
                case SyntaxKind.NewExpression:
                    return getContextualTypeForArgument(<CallExpression | NewExpression>parent, node);
                case SyntaxKind.TypeAssertionExpression:
                case SyntaxKind.AsExpression:
                    return getTypeFromTypeNode((<AssertionExpression>parent).type);
                case SyntaxKind.BinaryExpression:
                    return getContextualTypeForBinaryOperand(node);
                case SyntaxKind.PropertyAssignment:
                case SyntaxKind.ShorthandPropertyAssignment:
                    return getContextualTypeForObjectLiteralElement(<PropertyAssignment | ShorthandPropertyAssignment>parent);
                case SyntaxKind.SpreadAssignment:
                    return getApparentTypeOfContextualType(parent.parent as ObjectLiteralExpression);
                case SyntaxKind.ArrayLiteralExpression: {
                    const arrayLiteral = <ArrayLiteralExpression>parent;
                    const type = getApparentTypeOfContextualType(arrayLiteral);
                    return getContextualTypeForElementExpression(type, indexOfNode(arrayLiteral.elements, node));
                }
                case SyntaxKind.ConditionalExpression:
                    return getContextualTypeForConditionalOperand(node);
                case SyntaxKind.TemplateSpan:
                    Debug.assert(parent.parent.kind === SyntaxKind.TemplateExpression);
                    return getContextualTypeForSubstitutionExpression(<TemplateExpression>parent.parent, node);
                case SyntaxKind.ParenthesizedExpression: {
                    // Like in `checkParenthesizedExpression`, an `/** @type {xyz} */` comment before a parenthesized expression acts as a type cast.
                    const tag = isInJSFile(parent) ? getJSDocTypeTag(parent) : undefined;
                    return tag ? getTypeFromTypeNode(tag.typeExpression!.type) : getContextualType(<ParenthesizedExpression>parent);
                }
                case SyntaxKind.JsxExpression:
                    return getContextualTypeForJsxExpression(<JsxExpression>parent);
                case SyntaxKind.JsxAttribute:
                case SyntaxKind.JsxSpreadAttribute:
                    return getContextualTypeForJsxAttribute(<JsxAttribute | JsxSpreadAttribute>parent);
                case SyntaxKind.JsxOpeningElement:
                case SyntaxKind.JsxSelfClosingElement:
                    return getContextualJsxElementAttributesType(<JsxOpeningLikeElement>parent);
            }
            return undefined;
        }

        function getContextualMapper(node: Node) {
            const ancestor = findAncestor(node, n => !!n.contextualMapper);
            return ancestor ? ancestor.contextualMapper! : identityMapper;
        }

        function getContextualJsxElementAttributesType(node: JsxOpeningLikeElement) {
            if (isJsxIntrinsicIdentifier(node.tagName)) {
                return getIntrinsicAttributesTypeFromJsxOpeningLikeElement(node);
            }
            const valueType = checkExpression(node.tagName);
            if (isTypeAny(valueType)) {
                // Short-circuit if the class tag is using an element type 'any'
                return anyType;
            }

            const isJs = isInJSFile(node);
            return mapType(valueType, t => getJsxSignaturesParameterTypes(t, isJs, node));
        }

        function getJsxSignaturesParameterTypes(valueType: Type, isJs: boolean, context: JsxOpeningLikeElement) {
            // If the elemType is a string type, we have to return anyType to prevent an error downstream as we will try to find construct or call signature of the type
            if (valueType.flags & TypeFlags.String) {
                return anyType;
            }
            else if (valueType.flags & TypeFlags.StringLiteral) {
                // If the elemType is a stringLiteral type, we can then provide a check to make sure that the string literal type is one of the Jsx intrinsic element type
                // For example:
                //      var CustomTag: "h1" = "h1";
                //      <CustomTag> Hello World </CustomTag>
                const intrinsicElementsType = getJsxType(JsxNames.IntrinsicElements, context);
                if (intrinsicElementsType !== errorType) {
                    const stringLiteralTypeName = (<StringLiteralType>valueType).value;
                    const intrinsicProp = getPropertyOfType(intrinsicElementsType, escapeLeadingUnderscores(stringLiteralTypeName));
                    if (intrinsicProp) {
                        return getTypeOfSymbol(intrinsicProp);
                    }
                    const indexSignatureType = getIndexTypeOfType(intrinsicElementsType, IndexKind.String);
                    if (indexSignatureType) {
                        return indexSignatureType;
                    }
                }
                return anyType;
            }

            // Resolve the signatures, preferring constructor
            let signatures = getSignaturesOfType(valueType, SignatureKind.Construct);
            let ctor = true;
            if (signatures.length === 0) {
                // No construct signatures, try call signatures
                signatures = getSignaturesOfType(valueType, SignatureKind.Call);
                ctor = false;
                if (signatures.length === 0) {
                    // We found no signatures at all, which is an error
                    return errorType;
                }
            }

            const links = getNodeLinks(context);
            if (!links.resolvedSignatures) {
                links.resolvedSignatures = createMap();
            }
            const cacheKey = "" + getTypeId(valueType);
            const cachedResolved = links.resolvedSignatures.get(cacheKey);
            if (cachedResolved && cachedResolved !== resolvingSignaturesArray) {
                signatures = cachedResolved;
            }
            else if (!cachedResolved) {
                links.resolvedSignatures.set(cacheKey, resolvingSignaturesArray);
                links.resolvedSignatures.set(cacheKey, signatures = instantiateJsxSignatures(context, signatures));
            }

            return getUnionType(map(signatures, ctor ? t => getJsxPropsTypeFromClassType(t, isJs, context, /*reportErrors*/ false) : t => getJsxPropsTypeFromCallSignature(t, context)), UnionReduction.None);
        }

        function getJsxPropsTypeFromCallSignature(sig: Signature, context: JsxOpeningLikeElement) {
            let propsType = getTypeOfFirstParameterOfSignatureWithFallback(sig, emptyObjectType);
            propsType = getJsxManagedAttributesFromLocatedAttributes(context, getJsxNamespaceAt(context), propsType);
            const intrinsicAttribs = getJsxType(JsxNames.IntrinsicAttributes, context);
            if (intrinsicAttribs !== errorType) {
                propsType = intersectTypes(intrinsicAttribs, propsType);
            }
            return propsType;
        }

        function getJsxPropsTypeForSignatureFromMember(sig: Signature, forcedLookupLocation: __String) {
            const instanceType = getReturnTypeOfSignature(sig);
            return isTypeAny(instanceType) ? instanceType : getTypeOfPropertyOfType(instanceType, forcedLookupLocation);
        }

        function getJsxManagedAttributesFromLocatedAttributes(context: JsxOpeningLikeElement, ns: Symbol, attributesType: Type) {
            const managedSym = getJsxLibraryManagedAttributes(ns);
            if (managedSym) {
                const declaredManagedType = getDeclaredTypeOfSymbol(managedSym);
                if (length((declaredManagedType as GenericType).typeParameters) >= 2) {
                    const args = fillMissingTypeArguments([checkExpressionCached(context.tagName), attributesType], (declaredManagedType as GenericType).typeParameters, 2, isInJSFile(context));
                    return createTypeReference((declaredManagedType as GenericType), args);
                }
                else if (length(declaredManagedType.aliasTypeArguments) >= 2) {
                    const args = fillMissingTypeArguments([checkExpressionCached(context.tagName), attributesType], declaredManagedType.aliasTypeArguments!, 2, isInJSFile(context));
                    return getTypeAliasInstantiation(declaredManagedType.aliasSymbol!, args);
                }
            }
            return attributesType;
        }

        function getJsxPropsTypeFromClassType(sig: Signature, isJs: boolean, context: JsxOpeningLikeElement, reportErrors: boolean) {
            const ns = getJsxNamespaceAt(context);
            const forcedLookupLocation = getJsxElementPropertiesName(ns);
            let attributesType = forcedLookupLocation === undefined
                // If there is no type ElementAttributesProperty, return the type of the first parameter of the signature, which should be the props type
                ? getTypeOfFirstParameterOfSignatureWithFallback(sig, emptyObjectType)
                : forcedLookupLocation === ""
                    // If there is no e.g. 'props' member in ElementAttributesProperty, use the element class type instead
                    ? getReturnTypeOfSignature(sig)
                    // Otherwise get the type of the property on the signature return type
                    : getJsxPropsTypeForSignatureFromMember(sig, forcedLookupLocation);

            if (!attributesType) {
                // There is no property named 'props' on this instance type
                if (reportErrors && !!forcedLookupLocation && !!length(context.attributes.properties)) {
                    error(context, Diagnostics.JSX_element_class_does_not_support_attributes_because_it_does_not_have_a_0_property, unescapeLeadingUnderscores(forcedLookupLocation));
                }
                return emptyObjectType;
            }

            attributesType = getJsxManagedAttributesFromLocatedAttributes(context, ns, attributesType);

            if (isTypeAny(attributesType)) {
                // Props is of type 'any' or unknown
                return attributesType;
            }
            else {
                // Normal case -- add in IntrinsicClassElements<T> and IntrinsicElements
                let apparentAttributesType = attributesType;
                const intrinsicClassAttribs = getJsxType(JsxNames.IntrinsicClassAttributes, context);
                if (intrinsicClassAttribs !== errorType) {
                    const typeParams = getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(intrinsicClassAttribs.symbol);
                    const hostClassType = getReturnTypeOfSignature(sig);
                    apparentAttributesType = intersectTypes(
                        typeParams
                            ? createTypeReference(<GenericType>intrinsicClassAttribs, fillMissingTypeArguments([hostClassType], typeParams, getMinTypeArgumentCount(typeParams), isJs))
                            : intrinsicClassAttribs,
                        apparentAttributesType
                    );
                }

                const intrinsicAttribs = getJsxType(JsxNames.IntrinsicAttributes, context);
                if (intrinsicAttribs !== errorType) {
                    apparentAttributesType = intersectTypes(intrinsicAttribs, apparentAttributesType);
                }

                return apparentAttributesType;
            }
        }

        // If the given type is an object or union type with a single signature, and if that signature has at
        // least as many parameters as the given function, return the signature. Otherwise return undefined.
        function getContextualCallSignature(type: Type, node: SignatureDeclaration): Signature | undefined {
            const signatures = getSignaturesOfType(type, SignatureKind.Call);
            if (signatures.length === 1) {
                const signature = signatures[0];
                if (!isAritySmaller(signature, node)) {
                    return signature;
                }
            }
        }

        /** If the contextual signature has fewer parameters than the function expression, do not use it */
        function isAritySmaller(signature: Signature, target: SignatureDeclaration) {
            let targetParameterCount = 0;
            for (; targetParameterCount < target.parameters.length; targetParameterCount++) {
                const param = target.parameters[targetParameterCount];
                if (param.initializer || param.questionToken || param.dotDotDotToken || isJSDocOptionalParameter(param)) {
                    break;
                }
            }
            if (target.parameters.length && parameterIsThisKeyword(target.parameters[0])) {
                targetParameterCount--;
            }
            return !hasEffectiveRestParameter(signature) && getParameterCount(signature) < targetParameterCount;
        }

        function isFunctionExpressionOrArrowFunction(node: Node): node is FunctionExpression | ArrowFunction {
            return node.kind === SyntaxKind.FunctionExpression || node.kind === SyntaxKind.ArrowFunction;
        }

        function getContextualSignatureForFunctionLikeDeclaration(node: FunctionLikeDeclaration): Signature | undefined {
            // Only function expressions, arrow functions, and object literal methods are contextually typed.
            return isFunctionExpressionOrArrowFunction(node) || isObjectLiteralMethod(node)
                ? getContextualSignature(<FunctionExpression>node)
                : undefined;
        }

        function getContextualTypeForFunctionLikeDeclaration(node: FunctionExpression | ArrowFunction | MethodDeclaration) {
            return isObjectLiteralMethod(node) ?
                getContextualTypeForObjectLiteralMethod(node) :
                getApparentTypeOfContextualType(node);
        }

        // Return the contextual signature for a given expression node. A contextual type provides a
        // contextual signature if it has a single call signature and if that call signature is non-generic.
        // If the contextual type is a union type, get the signature from each type possible and if they are
        // all identical ignoring their return type, the result is same signature but with return type as
        // union type of return types from these signatures
        function getContextualSignature(node: FunctionExpression | ArrowFunction | MethodDeclaration): Signature | undefined {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));
            const typeTagSignature = getSignatureOfTypeTag(node);
            if (typeTagSignature) {
                return typeTagSignature;
            }
            const type = getContextualTypeForFunctionLikeDeclaration(node);
            if (!type) {
                return undefined;
            }
            if (!(type.flags & TypeFlags.Union)) {
                return getContextualCallSignature(type, node);
            }
            let signatureList: Signature[] | undefined;
            const types = (<UnionType>type).types;
            for (const current of types) {
                const signature = getContextualCallSignature(current, node);
                if (signature) {
                    if (!signatureList) {
                        // This signature will contribute to contextual union signature
                        signatureList = [signature];
                    }
                    else if (!compareSignaturesIdentical(signatureList[0], signature, /*partialMatch*/ false, /*ignoreThisTypes*/ true, /*ignoreReturnTypes*/ true, compareTypesIdentical)) {
                        // Signatures aren't identical, do not use
                        return undefined;
                    }
                    else {
                        // Use this signature for contextual union signature
                        signatureList.push(signature);
                    }
                }
            }

            // Result is union of signatures collected (return type is union of return types of this signature set)
            let result: Signature | undefined;
            if (signatureList) {
                result = cloneSignature(signatureList[0]);
                result.unionSignatures = signatureList;
            }
            return result;
        }

        function checkSpreadExpression(node: SpreadElement, checkMode?: CheckMode): Type {
            if (languageVersion < ScriptTarget.ES2015 && compilerOptions.downlevelIteration) {
                checkExternalEmitHelpers(node, ExternalEmitHelpers.SpreadIncludes);
            }

            const arrayOrIterableType = checkExpression(node.expression, checkMode);
            return checkIteratedTypeOrElementType(arrayOrIterableType, node.expression, /*allowStringInput*/ false, /*allowAsyncIterables*/ false);
        }

        function hasDefaultValue(node: BindingElement | Expression): boolean {
            return (node.kind === SyntaxKind.BindingElement && !!(<BindingElement>node).initializer) ||
                (node.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>node).operatorToken.kind === SyntaxKind.EqualsToken);
        }

        function checkArrayLiteral(node: ArrayLiteralExpression, checkMode: CheckMode | undefined): Type {
            const elements = node.elements;
            const elementCount = elements.length;
            let hasNonEndingSpreadElement = false;
            const elementTypes: Type[] = [];
            const inDestructuringPattern = isAssignmentTarget(node);
            const contextualType = getApparentTypeOfContextualType(node);
            for (let index = 0; index < elementCount; index++) {
                const e = elements[index];
                if (inDestructuringPattern && e.kind === SyntaxKind.SpreadElement) {
                    // Given the following situation:
                    //    var c: {};
                    //    [...c] = ["", 0];
                    //
                    // c is represented in the tree as a spread element in an array literal.
                    // But c really functions as a rest element, and its purpose is to provide
                    // a contextual type for the right hand side of the assignment. Therefore,
                    // instead of calling checkExpression on "...c", which will give an error
                    // if c is not iterable/array-like, we need to act as if we are trying to
                    // get the contextual element type from it. So we do something similar to
                    // getContextualTypeForElementExpression, which will crucially not error
                    // if there is no index type / iterated type.
                    const restArrayType = checkExpression((<SpreadElement>e).expression, checkMode);
                    const restElementType = getIndexTypeOfType(restArrayType, IndexKind.Number) ||
                        getIteratedTypeOrElementType(restArrayType, /*errorNode*/ undefined, /*allowStringInput*/ false, /*allowAsyncIterables*/ false, /*checkAssignability*/ false);
                    if (restElementType) {
                        elementTypes.push(restElementType);
                    }
                }
                else {
                    const elementContextualType = getContextualTypeForElementExpression(contextualType, index);
                    const type = checkExpressionForMutableLocation(e, checkMode, elementContextualType);
                    elementTypes.push(type);
                }
                if (index < elementCount - 1 && e.kind === SyntaxKind.SpreadElement) {
                    hasNonEndingSpreadElement = true;
                }
            }
            if (!hasNonEndingSpreadElement) {
                const hasRestElement = elementCount > 0 && elements[elementCount - 1].kind === SyntaxKind.SpreadElement;
                const minLength = elementCount - (hasRestElement ? 1 : 0);
                // If array literal is actually a destructuring pattern, mark it as an implied type. We do this such
                // that we get the same behavior for "var [x, y] = []" and "[x, y] = []".
                if (inDestructuringPattern && minLength > 0) {
                    const type = cloneTypeReference(<TypeReference>createTupleType(elementTypes, minLength, hasRestElement));
                    type.pattern = node;
                    return type;
                }
                if (contextualType && contextualTypeIsTupleLikeType(contextualType)) {
                    const pattern = contextualType.pattern;
                    // If array literal is contextually typed by a binding pattern or an assignment pattern, pad the resulting
                    // tuple type with the corresponding binding or assignment element types to make the lengths equal.
                    if (!hasRestElement && pattern && (pattern.kind === SyntaxKind.ArrayBindingPattern || pattern.kind === SyntaxKind.ArrayLiteralExpression)) {
                        const patternElements = (<BindingPattern | ArrayLiteralExpression>pattern).elements;
                        for (let i = elementCount; i < patternElements.length; i++) {
                            const e = patternElements[i];
                            if (hasDefaultValue(e)) {
                                elementTypes.push((<TypeReference>contextualType).typeArguments![i]);
                            }
                            else if (i < patternElements.length - 1 || !(e.kind === SyntaxKind.BindingElement && (<BindingElement>e).dotDotDotToken || e.kind === SyntaxKind.SpreadElement)) {
                                if (e.kind !== SyntaxKind.OmittedExpression) {
                                    error(e, Diagnostics.Initializer_provides_no_value_for_this_binding_element_and_the_binding_element_has_no_default_value);
                                }
                                elementTypes.push(strictNullChecks ? implicitNeverType : undefinedWideningType);
                            }
                        }
                    }
                    return createTupleType(elementTypes, minLength, hasRestElement);
                }
            }
            return getArrayLiteralType(elementTypes, UnionReduction.Subtype);
        }

        function getArrayLiteralType(elementTypes: Type[], unionReduction = UnionReduction.Literal) {
            return createArrayType(elementTypes.length ?
                getUnionType(elementTypes, unionReduction) :
                strictNullChecks ? implicitNeverType : undefinedWideningType);
        }

        function isNumericName(name: DeclarationName): boolean {
            switch (name.kind) {
                case SyntaxKind.ComputedPropertyName:
                    return isNumericComputedName(name);
                case SyntaxKind.Identifier:
                    return isNumericLiteralName(name.escapedText);
                case SyntaxKind.NumericLiteral:
                case SyntaxKind.StringLiteral:
                    return isNumericLiteralName(name.text);
                default:
                    return false;
            }
        }

        function isNumericComputedName(name: ComputedPropertyName): boolean {
            // It seems odd to consider an expression of type Any to result in a numeric name,
            // but this behavior is consistent with checkIndexedAccess
            return isTypeAssignableToKind(checkComputedPropertyName(name), TypeFlags.NumberLike);
        }

        function isInfinityOrNaNString(name: string | __String): boolean {
            return name === "Infinity" || name === "-Infinity" || name === "NaN";
        }

        function isNumericLiteralName(name: string | __String) {
            // The intent of numeric names is that
            //     - they are names with text in a numeric form, and that
            //     - setting properties/indexing with them is always equivalent to doing so with the numeric literal 'numLit',
            //         acquired by applying the abstract 'ToNumber' operation on the name's text.
            //
            // The subtlety is in the latter portion, as we cannot reliably say that anything that looks like a numeric literal is a numeric name.
            // In fact, it is the case that the text of the name must be equal to 'ToString(numLit)' for this to hold.
            //
            // Consider the property name '"0xF00D"'. When one indexes with '0xF00D', they are actually indexing with the value of 'ToString(0xF00D)'
            // according to the ECMAScript specification, so it is actually as if the user indexed with the string '"61453"'.
            // Thus, the text of all numeric literals equivalent to '61543' such as '0xF00D', '0xf00D', '0170015', etc. are not valid numeric names
            // because their 'ToString' representation is not equal to their original text.
            // This is motivated by ECMA-262 sections 9.3.1, 9.8.1, 11.1.5, and 11.2.1.
            //
            // Here, we test whether 'ToString(ToNumber(name))' is exactly equal to 'name'.
            // The '+' prefix operator is equivalent here to applying the abstract ToNumber operation.
            // Applying the 'toString()' method on a number gives us the abstract ToString operation on a number.
            //
            // Note that this accepts the values 'Infinity', '-Infinity', and 'NaN', and that this is intentional.
            // This is desired behavior, because when indexing with them as numeric entities, you are indexing
            // with the strings '"Infinity"', '"-Infinity"', and '"NaN"' respectively.
            return (+name).toString() === name;
        }

        function checkComputedPropertyName(node: ComputedPropertyName): Type {
            const links = getNodeLinks(node.expression);
            if (!links.resolvedType) {
                links.resolvedType = checkExpression(node.expression);
                // This will allow types number, string, symbol or any. It will also allow enums, the unknown
                // type, and any union of these types (like string | number).
                if (links.resolvedType.flags & TypeFlags.Nullable ||
                    !isTypeAssignableToKind(links.resolvedType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbolLike) &&
                    !isTypeAssignableTo(links.resolvedType, stringNumberSymbolType)) {
                    error(node, Diagnostics.A_computed_property_name_must_be_of_type_string_number_symbol_or_any);
                }
                else {
                    checkThatExpressionIsProperSymbolReference(node.expression, links.resolvedType, /*reportError*/ true);
                }
            }

            return links.resolvedType;
        }

        function getObjectLiteralIndexInfo(propertyNodes: NodeArray<ObjectLiteralElementLike>, offset: number, properties: Symbol[], kind: IndexKind): IndexInfo {
            const propTypes: Type[] = [];
            for (let i = 0; i < properties.length; i++) {
                if (kind === IndexKind.String || isNumericName(propertyNodes[i + offset].name!)) {
                    propTypes.push(getTypeOfSymbol(properties[i]));
                }
            }
            const unionType = propTypes.length ? getUnionType(propTypes, UnionReduction.Subtype) : undefinedType;
            return createIndexInfo(unionType, /*isReadonly*/ false);
        }

        function checkObjectLiteral(node: ObjectLiteralExpression, checkMode?: CheckMode): Type {
            const inDestructuringPattern = isAssignmentTarget(node);
            // Grammar checking
            checkGrammarObjectLiteralExpression(node, inDestructuringPattern);

            let propertiesTable: SymbolTable;
            let propertiesArray: Symbol[] = [];
            let spread: Type = emptyObjectType;
            let propagatedFlags: TypeFlags = TypeFlags.FreshLiteral;

            const contextualType = getApparentTypeOfContextualType(node);
            const contextualTypeHasPattern = contextualType && contextualType.pattern &&
                (contextualType.pattern.kind === SyntaxKind.ObjectBindingPattern || contextualType.pattern.kind === SyntaxKind.ObjectLiteralExpression);
            const isInJavascript = isInJSFile(node) && !isInJsonFile(node);
            const enumTag = getJSDocEnumTag(node);
            const isJSObjectLiteral = !contextualType && isInJavascript && !enumTag;
            let typeFlags: TypeFlags = 0;
            let patternWithComputedProperties = false;
            let hasComputedStringProperty = false;
            let hasComputedNumberProperty = false;
            propertiesTable = createSymbolTable();

            let offset = 0;
            for (let i = 0; i < node.properties.length; i++) {
                const memberDecl = node.properties[i];
                let member = getSymbolOfNode(memberDecl);
                const computedNameType = memberDecl.name && memberDecl.name.kind === SyntaxKind.ComputedPropertyName && !isWellKnownSymbolSyntactically(memberDecl.name.expression) ?
                    checkComputedPropertyName(memberDecl.name) : undefined;
                if (memberDecl.kind === SyntaxKind.PropertyAssignment ||
                    memberDecl.kind === SyntaxKind.ShorthandPropertyAssignment ||
                    isObjectLiteralMethod(memberDecl)) {
                    let type = memberDecl.kind === SyntaxKind.PropertyAssignment ? checkPropertyAssignment(memberDecl, checkMode) :
                        memberDecl.kind === SyntaxKind.ShorthandPropertyAssignment ? checkExpressionForMutableLocation(memberDecl.name, checkMode) :
                        checkObjectLiteralMethod(memberDecl, checkMode);
                    if (isInJavascript) {
                        const jsDocType = getTypeForDeclarationFromJSDocComment(memberDecl);
                        if (jsDocType) {
                            checkTypeAssignableTo(type, jsDocType, memberDecl);
                            type = jsDocType;
                        }
                        else if (enumTag && enumTag.typeExpression) {
                            checkTypeAssignableTo(type, getTypeFromTypeNode(enumTag.typeExpression), memberDecl);
                        }
                    }
                    typeFlags |= type.flags;
                    const nameType = computedNameType && computedNameType.flags & TypeFlags.StringOrNumberLiteralOrUnique ?
                        <LiteralType | UniqueESSymbolType>computedNameType : undefined;
                    const prop = nameType ?
                        createSymbol(SymbolFlags.Property | member.flags, getLateBoundNameFromType(nameType), CheckFlags.Late) :
                        createSymbol(SymbolFlags.Property | member.flags, member.escapedName);
                    if (nameType) {
                        prop.nameType = nameType;
                    }

                    if (inDestructuringPattern) {
                        // If object literal is an assignment pattern and if the assignment pattern specifies a default value
                        // for the property, make the property optional.
                        const isOptional =
                            (memberDecl.kind === SyntaxKind.PropertyAssignment && hasDefaultValue(memberDecl.initializer)) ||
                            (memberDecl.kind === SyntaxKind.ShorthandPropertyAssignment && memberDecl.objectAssignmentInitializer);
                        if (isOptional) {
                            prop.flags |= SymbolFlags.Optional;
                        }
                    }
                    else if (contextualTypeHasPattern && !(getObjectFlags(contextualType!) & ObjectFlags.ObjectLiteralPatternWithComputedProperties)) {
                        // If object literal is contextually typed by the implied type of a binding pattern, and if the
                        // binding pattern specifies a default value for the property, make the property optional.
                        const impliedProp = getPropertyOfType(contextualType!, member.escapedName);
                        if (impliedProp) {
                            prop.flags |= impliedProp.flags & SymbolFlags.Optional;
                        }

                        else if (!compilerOptions.suppressExcessPropertyErrors && !getIndexInfoOfType(contextualType!, IndexKind.String)) {
                            error(memberDecl.name, Diagnostics.Object_literal_may_only_specify_known_properties_and_0_does_not_exist_in_type_1,
                                symbolToString(member), typeToString(contextualType!));
                        }
                    }

                    prop.declarations = member.declarations;
                    prop.parent = member.parent;
                    if (member.valueDeclaration) {
                        prop.valueDeclaration = member.valueDeclaration;
                    }

                    prop.type = type;
                    prop.target = member;
                    member = prop;
                }
                else if (memberDecl.kind === SyntaxKind.SpreadAssignment) {
                    if (languageVersion < ScriptTarget.ES2015) {
                        checkExternalEmitHelpers(memberDecl, ExternalEmitHelpers.Assign);
                    }
                    if (propertiesArray.length > 0) {
                        spread = getSpreadType(spread, createObjectLiteralType(), node.symbol, propagatedFlags, /*objectFlags*/ 0);
                        propertiesArray = [];
                        propertiesTable = createSymbolTable();
                        hasComputedStringProperty = false;
                        hasComputedNumberProperty = false;
                        typeFlags = 0;
                    }
                    const type = checkExpression(memberDecl.expression);
                    if (!isValidSpreadType(type)) {
                        error(memberDecl, Diagnostics.Spread_types_may_only_be_created_from_object_types);
                        return errorType;
                    }
                    spread = getSpreadType(spread, type, node.symbol, propagatedFlags, /*objectFlags*/ 0);
                    offset = i + 1;
                    continue;
                }
                else {
                    // TypeScript 1.0 spec (April 2014)
                    // A get accessor declaration is processed in the same manner as
                    // an ordinary function declaration(section 6.1) with no parameters.
                    // A set accessor declaration is processed in the same manner
                    // as an ordinary function declaration with a single parameter and a Void return type.
                    Debug.assert(memberDecl.kind === SyntaxKind.GetAccessor || memberDecl.kind === SyntaxKind.SetAccessor);
                    checkNodeDeferred(memberDecl);
                }

                if (computedNameType && !(computedNameType.flags & TypeFlags.StringOrNumberLiteralOrUnique)) {
                    if (isTypeAssignableTo(computedNameType, stringNumberSymbolType)) {
                        if (isTypeAssignableTo(computedNameType, numberType)) {
                            hasComputedNumberProperty = true;
                        }
                        else {
                            hasComputedStringProperty = true;
                        }
                        if (inDestructuringPattern) {
                            patternWithComputedProperties = true;
                        }
                    }
                }
                else {
                    propertiesTable.set(member.escapedName, member);
                }
                propertiesArray.push(member);
            }

            // If object literal is contextually typed by the implied type of a binding pattern, augment the result
            // type with those properties for which the binding pattern specifies a default value.
            if (contextualTypeHasPattern) {
                for (const prop of getPropertiesOfType(contextualType!)) {
                    if (!propertiesTable.get(prop.escapedName) && !(spread && getPropertyOfType(spread, prop.escapedName))) {
                        if (!(prop.flags & SymbolFlags.Optional)) {
                            error(prop.valueDeclaration || (<TransientSymbol>prop).bindingElement,
                                Diagnostics.Initializer_provides_no_value_for_this_binding_element_and_the_binding_element_has_no_default_value);
                        }
                        propertiesTable.set(prop.escapedName, prop);
                        propertiesArray.push(prop);
                    }
                }
            }

            if (spread !== emptyObjectType) {
                if (propertiesArray.length > 0) {
                    spread = getSpreadType(spread, createObjectLiteralType(), node.symbol, propagatedFlags, /*objectFlags*/ 0);
                }
                return spread;
            }

            return createObjectLiteralType();

            function createObjectLiteralType() {
                const stringIndexInfo = hasComputedStringProperty ? getObjectLiteralIndexInfo(node.properties, offset, propertiesArray, IndexKind.String) : undefined;
                const numberIndexInfo = hasComputedNumberProperty ? getObjectLiteralIndexInfo(node.properties, offset, propertiesArray, IndexKind.Number) : undefined;
                const result = createAnonymousType(node.symbol, propertiesTable, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
                const freshObjectLiteralFlag = compilerOptions.suppressExcessPropertyErrors ? 0 : TypeFlags.FreshLiteral;
                result.flags |= TypeFlags.ContainsObjectLiteral | freshObjectLiteralFlag | (typeFlags & TypeFlags.PropagatingFlags);
                result.objectFlags |= ObjectFlags.ObjectLiteral;
                if (isJSObjectLiteral) {
                    result.objectFlags |= ObjectFlags.JSLiteral;
                }
                if (patternWithComputedProperties) {
                    result.objectFlags |= ObjectFlags.ObjectLiteralPatternWithComputedProperties;
                }
                if (inDestructuringPattern) {
                    result.pattern = node;
                }
                if (!(result.flags & TypeFlags.Nullable)) {
                    propagatedFlags |= (result.flags & TypeFlags.PropagatingFlags);
                }
                return result;
            }
        }

        function isValidSpreadType(type: Type): boolean {
            return !!(type.flags & (TypeFlags.AnyOrUnknown | TypeFlags.NonPrimitive) ||
                getFalsyFlags(type) & TypeFlags.DefinitelyFalsy && isValidSpreadType(removeDefinitelyFalsyTypes(type)) ||
                type.flags & TypeFlags.Object && !isGenericMappedType(type) ||
                type.flags & TypeFlags.UnionOrIntersection && every((<UnionOrIntersectionType>type).types, isValidSpreadType));
        }

        function checkJsxSelfClosingElementDeferred(node: JsxSelfClosingElement) {
            checkJsxOpeningLikeElementOrOpeningFragment(node, CheckMode.Normal);
        }

        function checkJsxSelfClosingElement(node: JsxSelfClosingElement, _checkMode: CheckMode | undefined): Type {
            checkNodeDeferred(node);
            return getJsxElementTypeAt(node) || anyType;
        }

        function checkJsxElementDeferred(node: JsxElement) {
            // Check attributes
            checkJsxOpeningLikeElementOrOpeningFragment(node.openingElement, CheckMode.Normal);

            // Perform resolution on the closing tag so that rename/go to definition/etc work
            if (isJsxIntrinsicIdentifier(node.closingElement.tagName)) {
                getIntrinsicTagSymbol(node.closingElement);
            }
            else {
                checkExpression(node.closingElement.tagName);
            }
        }

        function checkJsxElement(node: JsxElement, _checkMode: CheckMode | undefined): Type {
            checkNodeDeferred(node);

            return getJsxElementTypeAt(node) || anyType;
        }

        function checkJsxFragment(node: JsxFragment, checkMode: CheckMode | undefined): Type {
            checkJsxOpeningLikeElementOrOpeningFragment(node.openingFragment, checkMode);

            if (compilerOptions.jsx === JsxEmit.React && (compilerOptions.jsxFactory || getSourceFileOfNode(node).pragmas.has("jsx"))) {
                error(node, compilerOptions.jsxFactory
                    ? Diagnostics.JSX_fragment_is_not_supported_when_using_jsxFactory
                    : Diagnostics.JSX_fragment_is_not_supported_when_using_an_inline_JSX_factory_pragma);
            }

            return getJsxElementTypeAt(node) || anyType;
        }

        /**
         * Returns true iff the JSX element name would be a valid JS identifier, ignoring restrictions about keywords not being identifiers
         */
        function isUnhyphenatedJsxName(name: string | __String) {
            // - is the only character supported in JSX attribute names that isn't valid in JavaScript identifiers
            return !stringContains(name as string, "-");
        }

        /**
         * Returns true iff React would emit this tag name as a string rather than an identifier or qualified name
         */
        function isJsxIntrinsicIdentifier(tagName: JsxTagNameExpression): boolean {
            return tagName.kind === SyntaxKind.Identifier && isIntrinsicJsxName(tagName.escapedText);
        }

        function checkJsxAttribute(node: JsxAttribute, checkMode?: CheckMode) {
            return node.initializer
                ? checkExpressionForMutableLocation(node.initializer, checkMode)
                : trueType;  // <Elem attr /> is sugar for <Elem attr={true} />
        }

        /**
         * Get attributes type of the JSX opening-like element. The result is from resolving "attributes" property of the opening-like element.
         *
         * @param openingLikeElement a JSX opening-like element
         * @param filter a function to remove attributes that will not participate in checking whether attributes are assignable
         * @return an anonymous type (similar to the one returned by checkObjectLiteral) in which its properties are attributes property.
         * @remarks Because this function calls getSpreadType, it needs to use the same checks as checkObjectLiteral,
         * which also calls getSpreadType.
         */
        function createJsxAttributesTypeFromAttributesProperty(openingLikeElement: JsxOpeningLikeElement, checkMode: CheckMode | undefined) {
            const attributes = openingLikeElement.attributes;
            let attributesTable = createSymbolTable();
            let spread: Type = emptyObjectType;
            let hasSpreadAnyType = false;
            let typeToIntersect: Type | undefined;
            let explicitlySpecifyChildrenAttribute = false;
            const jsxChildrenPropertyName = getJsxElementChildrenPropertyName(getJsxNamespaceAt(openingLikeElement));

            for (const attributeDecl of attributes.properties) {
                const member = attributeDecl.symbol;
                if (isJsxAttribute(attributeDecl)) {
                    const exprType = checkJsxAttribute(attributeDecl, checkMode);

                    const attributeSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient | member.flags, member.escapedName);
                    attributeSymbol.declarations = member.declarations;
                    attributeSymbol.parent = member.parent;
                    if (member.valueDeclaration) {
                        attributeSymbol.valueDeclaration = member.valueDeclaration;
                    }
                    attributeSymbol.type = exprType;
                    attributeSymbol.target = member;
                    attributesTable.set(attributeSymbol.escapedName, attributeSymbol);
                    if (attributeDecl.name.escapedText === jsxChildrenPropertyName) {
                        explicitlySpecifyChildrenAttribute = true;
                    }
                }
                else {
                    Debug.assert(attributeDecl.kind === SyntaxKind.JsxSpreadAttribute);
                    if (attributesTable.size > 0) {
                        spread = getSpreadType(spread, createJsxAttributesType(), attributes.symbol, /*typeFlags*/ 0, ObjectFlags.JsxAttributes);
                        attributesTable = createSymbolTable();
                    }
                    const exprType = checkExpressionCached(attributeDecl.expression, checkMode);
                    if (isTypeAny(exprType)) {
                        hasSpreadAnyType = true;
                    }
                    if (isValidSpreadType(exprType)) {
                        spread = getSpreadType(spread, exprType, openingLikeElement.symbol, /*typeFlags*/ 0, ObjectFlags.JsxAttributes);
                    }
                    else {
                        typeToIntersect = typeToIntersect ? getIntersectionType([typeToIntersect, exprType]) : exprType;
                    }
                }
            }

            if (!hasSpreadAnyType) {
                if (attributesTable.size > 0) {
                    spread = getSpreadType(spread, createJsxAttributesType(), attributes.symbol, /*typeFlags*/ 0, ObjectFlags.JsxAttributes);
                }
            }

            // Handle children attribute
            const parent = openingLikeElement.parent.kind === SyntaxKind.JsxElement ? openingLikeElement.parent as JsxElement : undefined;
            // We have to check that openingElement of the parent is the one we are visiting as this may not be true for selfClosingElement
            if (parent && parent.openingElement === openingLikeElement && parent.children.length > 0) {
                const childrenTypes: Type[] = checkJsxChildren(parent, checkMode);

                if (!hasSpreadAnyType && jsxChildrenPropertyName && jsxChildrenPropertyName !== "") {
                    // Error if there is a attribute named "children" explicitly specified and children element.
                    // This is because children element will overwrite the value from attributes.
                    // Note: we will not warn "children" attribute overwritten if "children" attribute is specified in object spread.
                    if (explicitlySpecifyChildrenAttribute) {
                        error(attributes, Diagnostics._0_are_specified_twice_The_attribute_named_0_will_be_overwritten, unescapeLeadingUnderscores(jsxChildrenPropertyName));
                    }

                    // If there are children in the body of JSX element, create dummy attribute "children" with the union of children types so that it will pass the attribute checking process
                    const childrenPropSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, jsxChildrenPropertyName);
                    childrenPropSymbol.type = childrenTypes.length === 1 ?
                        childrenTypes[0] :
                        createArrayType(getUnionType(childrenTypes));
                    const childPropMap = createSymbolTable();
                    childPropMap.set(jsxChildrenPropertyName, childrenPropSymbol);
                    spread = getSpreadType(spread, createAnonymousType(attributes.symbol, childPropMap, emptyArray, emptyArray, /*stringIndexInfo*/ undefined, /*numberIndexInfo*/ undefined),
                        attributes.symbol, /*typeFlags*/ 0, ObjectFlags.JsxAttributes);

                }
            }

            if (hasSpreadAnyType) {
                return anyType;
            }
            if (typeToIntersect && spread !== emptyObjectType) {
                return getIntersectionType([typeToIntersect, spread]);
            }
            return typeToIntersect || (spread === emptyObjectType ? createJsxAttributesType() : spread);

            /**
             * Create anonymous type from given attributes symbol table.
             * @param symbol a symbol of JsxAttributes containing attributes corresponding to attributesTable
             * @param attributesTable a symbol table of attributes property
             */
            function createJsxAttributesType() {
                const result = createAnonymousType(attributes.symbol, attributesTable, emptyArray, emptyArray, /*stringIndexInfo*/ undefined, /*numberIndexInfo*/ undefined);
                result.flags |= TypeFlags.ContainsObjectLiteral;
                result.objectFlags |= ObjectFlags.ObjectLiteral | ObjectFlags.JsxAttributes;
                return result;
            }
        }

        function checkJsxChildren(node: JsxElement | JsxFragment, checkMode?: CheckMode) {
            const childrenTypes: Type[] = [];
            for (const child of node.children) {
                // In React, JSX text that contains only whitespaces will be ignored so we don't want to type-check that
                // because then type of children property will have constituent of string type.
                if (child.kind === SyntaxKind.JsxText) {
                    if (!child.containsOnlyWhiteSpaces) {
                        childrenTypes.push(stringType);
                    }
                }
                else {
                    childrenTypes.push(checkExpressionForMutableLocation(child, checkMode));
                }
            }
            return childrenTypes;
        }

        /**
         * Check attributes property of opening-like element. This function is called during chooseOverload to get call signature of a JSX opening-like element.
         * (See "checkApplicableSignatureForJsxOpeningLikeElement" for how the function is used)
         * @param node a JSXAttributes to be resolved of its type
         */
        function checkJsxAttributes(node: JsxAttributes, checkMode: CheckMode | undefined) {
            return createJsxAttributesTypeFromAttributesProperty(node.parent, checkMode);
        }

        function getJsxType(name: __String, location: Node | undefined) {
            const namespace = getJsxNamespaceAt(location);
            const exports = namespace && getExportsOfSymbol(namespace);
            const typeSymbol = exports && getSymbol(exports, name, SymbolFlags.Type);
            return typeSymbol ? getDeclaredTypeOfSymbol(typeSymbol) : errorType;
        }

        /**
         * Looks up an intrinsic tag name and returns a symbol that either points to an intrinsic
         * property (in which case nodeLinks.jsxFlags will be IntrinsicNamedElement) or an intrinsic
         * string index signature (in which case nodeLinks.jsxFlags will be IntrinsicIndexedElement).
         * May also return unknownSymbol if both of these lookups fail.
         */
        function getIntrinsicTagSymbol(node: JsxOpeningLikeElement | JsxClosingElement): Symbol {
            const links = getNodeLinks(node);
            if (!links.resolvedSymbol) {
                const intrinsicElementsType = getJsxType(JsxNames.IntrinsicElements, node);
                if (intrinsicElementsType !== errorType) {
                    // Property case
                    if (!isIdentifier(node.tagName)) return Debug.fail();
                    const intrinsicProp = getPropertyOfType(intrinsicElementsType, node.tagName.escapedText);
                    if (intrinsicProp) {
                        links.jsxFlags |= JsxFlags.IntrinsicNamedElement;
                        return links.resolvedSymbol = intrinsicProp;
                    }

                    // Intrinsic string indexer case
                    const indexSignatureType = getIndexTypeOfType(intrinsicElementsType, IndexKind.String);
                    if (indexSignatureType) {
                        links.jsxFlags |= JsxFlags.IntrinsicIndexedElement;
                        return links.resolvedSymbol = intrinsicElementsType.symbol;
                    }

                    // Wasn't found
                    error(node, Diagnostics.Property_0_does_not_exist_on_type_1, idText(node.tagName), "JSX." + JsxNames.IntrinsicElements);
                    return links.resolvedSymbol = unknownSymbol;
                }
                else {
                    if (noImplicitAny) {
                        error(node, Diagnostics.JSX_element_implicitly_has_type_any_because_no_interface_JSX_0_exists, unescapeLeadingUnderscores(JsxNames.IntrinsicElements));
                    }
                    return links.resolvedSymbol = unknownSymbol;
                }
            }
            return links.resolvedSymbol;
        }

        function instantiateJsxSignatures(node: JsxOpeningLikeElement, signatures: ReadonlyArray<Signature>) {
            const instantiatedSignatures = [];
            let candidateForTypeArgumentError: Signature | undefined;
            let hasTypeArgumentError: boolean = !!node.typeArguments;
            for (const signature of signatures) {
                if (signature.typeParameters) {
                    const isJavascript = isInJSFile(node);
                    const typeArgumentInstantiated = getJsxSignatureTypeArgumentInstantiation(signature, node, isJavascript, /*reportErrors*/ false);
                    if (typeArgumentInstantiated) {
                        hasTypeArgumentError = false;
                        instantiatedSignatures.push(typeArgumentInstantiated);
                    }
                    else {
                        if (node.typeArguments && hasCorrectTypeArgumentArity(signature, node.typeArguments)) {
                            candidateForTypeArgumentError = signature;
                        }
                        const inferenceContext = createInferenceContext(signature.typeParameters, signature, /*flags*/ isJavascript ? InferenceFlags.AnyDefault : InferenceFlags.None);
                        const typeArguments = inferJsxTypeArguments(signature, node, inferenceContext);
                        instantiatedSignatures.push(getSignatureInstantiation(signature, typeArguments, isJavascript));
                    }
                }
                else {
                    instantiatedSignatures.push(signature);
                }
            }
            if (node.typeArguments && hasTypeArgumentError) {
                if (candidateForTypeArgumentError) {
                    checkTypeArguments(candidateForTypeArgumentError, node.typeArguments, /*reportErrors*/ true);
                }
                // Length check to avoid issuing an arity error on length=0, the "Type argument list cannot be empty" grammar error alone is fine
                else if (node.typeArguments.length !== 0) {
                    diagnostics.add(getTypeArgumentArityError(node, signatures, node.typeArguments));
                }
            }
            return instantiatedSignatures;
        }

        function getJsxSignatureTypeArgumentInstantiation(signature: Signature, node: JsxOpeningLikeElement, isJavascript: boolean, reportErrors = false) {
            if (!node.typeArguments) {
                return;
            }
            if (!hasCorrectTypeArgumentArity(signature, node.typeArguments)) {
                return;
            }
            const args = checkTypeArguments(signature, node.typeArguments, reportErrors);
            if (!args) {
                return;
            }
            return getSignatureInstantiation(signature, args, isJavascript);
        }

        function getJsxNamespaceAt(location: Node | undefined): Symbol {
            const links = location && getNodeLinks(location);
            if (links && links.jsxNamespace) {
                return links.jsxNamespace;
            }
            if (!links || links.jsxNamespace !== false) {
                const namespaceName = getJsxNamespace(location);
                const resolvedNamespace = resolveName(location, namespaceName, SymbolFlags.Namespace, /*diagnosticMessage*/ undefined, namespaceName, /*isUse*/ false);
                if (resolvedNamespace) {
                    const candidate = getSymbol(getExportsOfSymbol(resolveSymbol(resolvedNamespace)), JsxNames.JSX, SymbolFlags.Namespace);
                    if (candidate) {
                        if (links) {
                            links.jsxNamespace = candidate;
                        }
                        return candidate;
                    }
                    if (links) {
                        links.jsxNamespace = false;
                    }
                }
            }
            // JSX global fallback
            return getGlobalSymbol(JsxNames.JSX, SymbolFlags.Namespace, /*diagnosticMessage*/ undefined)!; // TODO: GH#18217
        }

        /**
         * Look into JSX namespace and then look for container with matching name as nameOfAttribPropContainer.
         * Get a single property from that container if existed. Report an error if there are more than one property.
         *
         * @param nameOfAttribPropContainer a string of value JsxNames.ElementAttributesPropertyNameContainer or JsxNames.ElementChildrenAttributeNameContainer
         *          if other string is given or the container doesn't exist, return undefined.
         */
        function getNameFromJsxElementAttributesContainer(nameOfAttribPropContainer: __String, jsxNamespace: Symbol): __String | undefined {
            // JSX.ElementAttributesProperty | JSX.ElementChildrenAttribute [symbol]
            const jsxElementAttribPropInterfaceSym = jsxNamespace && getSymbol(jsxNamespace.exports!, nameOfAttribPropContainer, SymbolFlags.Type);
            // JSX.ElementAttributesProperty | JSX.ElementChildrenAttribute [type]
            const jsxElementAttribPropInterfaceType = jsxElementAttribPropInterfaceSym && getDeclaredTypeOfSymbol(jsxElementAttribPropInterfaceSym);
            // The properties of JSX.ElementAttributesProperty | JSX.ElementChildrenAttribute
            const propertiesOfJsxElementAttribPropInterface = jsxElementAttribPropInterfaceType && getPropertiesOfType(jsxElementAttribPropInterfaceType);
            if (propertiesOfJsxElementAttribPropInterface) {
                // Element Attributes has zero properties, so the element attributes type will be the class instance type
                if (propertiesOfJsxElementAttribPropInterface.length === 0) {
                    return "" as __String;
                }
                // Element Attributes has one property, so the element attributes type will be the type of the corresponding
                // property of the class instance type
                else if (propertiesOfJsxElementAttribPropInterface.length === 1) {
                    return propertiesOfJsxElementAttribPropInterface[0].escapedName;
                }
                else if (propertiesOfJsxElementAttribPropInterface.length > 1) {
                    // More than one property on ElementAttributesProperty is an error
                    error(jsxElementAttribPropInterfaceSym!.declarations[0], Diagnostics.The_global_type_JSX_0_may_not_have_more_than_one_property, unescapeLeadingUnderscores(nameOfAttribPropContainer));
                }
            }
            return undefined;
        }

        function getJsxLibraryManagedAttributes(jsxNamespace: Symbol) {
            // JSX.LibraryManagedAttributes [symbol]
            return jsxNamespace && getSymbol(jsxNamespace.exports!, JsxNames.LibraryManagedAttributes, SymbolFlags.Type);
        }

        /// e.g. "props" for React.d.ts,
        /// or 'undefined' if ElementAttributesProperty doesn't exist (which means all
        ///     non-intrinsic elements' attributes type is 'any'),
        /// or '' if it has 0 properties (which means every
        ///     non-intrinsic elements' attributes type is the element instance type)
        function getJsxElementPropertiesName(jsxNamespace: Symbol) {
            return getNameFromJsxElementAttributesContainer(JsxNames.ElementAttributesPropertyNameContainer, jsxNamespace);
        }

        function getJsxElementChildrenPropertyName(jsxNamespace: Symbol): __String | undefined {
            return getNameFromJsxElementAttributesContainer(JsxNames.ElementChildrenAttributeNameContainer, jsxNamespace);
        }

        function getApparentTypeOfJsxPropsType(propsType: Type | undefined): Type | undefined {
            if (!propsType) {
                return undefined;
            }
            if (propsType.flags & TypeFlags.Intersection) {
                const propsApparentType: Type[] = [];
                for (const t of (<UnionOrIntersectionType>propsType).types) {
                    propsApparentType.push(getApparentType(t));
                }
                return getIntersectionType(propsApparentType);
            }
            return getApparentType(propsType);
        }

        /**
         * Get JSX attributes type by trying to resolve openingLikeElement as a stateless function component.
         * Return only attributes type of successfully resolved call signature.
         * This function assumes that the caller handled other possible element type of the JSX element (e.g. stateful component)
         * Unlike tryGetAllJsxStatelessFunctionAttributesType, this function is a default behavior of type-checkers.
         * @param openingLikeElement a JSX opening-like element to find attributes type
         * @param elementType a type of the opening-like element. This elementType can't be an union type
         * @param elemInstanceType an element instance type (the result of newing or invoking this tag)
         * @param elementClassType a JSX-ElementClass type. This is a result of looking up ElementClass interface in the JSX global
         */
        function defaultTryGetJsxStatelessFunctionAttributesType(openingLikeElement: JsxOpeningLikeElement, elementType: Type, elemInstanceType: Type, elementClassType?: Type): Type | undefined {
            Debug.assert(!(elementType.flags & TypeFlags.Union));
            if (!elementClassType || !isTypeAssignableTo(elemInstanceType, elementClassType)) {
                const jsxStatelessElementType = getJsxStatelessElementTypeAt(openingLikeElement);
                if (jsxStatelessElementType) {
                    // We don't call getResolvedSignature here because we have already resolve the type of JSX Element.
                    const callSignature = getResolvedJsxStatelessFunctionSignature(openingLikeElement, elementType, /*candidatesOutArray*/ undefined, /*isForSignatureHelp*/ false);
                    if (callSignature !== unknownSignature) {
                        const callReturnType = callSignature && getReturnTypeOfSignature(callSignature);
                        let paramType = callReturnType && (callSignature!.parameters.length === 0 ? emptyObjectType : getTypeOfSymbol(callSignature!.parameters[0]));
                        paramType = getApparentTypeOfJsxPropsType(paramType);
                        if (callReturnType && isTypeAssignableTo(callReturnType, jsxStatelessElementType)) {
                            // Intersect in JSX.IntrinsicAttributes if it exists
                            const intrinsicAttributes = getJsxType(JsxNames.IntrinsicAttributes, openingLikeElement);
                            if (intrinsicAttributes !== errorType) {
                                paramType = intersectTypes(intrinsicAttributes, paramType);
                            }
                            return paramType;
                        }
                    }
                }
            }
            return undefined;
        }

        /**
         * Get JSX attributes type by trying to resolve openingLikeElement as a stateless function component.
         * Return all attributes type of resolved call signature including candidate signatures.
         * This function assumes that the caller handled other possible element type of the JSX element.
         * This function is a behavior used by language service when looking up completion in JSX element.
         * @param openingLikeElement a JSX opening-like element to find attributes type
         * @param elementType a type of the opening-like element. This elementType can't be an union type
         * @param elemInstanceType an element instance type (the result of newing or invoking this tag)
         * @param elementClassType a JSX-ElementClass type. This is a result of looking up ElementClass interface in the JSX global
         */
        function tryGetAllJsxStatelessFunctionAttributesType(openingLikeElement: JsxOpeningLikeElement, elementType: Type, elemInstanceType: Type, elementClassType?: Type): Type | undefined {
            Debug.assert(!(elementType.flags & TypeFlags.Union));
            if (!elementClassType || !isTypeAssignableTo(elemInstanceType, elementClassType)) {
                // Is this is a stateless function component? See if its single signature's return type is assignable to the JSX Element Type
                const jsxStatelessElementType = getJsxStatelessElementTypeAt(openingLikeElement);
                if (jsxStatelessElementType) {
                    // We don't call getResolvedSignature because here we have already resolve the type of JSX Element.
                    const candidatesOutArray: Signature[] = [];
                    getResolvedJsxStatelessFunctionSignature(openingLikeElement, elementType, candidatesOutArray, /*isForSignatureHelp*/ false);
                    let result: Type | undefined;
                    let allMatchingAttributesType: Type | undefined;
                    for (const candidate of candidatesOutArray) {
                        const callReturnType = getReturnTypeOfSignature(candidate);
                        // TODO: GH#18217: callReturnType should always be defined...
                        let paramType: Type | undefined = callReturnType && (candidate.parameters.length === 0 ? emptyObjectType : getTypeOfSymbol(candidate.parameters[0]));
                        paramType = getApparentTypeOfJsxPropsType(paramType);
                        if (callReturnType && isTypeAssignableTo(callReturnType, jsxStatelessElementType)) {
                            let shouldBeCandidate = true;
                            for (const attribute of openingLikeElement.attributes.properties) {
                                if (isJsxAttribute(attribute) &&
                                    isUnhyphenatedJsxName(attribute.name.escapedText) &&
                                    !getPropertyOfType(paramType!, attribute.name.escapedText)) { // TODO: GH#18217
                                    shouldBeCandidate = false;
                                    break;
                                }
                            }
                            if (shouldBeCandidate) {
                                result = intersectTypes(result, paramType);
                            }
                            allMatchingAttributesType = intersectTypes(allMatchingAttributesType, paramType);
                        }
                    }

                    // If we can't find any matching, just return everything.
                    if (!result) {
                        result = allMatchingAttributesType;
                    }
                    // Intersect in JSX.IntrinsicAttributes if it exists
                    const intrinsicAttributes = getJsxType(JsxNames.IntrinsicAttributes, openingLikeElement);
                    if (intrinsicAttributes !== errorType) {
                        result = intersectTypes(intrinsicAttributes, result);
                    }
                    return result;
                }
            }
            return undefined;
        }

        function getInstantiatedJsxSignatures(openingLikeElement: JsxOpeningLikeElement, elementType: Type, reportErrors?: boolean) {
            const links = getNodeLinks(openingLikeElement);
            if (!links.resolvedSignatures) {
                links.resolvedSignatures = createMap();
            }
            const cacheKey = "" + getTypeId(elementType);
            if (links.resolvedSignatures.get(cacheKey) && links.resolvedSignatures.get(cacheKey) === resolvingSignaturesArray) {
                return;
            }
            else if (links.resolvedSignatures.get(cacheKey)) {
                return links.resolvedSignatures.get(cacheKey);
            }

            links.resolvedSignatures.set(cacheKey, resolvingSignaturesArray);
            // Resolve the signatures, preferring constructor
            let signatures = getSignaturesOfType(elementType, SignatureKind.Construct);
            if (signatures.length === 0) {
                // No construct signatures, try call signatures
                signatures = getSignaturesOfType(elementType, SignatureKind.Call);
                if (signatures.length === 0) {
                    // We found no signatures at all, which is an error
                    if (reportErrors) {
                        error(openingLikeElement.tagName, Diagnostics.JSX_element_type_0_does_not_have_any_construct_or_call_signatures, getTextOfNode(openingLikeElement.tagName));
                    }
                    return;
                }
            }

            // Instantiate in context of source type
            const results = instantiateJsxSignatures(openingLikeElement, signatures);
            links.resolvedSignatures.set(cacheKey, results);
            return results;
        }

        /**
         * Resolve attributes type of the given opening-like element. The attributes type is a type of attributes associated with the given elementType.
         * For instance:
         *      declare function Foo(attr: { p1: string}): JSX.Element;
         *      <Foo p1={10} />;  // This function will try resolve "Foo" and return an attributes type of "Foo" which is "{ p1: string }"
         *
         * The function is intended to initially be called from getAttributesTypeFromJsxOpeningLikeElement which already handle JSX-intrinsic-element..
         * This function will try to resolve custom JSX attributes type in following order: string literal, stateless function, and stateful component
         *
         * @param openingLikeElement a non-intrinsic JSXOPeningLikeElement
         * @param shouldIncludeAllStatelessAttributesType a boolean indicating whether to include all attributes types from all stateless function signature
         * @param sourceAttributesType Is the attributes type the user passed, and is used to create inferences in the target type if present
         * @param elementType an instance type of the given opening-like element. If undefined, the function will check type openinglikeElement's tagname.
         * @param elementClassType a JSX-ElementClass type. This is a result of looking up ElementClass interface in the JSX global (imported from react.d.ts)
         * @return attributes type if able to resolve the type of node
         *         anyType if there is no type ElementAttributesProperty or there is an error
         *         emptyObjectType if there is no "prop" in the element instance type
         */
        function resolveCustomJsxElementAttributesType(openingLikeElement: JsxOpeningLikeElement,
            shouldIncludeAllStatelessAttributesType: boolean,
            elementType: Type,
            elementClassType?: Type): Type {

            if (elementType.flags & TypeFlags.Union) {
                const types = (elementType as UnionType).types;
                return getUnionType(types.map(type => {
                    return resolveCustomJsxElementAttributesType(openingLikeElement, shouldIncludeAllStatelessAttributesType, type, elementClassType);
                }), UnionReduction.Subtype);
            }

            // Shortcircuit any
            if (isTypeAny(elementType)) {
                return elementType;
            }
            // If the elemType is a string type, we have to return anyType to prevent an error downstream as we will try to find construct or call signature of the type
            else if (elementType.flags & TypeFlags.String) {
                return anyType;
            }
            else if (elementType.flags & TypeFlags.StringLiteral) {
                // If the elemType is a stringLiteral type, we can then provide a check to make sure that the string literal type is one of the Jsx intrinsic element type
                // For example:
                //      var CustomTag: "h1" = "h1";
                //      <CustomTag> Hello World </CustomTag>
                const intrinsicElementsType = getJsxType(JsxNames.IntrinsicElements, openingLikeElement);
                if (intrinsicElementsType !== errorType) {
                    const stringLiteralTypeName = (<StringLiteralType>elementType).value;
                    const intrinsicProp = getPropertyOfType(intrinsicElementsType, escapeLeadingUnderscores(stringLiteralTypeName));
                    if (intrinsicProp) {
                        return getTypeOfSymbol(intrinsicProp);
                    }
                    const indexSignatureType = getIndexTypeOfType(intrinsicElementsType, IndexKind.String);
                    if (indexSignatureType) {
                        return indexSignatureType;
                    }
                    error(openingLikeElement, Diagnostics.Property_0_does_not_exist_on_type_1, stringLiteralTypeName, "JSX." + JsxNames.IntrinsicElements);
                }
                // If we need to report an error, we already done so here. So just return any to prevent any more error downstream
                return anyType;
            }

            // Get the element instance type (the result of newing or invoking this tag)

            const instantiatedSignatures = getInstantiatedJsxSignatures(openingLikeElement, elementType, /*reportErrors*/ true);
            if (!length(instantiatedSignatures)) {
                return errorType;
            }
            const elemInstanceType = getUnionType(instantiatedSignatures!.map(getReturnTypeOfSignature), UnionReduction.Subtype);

            // If we should include all stateless attributes type, then get all attributes type from all stateless function signature.
            // Otherwise get only attributes type from the signature picked by choose-overload logic.
            const statelessAttributesType = shouldIncludeAllStatelessAttributesType ?
                tryGetAllJsxStatelessFunctionAttributesType(openingLikeElement, elementType, elemInstanceType, elementClassType) :
                defaultTryGetJsxStatelessFunctionAttributesType(openingLikeElement, elementType, elemInstanceType, elementClassType);

            if (statelessAttributesType) {
                return statelessAttributesType;
            }

            // Issue an error if this return type isn't assignable to JSX.ElementClass
            if (elementClassType) {
                checkTypeRelatedTo(elemInstanceType, elementClassType, assignableRelation, openingLikeElement, Diagnostics.JSX_element_type_0_is_not_a_constructor_function_for_JSX_elements);
            }

            const isJs = isInJSFile(openingLikeElement);
            return getUnionType(instantiatedSignatures!.map(sig => getJsxPropsTypeFromClassType(sig, isJs, openingLikeElement, /*reportErrors*/ true)));
        }

        /**
         * Get attributes type of the given intrinsic opening-like Jsx element by resolving the tag name.
         * The function is intended to be called from a function which has checked that the opening element is an intrinsic element.
         * @param node an intrinsic JSX opening-like element
         */
        function getIntrinsicAttributesTypeFromJsxOpeningLikeElement(node: JsxOpeningLikeElement): Type {
            Debug.assert(isJsxIntrinsicIdentifier(node.tagName));
            const links = getNodeLinks(node);
            if (!links.resolvedJsxElementAttributesType) {
                const symbol = getIntrinsicTagSymbol(node);
                if (links.jsxFlags & JsxFlags.IntrinsicNamedElement) {
                    return links.resolvedJsxElementAttributesType = getTypeOfSymbol(symbol);
                }
                else if (links.jsxFlags & JsxFlags.IntrinsicIndexedElement) {
                    return links.resolvedJsxElementAttributesType = getIndexInfoOfSymbol(symbol, IndexKind.String)!.type;
                }
                else {
                    return links.resolvedJsxElementAttributesType = errorType;
                }
            }
            return links.resolvedJsxElementAttributesType;
        }

        /**
         * Get attributes type of the given custom opening-like JSX element.
         * This function is intended to be called from a caller that handles intrinsic JSX element already.
         * @param node a custom JSX opening-like element
         * @param shouldIncludeAllStatelessAttributesType a boolean value used by language service to get all possible attributes type from an overload stateless function component
         */
        function getCustomJsxElementAttributesType(node: JsxOpeningLikeElement, shouldIncludeAllStatelessAttributesType: boolean): Type {
            return resolveCustomJsxElementAttributesType(node, shouldIncludeAllStatelessAttributesType, checkExpression(node.tagName), getJsxElementClassTypeAt(node));
        }

        /**
         * Get all possible attributes type, especially from an overload stateless function component, of the given JSX opening-like element.
         * This function is called by language service (see: completions-tryGetGlobalSymbols).
         * @param node a JSX opening-like element to get attributes type for
         */
        function getAllAttributesTypeFromJsxOpeningLikeElement(node: JsxOpeningLikeElement): Type {
            if (isJsxIntrinsicIdentifier(node.tagName)) {
                return getIntrinsicAttributesTypeFromJsxOpeningLikeElement(node);
            }
            else {
                // Because in language service, the given JSX opening-like element may be incomplete and therefore,
                // we can't resolve to exact signature if the element is a stateless function component so the best thing to do is return all attributes type from all overloads.
                return getCustomJsxElementAttributesType(node, /*shouldIncludeAllStatelessAttributesType*/ true);
            }
        }

        /**
         * Get the attributes type, which indicates the attributes that are valid on the given JSXOpeningLikeElement.
         * @param node a JSXOpeningLikeElement node
         * @return an attributes type of the given node
         */
        function getAttributesTypeFromJsxOpeningLikeElement(node: JsxOpeningLikeElement): Type {
            if (isJsxIntrinsicIdentifier(node.tagName)) {
                return getIntrinsicAttributesTypeFromJsxOpeningLikeElement(node);
            }
            else {
                return getCustomJsxElementAttributesType(node, /*shouldIncludeAllStatelessAttributesType*/ false);
            }
        }

        /**
         * Given a JSX attribute, returns the symbol for the corresponds property
         * of the element attributes type. Will return unknownSymbol for attributes
         * that have no matching element attributes type property.
         */
        function getJsxAttributePropertySymbol(attrib: JsxAttribute): Symbol {
            const attributesType = getAttributesTypeFromJsxOpeningLikeElement(attrib.parent.parent as JsxOpeningElement);
            const prop = getPropertyOfType(attributesType, attrib.name.escapedText);
            return prop || unknownSymbol;
        }

        function getJsxElementClassTypeAt(location: Node): Type | undefined {
            const type = getJsxType(JsxNames.ElementClass, location);
            if (type === errorType) return undefined;
            return type;
        }

        function getJsxElementTypeAt(location: Node): Type {
            return getJsxType(JsxNames.Element, location);
        }

        function getJsxStatelessElementTypeAt(location: Node): Type | undefined {
            const jsxElementType = getJsxElementTypeAt(location);
            if (jsxElementType) {
                return getUnionType([jsxElementType, nullType]);
            }
        }

        /**
         * Returns all the properties of the Jsx.IntrinsicElements interface
         */
        function getJsxIntrinsicTagNamesAt(location: Node): Symbol[] {
            const intrinsics = getJsxType(JsxNames.IntrinsicElements, location);
            return intrinsics ? getPropertiesOfType(intrinsics) : emptyArray;
        }

        function checkJsxPreconditions(errorNode: Node) {
            // Preconditions for using JSX
            if ((compilerOptions.jsx || JsxEmit.None) === JsxEmit.None) {
                error(errorNode, Diagnostics.Cannot_use_JSX_unless_the_jsx_flag_is_provided);
            }

            if (getJsxElementTypeAt(errorNode) === undefined) {
                if (noImplicitAny) {
                    error(errorNode, Diagnostics.JSX_element_implicitly_has_type_any_because_the_global_type_JSX_Element_does_not_exist);
                }
            }
        }

        function checkJsxOpeningLikeElementOrOpeningFragment(node: JsxOpeningLikeElement | JsxOpeningFragment, checkMode: CheckMode | undefined) {
            const isNodeOpeningLikeElement = isJsxOpeningLikeElement(node);

            if (isNodeOpeningLikeElement) {
                checkGrammarJsxElement(<JsxOpeningLikeElement>node);
            }
            checkJsxPreconditions(node);
            // The reactNamespace/jsxFactory's root symbol should be marked as 'used' so we don't incorrectly elide its import.
            // And if there is no reactNamespace/jsxFactory's symbol in scope when targeting React emit, we should issue an error.
            const reactRefErr = diagnostics && compilerOptions.jsx === JsxEmit.React ? Diagnostics.Cannot_find_name_0 : undefined;
            const reactNamespace = getJsxNamespace(node);
            const reactLocation = isNodeOpeningLikeElement ? (<JsxOpeningLikeElement>node).tagName : node;
            const reactSym = resolveName(reactLocation, reactNamespace, SymbolFlags.Value, reactRefErr, reactNamespace, /*isUse*/ true);
            if (reactSym) {
                // Mark local symbol as referenced here because it might not have been marked
                // if jsx emit was not react as there wont be error being emitted
                reactSym.isReferenced = SymbolFlags.All;

                // If react symbol is alias, mark it as refereced
                if (reactSym.flags & SymbolFlags.Alias && !isConstEnumOrConstEnumOnlyModule(resolveAlias(reactSym))) {
                    markAliasSymbolAsReferenced(reactSym);
                }
            }

            if (isNodeOpeningLikeElement) {
                checkJsxAttributesAssignableToTagNameAttributes(<JsxOpeningLikeElement>node, checkMode);
            }
            else {
                checkJsxChildren((node as JsxOpeningFragment).parent);
            }
        }

        /**
         * Check if a property with the given name is known anywhere in the given type. In an object type, a property
         * is considered known if
         * 1. the object type is empty and the check is for assignability, or
         * 2. if the object type has index signatures, or
         * 3. if the property is actually declared in the object type
         *    (this means that 'toString', for example, is not usually a known property).
         * 4. In a union or intersection type,
         *    a property is considered known if it is known in any constituent type.
         * @param targetType a type to search a given name in
         * @param name a property name to search
         * @param isComparingJsxAttributes a boolean flag indicating whether we are searching in JsxAttributesType
         */
        function isKnownProperty(targetType: Type, name: __String, isComparingJsxAttributes: boolean): boolean {
            if (targetType.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(targetType as ObjectType);
                if (resolved.stringIndexInfo ||
                    resolved.numberIndexInfo && isNumericLiteralName(name) ||
                    getPropertyOfObjectType(targetType, name) ||
                    isComparingJsxAttributes && !isUnhyphenatedJsxName(name)) {
                    // For JSXAttributes, if the attribute has a hyphenated name, consider that the attribute to be known.
                    return true;
                }
            }
            else if (targetType.flags & TypeFlags.UnionOrIntersection) {
                for (const t of (targetType as UnionOrIntersectionType).types) {
                    if (isKnownProperty(t, name, isComparingJsxAttributes)) {
                        return true;
                    }
                }
            }
            else if (targetType.flags & TypeFlags.Conditional) {
                return isKnownProperty((targetType as ConditionalType).root.trueType, name, isComparingJsxAttributes) ||
                    isKnownProperty((targetType as ConditionalType).root.falseType, name, isComparingJsxAttributes);
            }
            return false;
        }

         /**
          * Check whether the given attributes of JSX opening-like element is assignable to the tagName attributes.
          *      Get the attributes type of the opening-like element through resolving the tagName, "target attributes"
          *      Check assignablity between given attributes property, "source attributes", and the "target attributes"
          * @param openingLikeElement an opening-like JSX element to check its JSXAttributes
          */
        function checkJsxAttributesAssignableToTagNameAttributes(openingLikeElement: JsxOpeningLikeElement, checkMode: CheckMode | undefined) {
            // The function involves following steps:
            //      1. Figure out expected attributes type by resolving tagName of the JSX opening-like element, targetAttributesType.
            //         During these steps, we will try to resolve the tagName as intrinsic name, stateless function, stateful component (in the order)
            //      2. Solved JSX attributes type given by users, sourceAttributesType, which is by resolving "attributes" property of the JSX opening-like element.
            //      3. Check if the two are assignable to each other


            // targetAttributesType is a type of an attribute from resolving tagName of an opening-like JSX element.
            const targetAttributesType = isJsxIntrinsicIdentifier(openingLikeElement.tagName) ?
                getIntrinsicAttributesTypeFromJsxOpeningLikeElement(openingLikeElement) :
                getCustomJsxElementAttributesType(openingLikeElement, /*shouldIncludeAllStatelessAttributesType*/ false);

            // sourceAttributesType is a type of an attributes properties.
            // i.e <div attr1={10} attr2="string" />
            //     attr1 and attr2 are treated as JSXAttributes attached in the JsxOpeningLikeElement as "attributes".
            const sourceAttributesType = checkExpressionCached(openingLikeElement.attributes, checkMode);

            // Check if sourceAttributesType assignable to targetAttributesType though this check will allow excess properties
            const isSourceAttributeTypeAssignableToTarget = isTypeAssignableTo(sourceAttributesType, targetAttributesType);
            // After we check for assignability, we will do another pass to check that all explicitly specified attributes have correct name corresponding in targetAttributeType.
            // This will allow excess properties in spread type as it is very common pattern to spread outer attributes into React component in its render method.
            if (isSourceAttributeTypeAssignableToTarget && !isTypeAny(sourceAttributesType) && !isTypeAny(targetAttributesType)) {
                for (const attribute of openingLikeElement.attributes.properties) {
                    if (!isJsxAttribute(attribute)) {
                        continue;
                    }
                    const attrName = attribute.name;
                    const isNotIgnoredJsxProperty = (isUnhyphenatedJsxName(idText(attrName)) || !!(getPropertyOfType(targetAttributesType, attrName.escapedText)));
                    if (isNotIgnoredJsxProperty && !isKnownProperty(targetAttributesType, attrName.escapedText, /*isComparingJsxAttributes*/ true)) {
                        error(attribute, Diagnostics.Property_0_does_not_exist_on_type_1, idText(attrName), typeToString(targetAttributesType));
                        // We break here so that errors won't be cascading
                        break;
                    }
                }
            }
            else if (!isSourceAttributeTypeAssignableToTarget) {
                checkTypeAssignableToAndOptionallyElaborate(sourceAttributesType, targetAttributesType, openingLikeElement.tagName, openingLikeElement.attributes);
            }
        }

        function checkJsxExpression(node: JsxExpression, checkMode?: CheckMode) {
            if (node.expression) {
                const type = checkExpression(node.expression, checkMode);
                if (node.dotDotDotToken && type !== anyType && !isArrayType(type)) {
                    error(node, Diagnostics.JSX_spread_child_must_be_an_array_type);
                }
                return type;
            }
            else {
                return errorType;
            }
        }

        function getDeclarationNodeFlagsFromSymbol(s: Symbol): NodeFlags {
            return s.valueDeclaration ? getCombinedNodeFlags(s.valueDeclaration) : 0;
        }

        /**
         * Return whether this symbol is a member of a prototype somewhere
         * Note that this is not tracked well within the compiler, so the answer may be incorrect.
         */
        function isPrototypeProperty(symbol: Symbol) {
            if (symbol.flags & SymbolFlags.Method || getCheckFlags(symbol) & CheckFlags.SyntheticMethod) {
                return true;
            }
            if (isInJSFile(symbol.valueDeclaration)) {
                const parent = symbol.valueDeclaration.parent;
                return parent && isBinaryExpression(parent) &&
                    getAssignmentDeclarationKind(parent) === AssignmentDeclarationKind.PrototypeProperty;
            }
        }

        /**
         * Check whether the requested property access is valid.
         * Returns true if node is a valid property access, and false otherwise.
         * @param node The node to be checked.
         * @param isSuper True if the access is from `super.`.
         * @param type The type of the object whose property is being accessed. (Not the type of the property.)
         * @param prop The symbol for the property being accessed.
         */
        function checkPropertyAccessibility(
            node: PropertyAccessExpression | QualifiedName | PropertyAccessExpression | VariableDeclaration | ParameterDeclaration | ImportTypeNode | PropertyAssignment | ShorthandPropertyAssignment | BindingElement,
            isSuper: boolean, type: Type, prop: Symbol): boolean {
            const flags = getDeclarationModifierFlagsFromSymbol(prop);
            const errorNode = node.kind === SyntaxKind.QualifiedName ? node.right : node.kind === SyntaxKind.ImportType ? node : node.name;

            if (getCheckFlags(prop) & CheckFlags.ContainsPrivate) {
                // Synthetic property with private constituent property
                error(errorNode, Diagnostics.Property_0_has_conflicting_declarations_and_is_inaccessible_in_type_1, symbolToString(prop), typeToString(type));
                return false;
            }

            if (isSuper) {
                // TS 1.0 spec (April 2014): 4.8.2
                // - In a constructor, instance member function, instance member accessor, or
                //   instance member variable initializer where this references a derived class instance,
                //   a super property access is permitted and must specify a public instance member function of the base class.
                // - In a static member function or static member accessor
                //   where this references the constructor function object of a derived class,
                //   a super property access is permitted and must specify a public static member function of the base class.
                if (languageVersion < ScriptTarget.ES2015) {
                    if (symbolHasNonMethodDeclaration(prop)) {
                        error(errorNode, Diagnostics.Only_public_and_protected_methods_of_the_base_class_are_accessible_via_the_super_keyword);
                        return false;
                    }
                }
                if (flags & ModifierFlags.Abstract) {
                    // A method cannot be accessed in a super property access if the method is abstract.
                    // This error could mask a private property access error. But, a member
                    // cannot simultaneously be private and abstract, so this will trigger an
                    // additional error elsewhere.
                    error(errorNode, Diagnostics.Abstract_method_0_in_class_1_cannot_be_accessed_via_super_expression, symbolToString(prop), typeToString(getDeclaringClass(prop)!));
                    return false;
                }
            }

            // Referencing abstract properties within their own constructors is not allowed
            if ((flags & ModifierFlags.Abstract) && isThisProperty(node) && symbolHasNonMethodDeclaration(prop)) {
                const declaringClassDeclaration = getClassLikeDeclarationOfSymbol(getParentOfSymbol(prop)!);
                if (declaringClassDeclaration && isNodeUsedDuringClassInitialization(node, declaringClassDeclaration)) {
                    error(errorNode, Diagnostics.Abstract_property_0_in_class_1_cannot_be_accessed_in_the_constructor, symbolToString(prop), getTextOfIdentifierOrLiteral(declaringClassDeclaration.name!)); // TODO: GH#18217
                    return false;
                }
            }

            // Public properties are otherwise accessible.
            if (!(flags & ModifierFlags.NonPublicAccessibilityModifier)) {
                return true;
            }

            // Property is known to be private or protected at this point

            // Private property is accessible if the property is within the declaring class
            if (flags & ModifierFlags.Private) {
                const declaringClassDeclaration = getClassLikeDeclarationOfSymbol(getParentOfSymbol(prop)!)!;
                if (!isNodeWithinClass(node, declaringClassDeclaration)) {
                    error(errorNode, Diagnostics.Property_0_is_private_and_only_accessible_within_class_1, symbolToString(prop), typeToString(getDeclaringClass(prop)!));
                    return false;
                }
                return true;
            }

            // Property is known to be protected at this point

            // All protected properties of a supertype are accessible in a super access
            if (isSuper) {
                return true;
            }

            // Find the first enclosing class that has the declaring classes of the protected constituents
            // of the property as base classes
            let enclosingClass = forEachEnclosingClass(node, enclosingDeclaration => {
                const enclosingClass = <InterfaceType>getDeclaredTypeOfSymbol(getSymbolOfNode(enclosingDeclaration)!);
                return isClassDerivedFromDeclaringClasses(enclosingClass, prop) ? enclosingClass : undefined;
            });
            // A protected property is accessible if the property is within the declaring class or classes derived from it
            if (!enclosingClass) {
                // allow PropertyAccessibility if context is in function with this parameter
                // static member access is disallow
                let thisParameter: ParameterDeclaration | undefined;
                if (flags & ModifierFlags.Static || !(thisParameter = getThisParameterFromNodeContext(node)) || !thisParameter.type) {
                    error(errorNode, Diagnostics.Property_0_is_protected_and_only_accessible_within_class_1_and_its_subclasses, symbolToString(prop), typeToString(getDeclaringClass(prop) || type));
                    return false;
                }

                const thisType = getTypeFromTypeNode(thisParameter.type);
                enclosingClass = ((thisType.flags & TypeFlags.TypeParameter) ? getConstraintOfTypeParameter(<TypeParameter>thisType) : thisType) as InterfaceType;
            }
            // No further restrictions for static properties
            if (flags & ModifierFlags.Static) {
                return true;
            }
            if (type.flags & TypeFlags.TypeParameter) {
                // get the original type -- represented as the type constraint of the 'this' type
                type = (type as TypeParameter).isThisType ? getConstraintOfTypeParameter(<TypeParameter>type)! : getBaseConstraintOfType(<TypeParameter>type)!; // TODO: GH#18217 Use a different variable that's allowed to be undefined
            }
            if (!type || !hasBaseType(type, enclosingClass)) {
                error(errorNode, Diagnostics.Property_0_is_protected_and_only_accessible_through_an_instance_of_class_1, symbolToString(prop), typeToString(enclosingClass));
                return false;
            }
            return true;
        }

        function getThisParameterFromNodeContext (node: Node) {
            const thisContainer = getThisContainer(node, /* includeArrowFunctions */ false);
            return thisContainer && isFunctionLike(thisContainer) ? getThisParameter(thisContainer) : undefined;
        }

        function symbolHasNonMethodDeclaration(symbol: Symbol) {
            return !!forEachProperty(symbol, prop => !(prop.flags & SymbolFlags.Method));
        }

        function checkNonNullExpression(
            node: Expression | QualifiedName,
            nullDiagnostic?: DiagnosticMessage,
            undefinedDiagnostic?: DiagnosticMessage,
            nullOrUndefinedDiagnostic?: DiagnosticMessage,
        ) {
            return checkNonNullType(
                checkExpression(node),
                node,
                nullDiagnostic,
                undefinedDiagnostic,
                nullOrUndefinedDiagnostic
            );
        }

        function checkNonNullType(
            type: Type,
            node: Node,
            nullDiagnostic?: DiagnosticMessage,
            undefinedDiagnostic?: DiagnosticMessage,
            nullOrUndefinedDiagnostic?: DiagnosticMessage
        ): Type {
            if (type.flags & TypeFlags.Unknown) {
                error(node, Diagnostics.Object_is_of_type_unknown);
                return errorType;
            }
            const kind = (strictNullChecks ? getFalsyFlags(type) : type.flags) & TypeFlags.Nullable;
            if (kind) {
                error(node, kind & TypeFlags.Undefined ? kind & TypeFlags.Null ?
                    (nullOrUndefinedDiagnostic || Diagnostics.Object_is_possibly_null_or_undefined) :
                    (undefinedDiagnostic || Diagnostics.Object_is_possibly_undefined) :
                    (nullDiagnostic || Diagnostics.Object_is_possibly_null)
                );
                const t = getNonNullableType(type);
                return t.flags & (TypeFlags.Nullable | TypeFlags.Never) ? errorType : t;
            }
            return type;
        }

        function checkPropertyAccessExpression(node: PropertyAccessExpression) {
            return checkPropertyAccessExpressionOrQualifiedName(node, node.expression, node.name);
        }

        function checkQualifiedName(node: QualifiedName) {
            return checkPropertyAccessExpressionOrQualifiedName(node, node.left, node.right);
        }

        function checkPropertyAccessExpressionOrQualifiedName(node: PropertyAccessExpression | QualifiedName, left: Expression | QualifiedName, right: Identifier) {
            let propType: Type;
            const leftType = checkNonNullExpression(left);
            const parentSymbol = getNodeLinks(left).resolvedSymbol;
            const apparentType = getApparentType(getWidenedType(leftType));
            if (isTypeAny(apparentType) || apparentType === silentNeverType) {
                if (isIdentifier(left) && parentSymbol) {
                    markAliasReferenced(parentSymbol, node);
                }
                return apparentType;
            }
            const assignmentKind = getAssignmentTargetKind(node);
            const prop = getPropertyOfType(apparentType, right.escapedText);
            if (isIdentifier(left) && parentSymbol && !(prop && isConstEnumOrConstEnumOnlyModule(prop))) {
                markAliasReferenced(parentSymbol, node);
            }
            if (!prop) {
                const indexInfo = getIndexInfoOfType(apparentType, IndexKind.String);
                if (!(indexInfo && indexInfo.type)) {
                    if (isJSLiteralType(leftType)) {
                        return anyType;
                    }
                    if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
                        reportNonexistentProperty(right, leftType.flags & TypeFlags.TypeParameter && (leftType as TypeParameter).isThisType ? apparentType : leftType);
                    }
                    return errorType;
                }
                if (indexInfo.isReadonly && (isAssignmentTarget(node) || isDeleteTarget(node))) {
                    error(node, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(apparentType));
                }
                propType = indexInfo.type;
            }
            else {
                checkPropertyNotUsedBeforeDeclaration(prop, node, right);
                markPropertyAsReferenced(prop, node, left.kind === SyntaxKind.ThisKeyword);
                getNodeLinks(node).resolvedSymbol = prop;
                checkPropertyAccessibility(node, left.kind === SyntaxKind.SuperKeyword, apparentType, prop);
                if (assignmentKind) {
                    if (isReferenceToReadonlyEntity(<Expression>node, prop) || isReferenceThroughNamespaceImport(<Expression>node)) {
                        error(right, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, idText(right));
                        return errorType;
                    }
                }
                propType = getConstraintForLocation(getTypeOfSymbol(prop), node);
            }
            // Only compute control flow type if this is a property access expression that isn't an
            // assignment target, and the referenced property was declared as a variable, property,
            // accessor, or optional method.
            if (node.kind !== SyntaxKind.PropertyAccessExpression ||
                assignmentKind === AssignmentKind.Definite ||
                prop && !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) && !(prop.flags & SymbolFlags.Method && propType.flags & TypeFlags.Union)) {
                return propType;
            }
            // If strict null checks and strict property initialization checks are enabled, if we have
            // a this.xxx property access, if the property is an instance property without an initializer,
            // and if we are in a constructor of the same class as the property declaration, assume that
            // the property is uninitialized at the top of the control flow.
            let assumeUninitialized = false;
            if (strictNullChecks && strictPropertyInitialization && left.kind === SyntaxKind.ThisKeyword) {
                const declaration = prop && prop.valueDeclaration;
                if (declaration && isInstancePropertyWithoutInitializer(declaration)) {
                    const flowContainer = getControlFlowContainer(node);
                    if (flowContainer.kind === SyntaxKind.Constructor && flowContainer.parent === declaration.parent) {
                        assumeUninitialized = true;
                    }
                }
            }
            const flowType = getFlowTypeOfReference(node, propType, assumeUninitialized ? getOptionalType(propType) : propType);
            if (assumeUninitialized && !(getFalsyFlags(propType) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
                error(right, Diagnostics.Property_0_is_used_before_being_assigned, symbolToString(prop!)); // TODO: GH#18217
                // Return the declared type to reduce follow-on errors
                return propType;
            }
            return assignmentKind ? getBaseTypeOfLiteralType(flowType) : flowType;
        }

        function checkPropertyNotUsedBeforeDeclaration(prop: Symbol, node: PropertyAccessExpression | QualifiedName, right: Identifier): void {
            const { valueDeclaration } = prop;
            if (!valueDeclaration) {
                return;
            }

            let diagnosticMessage;
            const declarationName = idText(right);
            if (isInPropertyInitializer(node) &&
                !isBlockScopedNameDeclaredBeforeUse(valueDeclaration, right)
                && !isPropertyDeclaredInAncestorClass(prop)) {
                diagnosticMessage = error(right, Diagnostics.Property_0_is_used_before_its_initialization, declarationName);
            }
            else if (valueDeclaration.kind === SyntaxKind.ClassDeclaration &&
                node.parent.kind !== SyntaxKind.TypeReference &&
                !(valueDeclaration.flags & NodeFlags.Ambient) &&
                !isBlockScopedNameDeclaredBeforeUse(valueDeclaration, right)) {
                diagnosticMessage = error(right, Diagnostics.Class_0_used_before_its_declaration, declarationName);
            }

            if (diagnosticMessage) {
                addRelatedInfo(diagnosticMessage,
                    createDiagnosticForNode(valueDeclaration, Diagnostics._0_is_declared_here, declarationName)
                );
            }
        }

        function isInPropertyInitializer(node: Node): boolean {
            return !!findAncestor(node, node => {
                switch (node.kind) {
                    case SyntaxKind.PropertyDeclaration:
                        return true;
                    case SyntaxKind.PropertyAssignment:
                        // We might be in `a = { b: this.b }`, so keep looking. See `tests/cases/compiler/useBeforeDeclaration_propertyAssignment.ts`.
                        return false;
                    default:
                        return isExpressionNode(node) ? false : "quit";
                }
            });
        }

        /**
         * It's possible that "prop.valueDeclaration" is a local declaration, but the property was also declared in a superclass.
         * In that case we won't consider it used before its declaration, because it gets its value from the superclass' declaration.
         */
        function isPropertyDeclaredInAncestorClass(prop: Symbol): boolean {
            if (!(prop.parent!.flags & SymbolFlags.Class)) {
                return false;
            }
            let classType: InterfaceType | undefined = getTypeOfSymbol(prop.parent!) as InterfaceType;
            while (true) {
                classType = classType.symbol && getSuperClass(classType) as InterfaceType | undefined;
                if (!classType) {
                    return false;
                }
                const superProperty = getPropertyOfType(classType, prop.escapedName);
                if (superProperty && superProperty.valueDeclaration) {
                    return true;
                }
            }
        }

        function getSuperClass(classType: InterfaceType): Type | undefined {
            const x = getBaseTypes(classType);
            if (x.length === 0) {
                return undefined;
            }
            return getIntersectionType(x);
        }

        function reportNonexistentProperty(propNode: Identifier, containingType: Type) {
            let errorInfo: DiagnosticMessageChain | undefined;
            let relatedInfo: Diagnostic | undefined;
            if (containingType.flags & TypeFlags.Union && !(containingType.flags & TypeFlags.Primitive)) {
                for (const subtype of (containingType as UnionType).types) {
                    if (!getPropertyOfType(subtype, propNode.escapedText)) {
                        errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Property_0_does_not_exist_on_type_1, declarationNameToString(propNode), typeToString(subtype));
                        break;
                    }
                }
            }
            if (typeHasStaticProperty(propNode.escapedText, containingType)) {
                errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Property_0_is_a_static_member_of_type_1, declarationNameToString(propNode), typeToString(containingType));
            }
            else {
                const promisedType = getPromisedTypeOfPromise(containingType);
                if (promisedType && getPropertyOfType(promisedType, propNode.escapedText)) {
                    errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Property_0_does_not_exist_on_type_1_Did_you_forget_to_use_await, declarationNameToString(propNode), typeToString(containingType));
                }
                else {
                    const suggestion = getSuggestedSymbolForNonexistentProperty(propNode, containingType);
                    if (suggestion !== undefined) {
                        const suggestedName = symbolName(suggestion);
                        errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Property_0_does_not_exist_on_type_1_Did_you_mean_2, declarationNameToString(propNode), typeToString(containingType), suggestedName);
                        relatedInfo = suggestion.valueDeclaration && createDiagnosticForNode(suggestion.valueDeclaration, Diagnostics._0_is_declared_here, suggestedName);
                    }
                    else {
                        errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Property_0_does_not_exist_on_type_1, declarationNameToString(propNode), typeToString(containingType));
                    }
                }
            }
            const resultDiagnostic = createDiagnosticForNodeFromMessageChain(propNode, errorInfo);
            if (relatedInfo) {
                addRelatedInfo(resultDiagnostic, relatedInfo);
            }
            diagnostics.add(resultDiagnostic);
        }

        function typeHasStaticProperty(propName: __String, containingType: Type): boolean {
            const prop = containingType.symbol && getPropertyOfType(getTypeOfSymbol(containingType.symbol), propName);
            return prop !== undefined && prop.valueDeclaration && hasModifier(prop.valueDeclaration, ModifierFlags.Static);
        }

        function getSuggestedSymbolForNonexistentProperty(name: Identifier | string, containingType: Type): Symbol | undefined {
            return getSpellingSuggestionForName(isString(name) ? name : idText(name), getPropertiesOfType(containingType), SymbolFlags.Value);
        }

        function getSuggestionForNonexistentProperty(name: Identifier | string, containingType: Type): string | undefined {
            const suggestion = getSuggestedSymbolForNonexistentProperty(name, containingType);
            return suggestion && symbolName(suggestion);
        }

        function getSuggestedSymbolForNonexistentSymbol(location: Node | undefined, outerName: __String, meaning: SymbolFlags): Symbol | undefined {
            Debug.assert(outerName !== undefined, "outername should always be defined");
            const result = resolveNameHelper(location, outerName, meaning, /*nameNotFoundMessage*/ undefined, outerName, /*isUse*/ false, /*excludeGlobals*/ false, (symbols, name, meaning) => {
                Debug.assertEqual(outerName, name, "name should equal outerName");
                const symbol = getSymbol(symbols, name, meaning);
                // Sometimes the symbol is found when location is a return type of a function: `typeof x` and `x` is declared in the body of the function
                // So the table *contains* `x` but `x` isn't actually in scope.
                // However, resolveNameHelper will continue and call this callback again, so we'll eventually get a correct suggestion.
                return symbol || getSpellingSuggestionForName(unescapeLeadingUnderscores(name), arrayFrom(symbols.values()), meaning);
            });
            return result;
        }

        function getSuggestionForNonexistentSymbol(location: Node | undefined, outerName: __String, meaning: SymbolFlags): string | undefined {
            const symbolResult = getSuggestedSymbolForNonexistentSymbol(location, outerName, meaning);
            return symbolResult && symbolName(symbolResult);
        }

        function getSuggestedSymbolForNonexistentModule(name: Identifier, targetModule: Symbol): Symbol | undefined {
            return targetModule.exports && getSpellingSuggestionForName(idText(name), getExportsOfModuleAsArray(targetModule), SymbolFlags.ModuleMember);
        }

        function getSuggestionForNonexistentExport(name: Identifier, targetModule: Symbol): string | undefined {
            const suggestion = getSuggestedSymbolForNonexistentModule(name, targetModule);
            return suggestion && symbolName(suggestion);
        }

        /**
         * Given a name and a list of symbols whose names are *not* equal to the name, return a spelling suggestion if there is one that is close enough.
         * Names less than length 3 only check for case-insensitive equality, not levenshtein distance.
         *
         * If there is a candidate that's the same except for case, return that.
         * If there is a candidate that's within one edit of the name, return that.
         * Otherwise, return the candidate with the smallest Levenshtein distance,
         *    except for candidates:
         *      * With no name
         *      * Whose meaning doesn't match the `meaning` parameter.
         *      * Whose length differs from the target name by more than 0.34 of the length of the name.
         *      * Whose levenshtein distance is more than 0.4 of the length of the name
         *        (0.4 allows 1 substitution/transposition for every 5 characters,
         *         and 1 insertion/deletion at 3 characters)
         */
        function getSpellingSuggestionForName(name: string, symbols: Symbol[], meaning: SymbolFlags): Symbol | undefined {
            return getSpellingSuggestion(name, symbols, getCandidateName);
            function getCandidateName(candidate: Symbol) {
                const candidateName = symbolName(candidate);
                return !startsWith(candidateName, "\"") && candidate.flags & meaning ? candidateName : undefined;
            }
        }

        function markPropertyAsReferenced(prop: Symbol, nodeForCheckWriteOnly: Node | undefined, isThisAccess: boolean) {
            if (!prop || !(prop.flags & SymbolFlags.ClassMember) || !prop.valueDeclaration || !hasModifier(prop.valueDeclaration, ModifierFlags.Private)) {
                return;
            }
            if (nodeForCheckWriteOnly && isWriteOnlyAccess(nodeForCheckWriteOnly) && !(prop.flags & SymbolFlags.SetAccessor && !(prop.flags & SymbolFlags.GetAccessor))) {
                return;
            }

            if (isThisAccess) {
                // Find any FunctionLikeDeclaration because those create a new 'this' binding. But this should only matter for methods (or getters/setters).
                const containingMethod = findAncestor(nodeForCheckWriteOnly, isFunctionLikeDeclaration);
                if (containingMethod && containingMethod.symbol === prop) {
                    return;
                }
            }

            (getCheckFlags(prop) & CheckFlags.Instantiated ? getSymbolLinks(prop).target : prop)!.isReferenced = SymbolFlags.All;
        }

        function isValidPropertyAccess(node: PropertyAccessExpression | QualifiedName | ImportTypeNode, propertyName: __String): boolean {
            switch (node.kind) {
                case SyntaxKind.PropertyAccessExpression:
                    return isValidPropertyAccessWithType(node, node.expression.kind === SyntaxKind.SuperKeyword, propertyName, getWidenedType(checkExpression(node.expression)));
                case SyntaxKind.QualifiedName:
                    return isValidPropertyAccessWithType(node, /*isSuper*/ false, propertyName, getWidenedType(checkExpression(node.left)));
                case SyntaxKind.ImportType:
                    return isValidPropertyAccessWithType(node, /*isSuper*/ false, propertyName, getTypeFromTypeNode(node));
            }
        }

        function isValidPropertyAccessForCompletions(node: PropertyAccessExpression | ImportTypeNode, type: Type, property: Symbol): boolean {
            return isValidPropertyAccessWithType(node, node.kind !== SyntaxKind.ImportType && node.expression.kind === SyntaxKind.SuperKeyword, property.escapedName, type)
                && (!(property.flags & SymbolFlags.Method) || isValidMethodAccess(property, type));
        }
        function isValidMethodAccess(method: Symbol, actualThisType: Type): boolean {
            const propType = getTypeOfPropertyOfType(actualThisType, method.escapedName)!;
            const signatures = getSignaturesOfType(getNonNullableType(propType), SignatureKind.Call);
            Debug.assert(signatures.length !== 0);
            return signatures.some(sig => {
                const signatureThisType = getThisTypeOfSignature(sig);
                return !signatureThisType || isTypeAssignableTo(actualThisType, getInstantiatedSignatureThisType(sig, signatureThisType, actualThisType));
            });
        }
        function getInstantiatedSignatureThisType(sig: Signature, signatureThisType: Type, actualThisType: Type): Type {
            if (!sig.typeParameters) {
                return signatureThisType;
            }
            const context = createInferenceContext(sig.typeParameters, sig, InferenceFlags.None);
            inferTypes(context.inferences, actualThisType, signatureThisType);
            return instantiateType(signatureThisType, createSignatureTypeMapper(sig, getInferredTypes(context)));
        }

        function isValidPropertyAccessWithType(
            node: PropertyAccessExpression | QualifiedName | ImportTypeNode,
            isSuper: boolean,
            propertyName: __String,
            type: Type): boolean {

            if (type === errorType || isTypeAny(type)) {
                return true;
            }
            const prop = getPropertyOfType(type, propertyName);
            return prop ? checkPropertyAccessibility(node, isSuper, type, prop)
                // In js files properties of unions are allowed in completion
                : isInJSFile(node) && (type.flags & TypeFlags.Union) !== 0 && (<UnionType>type).types.some(elementType => isValidPropertyAccessWithType(node, isSuper, propertyName, elementType));
        }

        /**
         * Return the symbol of the for-in variable declared or referenced by the given for-in statement.
         */
        function getForInVariableSymbol(node: ForInStatement): Symbol | undefined {
            const initializer = node.initializer;
            if (initializer.kind === SyntaxKind.VariableDeclarationList) {
                const variable = (<VariableDeclarationList>initializer).declarations[0];
                if (variable && !isBindingPattern(variable.name)) {
                    return getSymbolOfNode(variable);
                }
            }
            else if (initializer.kind === SyntaxKind.Identifier) {
                return getResolvedSymbol(<Identifier>initializer);
            }
            return undefined;
        }

        /**
         * Return true if the given type is considered to have numeric property names.
         */
        function hasNumericPropertyNames(type: Type) {
            return getIndexTypeOfType(type, IndexKind.Number) && !getIndexTypeOfType(type, IndexKind.String);
        }

        /**
         * Return true if given node is an expression consisting of an identifier (possibly parenthesized)
         * that references a for-in variable for an object with numeric property names.
         */
        function isForInVariableForNumericPropertyNames(expr: Expression) {
            const e = skipParentheses(expr);
            if (e.kind === SyntaxKind.Identifier) {
                const symbol = getResolvedSymbol(<Identifier>e);
                if (symbol.flags & SymbolFlags.Variable) {
                    let child: Node = expr;
                    let node = expr.parent;
                    while (node) {
                        if (node.kind === SyntaxKind.ForInStatement &&
                            child === (<ForInStatement>node).statement &&
                            getForInVariableSymbol(<ForInStatement>node) === symbol &&
                            hasNumericPropertyNames(getTypeOfExpression((<ForInStatement>node).expression))) {
                            return true;
                        }
                        child = node;
                        node = node.parent;
                    }
                }
            }
            return false;
        }

        function checkIndexedAccess(node: ElementAccessExpression): Type {
            const objectType = checkNonNullExpression(node.expression);

            const indexExpression = node.argumentExpression;
            if (!indexExpression) {
                const sourceFile = getSourceFileOfNode(node);
                if (node.parent.kind === SyntaxKind.NewExpression && (<NewExpression>node.parent).expression === node) {
                    const start = skipTrivia(sourceFile.text, node.expression.end);
                    const end = node.end;
                    grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.new_T_cannot_be_used_to_create_an_array_Use_new_Array_T_instead);
                }
                else {
                    const start = node.end - "]".length;
                    const end = node.end;
                    grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.Expression_expected);
                }
                return errorType;
            }

            const indexType = checkExpression(indexExpression);

            if (objectType === errorType || objectType === silentNeverType) {
                return objectType;
            }

            if (isConstEnumObjectType(objectType) && indexExpression.kind !== SyntaxKind.StringLiteral) {
                error(indexExpression, Diagnostics.A_const_enum_member_can_only_be_accessed_using_a_string_literal);
                return errorType;
            }
            if (isTupleType(objectType) && !objectType.target.hasRestElement && isNumericLiteral(indexExpression)) {
                const index = +indexExpression.text;
                const maximumIndex = length(objectType.target.typeParameters);
                if (index >= maximumIndex) {
                    error(indexExpression, Diagnostics.Index_0_is_out_of_bounds_in_tuple_of_length_1, index, maximumIndex);
                }
            }

            return checkIndexedAccessIndexType(getIndexedAccessType(objectType, isForInVariableForNumericPropertyNames(indexExpression) ? numberType : indexType, node), node);
        }

        function checkThatExpressionIsProperSymbolReference(expression: Expression, expressionType: Type, reportError: boolean): boolean {
            if (expressionType === errorType) {
                // There is already an error, so no need to report one.
                return false;
            }

            if (!isWellKnownSymbolSyntactically(expression)) {
                return false;
            }

            // Make sure the property type is the primitive symbol type
            if ((expressionType.flags & TypeFlags.ESSymbolLike) === 0) {
                if (reportError) {
                    error(expression, Diagnostics.A_computed_property_name_of_the_form_0_must_be_of_type_symbol, getTextOfNode(expression));
                }
                return false;
            }

            // The name is Symbol.<someName>, so make sure Symbol actually resolves to the
            // global Symbol object
            const leftHandSide = <Identifier>(<PropertyAccessExpression>expression).expression;
            const leftHandSideSymbol = getResolvedSymbol(leftHandSide);
            if (!leftHandSideSymbol) {
                return false;
            }

            const globalESSymbol = getGlobalESSymbolConstructorSymbol(/*reportErrors*/ true);
            if (!globalESSymbol) {
                // Already errored when we tried to look up the symbol
                return false;
            }

            if (leftHandSideSymbol !== globalESSymbol) {
                if (reportError) {
                    error(leftHandSide, Diagnostics.Symbol_reference_does_not_refer_to_the_global_Symbol_constructor_object);
                }
                return false;
            }

            return true;
        }

        function callLikeExpressionMayHaveTypeArguments(node: CallLikeExpression): node is CallExpression | NewExpression {
            // TODO: Also include tagged templates (https://github.com/Microsoft/TypeScript/issues/11947)
            return isCallOrNewExpression(node);
        }

        function resolveUntypedCall(node: CallLikeExpression): Signature {
            if (callLikeExpressionMayHaveTypeArguments(node)) {
                // Check type arguments even though we will give an error that untyped calls may not accept type arguments.
                // This gets us diagnostics for the type arguments and marks them as referenced.
                forEach(node.typeArguments, checkSourceElement);
            }

            if (node.kind === SyntaxKind.TaggedTemplateExpression) {
                checkExpression(node.template);
            }
            else if (node.kind !== SyntaxKind.Decorator) {
                forEach((<CallExpression>node).arguments, argument => {
                    checkExpression(argument);
                });
            }
            return anySignature;
        }

        function resolveErrorCall(node: CallLikeExpression): Signature {
            resolveUntypedCall(node);
            return unknownSignature;
        }

        // Re-order candidate signatures into the result array. Assumes the result array to be empty.
        // The candidate list orders groups in reverse, but within a group signatures are kept in declaration order
        // A nit here is that we reorder only signatures that belong to the same symbol,
        // so order how inherited signatures are processed is still preserved.
        // interface A { (x: string): void }
        // interface B extends A { (x: 'foo'): string }
        // const b: B;
        // b('foo') // <- here overloads should be processed as [(x:'foo'): string, (x: string): void]
        function reorderCandidates(signatures: ReadonlyArray<Signature>, result: Signature[]): void {
            let lastParent: Node | undefined;
            let lastSymbol: Symbol | undefined;
            let cutoffIndex = 0;
            let index: number | undefined;
            let specializedIndex = -1;
            let spliceIndex: number;
            Debug.assert(!result.length);
            for (const signature of signatures) {
                const symbol = signature.declaration && getSymbolOfNode(signature.declaration);
                const parent = signature.declaration && signature.declaration.parent;
                if (!lastSymbol || symbol === lastSymbol) {
                    if (lastParent && parent === lastParent) {
                        index = index! + 1;
                    }
                    else {
                        lastParent = parent;
                        index = cutoffIndex;
                    }
                }
                else {
                    // current declaration belongs to a different symbol
                    // set cutoffIndex so re-orderings in the future won't change result set from 0 to cutoffIndex
                    index = cutoffIndex = result.length;
                    lastParent = parent;
                }
                lastSymbol = symbol;

                // specialized signatures always need to be placed before non-specialized signatures regardless
                // of the cutoff position; see GH#1133
                if (signature.hasLiteralTypes) {
                    specializedIndex++;
                    spliceIndex = specializedIndex;
                    // The cutoff index always needs to be greater than or equal to the specialized signature index
                    // in order to prevent non-specialized signatures from being added before a specialized
                    // signature.
                    cutoffIndex++;
                }
                else {
                    spliceIndex = index;
                }

                result.splice(spliceIndex, 0, signature);
            }
        }

        function isSpreadArgument(arg: Expression | undefined): arg is Expression {
            return !!arg && (arg.kind === SyntaxKind.SpreadElement || arg.kind === SyntaxKind.SyntheticExpression && (<SyntheticExpression>arg).isSpread);
        }

        function getSpreadArgumentIndex(args: ReadonlyArray<Expression>): number {
            return findIndex(args, isSpreadArgument);
        }

        function hasCorrectArity(node: CallLikeExpression, args: ReadonlyArray<Expression>, signature: Signature, signatureHelpTrailingComma = false) {
            if (isJsxOpeningLikeElement(node)) {
                // The arity check will be done in "checkApplicableSignatureForJsxOpeningLikeElement".
                return true;
            }

            let argCount: number;
            let callIsIncomplete = false; // In incomplete call we want to be lenient when we have too few arguments

            if (node.kind === SyntaxKind.TaggedTemplateExpression) {
                argCount = args.length;
                if (node.template.kind === SyntaxKind.TemplateExpression) {
                    // If a tagged template expression lacks a tail literal, the call is incomplete.
                    // Specifically, a template only can end in a TemplateTail or a Missing literal.
                    const lastSpan = last(node.template.templateSpans); // we should always have at least one span.
                    callIsIncomplete = nodeIsMissing(lastSpan.literal) || !!lastSpan.literal.isUnterminated;
                }
                else {
                    // If the template didn't end in a backtick, or its beginning occurred right prior to EOF,
                    // then this might actually turn out to be a TemplateHead in the future;
                    // so we consider the call to be incomplete.
                    const templateLiteral = <LiteralExpression>node.template;
                    Debug.assert(templateLiteral.kind === SyntaxKind.NoSubstitutionTemplateLiteral);
                    callIsIncomplete = !!templateLiteral.isUnterminated;
                }
            }
            else if (node.kind === SyntaxKind.Decorator) {
                argCount = getDecoratorArgumentCount(node, signature);
            }
            else {
                if (!node.arguments) {
                    // This only happens when we have something of the form: 'new C'
                    Debug.assert(node.kind === SyntaxKind.NewExpression);
                    return getMinArgumentCount(signature) === 0;
                }

                argCount = signatureHelpTrailingComma ? args.length + 1 : args.length;

                // If we are missing the close parenthesis, the call is incomplete.
                callIsIncomplete = node.arguments.end === node.end;

                // If a spread argument is present, check that it corresponds to a rest parameter or at least that it's in the valid range.
                const spreadArgIndex = getSpreadArgumentIndex(args);
                if (spreadArgIndex >= 0) {
                    return spreadArgIndex >= getMinArgumentCount(signature) && (hasEffectiveRestParameter(signature) || spreadArgIndex < getParameterCount(signature));
                }
            }

            // Too many arguments implies incorrect arity.
            if (!hasEffectiveRestParameter(signature) && argCount > getParameterCount(signature)) {
                return false;
            }

            // If the call is incomplete, we should skip the lower bound check.
            const hasEnoughArguments = argCount >= getMinArgumentCount(signature);
            return callIsIncomplete || hasEnoughArguments;
        }

        function hasCorrectTypeArgumentArity(signature: Signature, typeArguments: NodeArray<TypeNode> | undefined) {
            // If the user supplied type arguments, but the number of type arguments does not match
            // the declared number of type parameters, the call has an incorrect arity.
            const numTypeParameters = length(signature.typeParameters);
            const minTypeArgumentCount = getMinTypeArgumentCount(signature.typeParameters);
            return !typeArguments ||
                (typeArguments.length >= minTypeArgumentCount && typeArguments.length <= numTypeParameters);
        }

        // If type has a single call signature and no other members, return that signature. Otherwise, return undefined.
        function getSingleCallSignature(type: Type): Signature | undefined {
            if (type.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                if (resolved.callSignatures.length === 1 && resolved.constructSignatures.length === 0 &&
                    resolved.properties.length === 0 && !resolved.stringIndexInfo && !resolved.numberIndexInfo) {
                    return resolved.callSignatures[0];
                }
            }
            return undefined;
        }

        // Instantiate a generic signature in the context of a non-generic signature (section 3.8.5 in TypeScript spec)
        function instantiateSignatureInContextOf(signature: Signature, contextualSignature: Signature, contextualMapper?: TypeMapper, compareTypes?: TypeComparer): Signature {
            const context = createInferenceContext(signature.typeParameters!, signature, InferenceFlags.None, compareTypes);
            const sourceSignature = contextualMapper ? instantiateSignature(contextualSignature, contextualMapper) : contextualSignature;
            forEachMatchingParameterType(sourceSignature, signature, (source, target) => {
                // Type parameters from outer context referenced by source type are fixed by instantiation of the source type
                inferTypes(context.inferences, source, target);
            });
            if (!contextualMapper) {
                inferTypes(context.inferences, getReturnTypeOfSignature(contextualSignature), getReturnTypeOfSignature(signature), InferencePriority.ReturnType);
            }
            return getSignatureInstantiation(signature, getInferredTypes(context), isInJSFile(contextualSignature.declaration));
        }

        function inferJsxTypeArguments(signature: Signature, node: JsxOpeningLikeElement, context: InferenceContext): Type[] {
            // Skip context sensitive pass
            const skipContextParamType = getTypeAtPosition(signature, 0);
            const checkAttrTypeSkipContextSensitive = checkExpressionWithContextualType(node.attributes, skipContextParamType, identityMapper);
            inferTypes(context.inferences, checkAttrTypeSkipContextSensitive, skipContextParamType);

            // Standard pass
            const paramType = getTypeAtPosition(signature, 0);
            const checkAttrType = checkExpressionWithContextualType(node.attributes, paramType, context);
            inferTypes(context.inferences, checkAttrType, paramType);

            return getInferredTypes(context);
        }

        function inferTypeArguments(node: CallLikeExpression, signature: Signature, args: ReadonlyArray<Expression>, excludeArgument: ReadonlyArray<boolean> | undefined, context: InferenceContext): Type[] {
            // Clear out all the inference results from the last time inferTypeArguments was called on this context
            for (const inference of context.inferences) {
                // As an optimization, we don't have to clear (and later recompute) inferred types
                // for type parameters that have already been fixed on the previous call to inferTypeArguments.
                // It would be just as correct to reset all of them. But then we'd be repeating the same work
                // for the type parameters that were fixed, namely the work done by getInferredType.
                if (!inference.isFixed) {
                    inference.inferredType = undefined;
                }
            }

            // If a contextual type is available, infer from that type to the return type of the call expression. For
            // example, given a 'function wrap<T, U>(cb: (x: T) => U): (x: T) => U' and a call expression
            // 'let f: (x: string) => number = wrap(s => s.length)', we infer from the declared type of 'f' to the
            // return type of 'wrap'.
            if (node.kind !== SyntaxKind.Decorator) {
                const contextualType = getContextualType(node);
                if (contextualType) {
                    // We clone the contextual mapper to avoid disturbing a resolution in progress for an
                    // outer call expression. Effectively we just want a snapshot of whatever has been
                    // inferred for any outer call expression so far.
                    const instantiatedType = instantiateType(contextualType, cloneTypeMapper(getContextualMapper(node)));
                    // If the contextual type is a generic function type with a single call signature, we
                    // instantiate the type with its own type parameters and type arguments. This ensures that
                    // the type parameters are not erased to type any during type inference such that they can
                    // be inferred as actual types from the contextual type. For example:
                    //   declare function arrayMap<T, U>(f: (x: T) => U): (a: T[]) => U[];
                    //   const boxElements: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));
                    // Above, the type of the 'value' parameter is inferred to be 'A'.
                    const contextualSignature = getSingleCallSignature(instantiatedType);
                    const inferenceSourceType = contextualSignature && contextualSignature.typeParameters ?
                        getOrCreateTypeFromSignature(getSignatureInstantiationWithoutFillingInTypeArguments(contextualSignature, contextualSignature.typeParameters)) :
                        instantiatedType;
                    const inferenceTargetType = getReturnTypeOfSignature(signature);
                     // Inferences made from return types have lower priority than all other inferences.
                    inferTypes(context.inferences, inferenceSourceType, inferenceTargetType, InferencePriority.ReturnType);
                }
            }

            const thisType = getThisTypeOfSignature(signature);
            if (thisType) {
                const thisArgumentNode = getThisArgumentOfCall(node);
                const thisArgumentType = thisArgumentNode ? checkExpression(thisArgumentNode) : voidType;
                inferTypes(context.inferences, thisArgumentType, thisType);
            }

            const restType = getNonArrayRestType(signature);
            const argCount = restType ? Math.min(getParameterCount(signature) - 1, args.length) : args.length;
            for (let i = 0; i < argCount; i++) {
                const arg = args[i];
                if (arg.kind !== SyntaxKind.OmittedExpression) {
                    const paramType = getTypeAtPosition(signature, i);
                    // For context sensitive arguments we pass the identityMapper, which is a signal to treat all
                    // context sensitive function expressions as wildcards
                    const mapper = excludeArgument && excludeArgument[i] !== undefined ? identityMapper : context;
                    const argType = checkExpressionWithContextualType(arg, paramType, mapper);
                    inferTypes(context.inferences, argType, paramType);
                }
            }

            if (restType) {
                const spreadType = getSpreadArgumentType(args, argCount, args.length, restType, context);
                inferTypes(context.inferences, spreadType, restType);
            }

            return getInferredTypes(context);
        }

        function getArrayifiedType(type: Type) {
            if (forEachType(type, t => !(t.flags & (TypeFlags.Any | TypeFlags.Instantiable) || isArrayType(t) || isTupleType(t)))) {
                return createArrayType(getIndexTypeOfType(type, IndexKind.Number) || errorType);
            }
            return type;
        }

        function getSpreadArgumentType(args: ReadonlyArray<Expression>, index: number, argCount: number, restType: TypeParameter, context: InferenceContext | undefined) {
            if (index >= argCount - 1) {
                const arg = args[argCount - 1];
                if (isSpreadArgument(arg)) {
                    // We are inferring from a spread expression in the last argument position, i.e. both the parameter
                    // and the argument are ...x forms.
                    return arg.kind === SyntaxKind.SyntheticExpression ?
                        createArrayType((<SyntheticExpression>arg).type) :
                        getArrayifiedType(checkExpressionWithContextualType((<SpreadElement>arg).expression, restType, context));
                }
            }
            const contextualType = getIndexTypeOfType(restType, IndexKind.Number) || anyType;
            const hasPrimitiveContextualType = maybeTypeOfKind(contextualType, TypeFlags.Primitive | TypeFlags.Index);
            const types = [];
            let spreadIndex = -1;
            for (let i = index; i < argCount; i++) {
                const argType = checkExpressionWithContextualType(args[i], contextualType, context);
                if (spreadIndex < 0 && isSpreadArgument(args[i])) {
                    spreadIndex = i - index;
                }
                types.push(hasPrimitiveContextualType ? getRegularTypeOfLiteralType(argType) : getWidenedLiteralType(argType));
            }
            return spreadIndex < 0 ?
                createTupleType(types) :
                createTupleType(append(types.slice(0, spreadIndex), getUnionType(types.slice(spreadIndex))), spreadIndex, /*hasRestElement*/ true);
        }

        function checkTypeArguments(signature: Signature, typeArgumentNodes: ReadonlyArray<TypeNode>, reportErrors: boolean, headMessage?: DiagnosticMessage): Type[] | undefined {
            const isJavascript = isInJSFile(signature.declaration);
            const typeParameters = signature.typeParameters!;
            const typeArgumentTypes = fillMissingTypeArguments(map(typeArgumentNodes, getTypeFromTypeNode), typeParameters, getMinTypeArgumentCount(typeParameters), isJavascript);
            let mapper: TypeMapper | undefined;
            for (let i = 0; i < typeArgumentNodes.length; i++) {
                Debug.assert(typeParameters[i] !== undefined, "Should not call checkTypeArguments with too many type arguments");
                const constraint = getConstraintOfTypeParameter(typeParameters[i]);
                if (constraint) {
                    const errorInfo = reportErrors && headMessage ? (() => chainDiagnosticMessages(/*details*/ undefined, Diagnostics.Type_0_does_not_satisfy_the_constraint_1)) : undefined;
                    const typeArgumentHeadMessage = headMessage || Diagnostics.Type_0_does_not_satisfy_the_constraint_1;
                    if (!mapper) {
                        mapper = createTypeMapper(typeParameters, typeArgumentTypes);
                    }
                    const typeArgument = typeArgumentTypes[i];
                    if (!checkTypeAssignableTo(
                        typeArgument,
                        getTypeWithThisArgument(instantiateType(constraint, mapper), typeArgument),
                        reportErrors ? typeArgumentNodes[i] : undefined,
                        typeArgumentHeadMessage,
                        errorInfo)) {
                        return undefined;
                    }
                }
            }
            return typeArgumentTypes;
        }

        /**
         * Check if the given signature can possibly be a signature called by the JSX opening-like element.
         * @param node a JSX opening-like element we are trying to figure its call signature
         * @param signature a candidate signature we are trying whether it is a call signature
         * @param relation a relationship to check parameter and argument type
         * @param excludeArgument
         */
        function checkApplicableSignatureForJsxOpeningLikeElement(node: JsxOpeningLikeElement, signature: Signature, relation: Map<RelationComparisonResult>) {
            // JSX opening-like element has correct arity for stateless-function component if the one of the following condition is true:
            //      1. callIsIncomplete
            //      2. attributes property has same number of properties as the parameter object type.
            //         We can figure that out by resolving attributes property and check number of properties in the resolved type
            // If the call has correct arity, we will then check if the argument type and parameter type is assignable

            const callIsIncomplete = node.attributes.end === node.end;  // If we are missing the close "/>", the call is incomplete
            if (callIsIncomplete) {
                return true;
            }

            const headMessage = Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1;
            // Stateless function components can have maximum of three arguments: "props", "context", and "updater".
            // However "context" and "updater" are implicit and can't be specify by users. Only the first parameter, props,
            // can be specified by users through attributes property.
            const paramType = getTypeAtPosition(signature, 0);
            const attributesType = checkExpressionWithContextualType(node.attributes, paramType, /*contextualMapper*/ undefined);
            const argProperties = getPropertiesOfType(attributesType);
            for (const arg of argProperties) {
                if (!getPropertyOfType(paramType, arg.escapedName) && isUnhyphenatedJsxName(arg.escapedName)) {
                    return false;
                }
            }
            return checkTypeRelatedTo(attributesType, paramType, relation, /*errorNode*/ undefined, headMessage);
        }

        function checkApplicableSignature(
            node: CallLikeExpression,
            args: ReadonlyArray<Expression>,
            signature: Signature,
            relation: Map<RelationComparisonResult>,
            excludeArgument: boolean[] | undefined,
            reportErrors: boolean) {
            if (isJsxOpeningLikeElement(node)) {
                return checkApplicableSignatureForJsxOpeningLikeElement(node, signature, relation);
            }
            const thisType = getThisTypeOfSignature(signature);
            if (thisType && thisType !== voidType && node.kind !== SyntaxKind.NewExpression) {
                // If the called expression is not of the form `x.f` or `x["f"]`, then sourceType = voidType
                // If the signature's 'this' type is voidType, then the check is skipped -- anything is compatible.
                // If the expression is a new expression, then the check is skipped.
                const thisArgumentNode = getThisArgumentOfCall(node);
                const thisArgumentType = thisArgumentNode ? checkExpression(thisArgumentNode) : voidType;
                const errorNode = reportErrors ? (thisArgumentNode || node) : undefined;
                const headMessage = Diagnostics.The_this_context_of_type_0_is_not_assignable_to_method_s_this_of_type_1;
                if (!checkTypeRelatedTo(thisArgumentType, thisType, relation, errorNode, headMessage)) {
                    return false;
                }
            }
            const headMessage = Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1;
            const restType = getNonArrayRestType(signature);
            const argCount = restType ? Math.min(getParameterCount(signature) - 1, args.length) : args.length;
            for (let i = 0; i < argCount; i++) {
                const arg = args[i];
                if (arg.kind !== SyntaxKind.OmittedExpression) {
                    const paramType = getTypeAtPosition(signature, i);
                    const argType = checkExpressionWithContextualType(arg, paramType, excludeArgument && excludeArgument[i] ? identityMapper : undefined);
                    // If one or more arguments are still excluded (as indicated by a non-null excludeArgument parameter),
                    // we obtain the regular type of any object literal arguments because we may not have inferred complete
                    // parameter types yet and therefore excess property checks may yield false positives (see #17041).
                    const checkArgType = excludeArgument ? getRegularTypeOfObjectLiteral(argType) : argType;
                    if (!checkTypeRelatedToAndOptionallyElaborate(checkArgType, paramType, relation, reportErrors ? arg : undefined, arg, headMessage)) {
                        return false;
                    }
                }
            }
            if (restType) {
                const spreadType = getSpreadArgumentType(args, argCount, args.length, restType, /*context*/ undefined);
                const errorNode = reportErrors ? argCount < args.length ? args[argCount] : node : undefined;
                return checkTypeRelatedTo(spreadType, restType, relation, errorNode, headMessage);
            }
            return true;
        }

        /**
         * Returns the this argument in calls like x.f(...) and x[f](...). Undefined otherwise.
         */
        function getThisArgumentOfCall(node: CallLikeExpression): LeftHandSideExpression | undefined {
            if (node.kind === SyntaxKind.CallExpression) {
                const callee = skipOuterExpressions(node.expression);
                if (callee.kind === SyntaxKind.PropertyAccessExpression || callee.kind === SyntaxKind.ElementAccessExpression) {
                    return (callee as PropertyAccessExpression | ElementAccessExpression).expression;
                }
            }
        }

        function createSyntheticExpression(parent: Node, type: Type, isSpread?: boolean) {
            const result = <SyntheticExpression>createNode(SyntaxKind.SyntheticExpression, parent.pos, parent.end);
            result.parent = parent;
            result.type = type;
            result.isSpread = isSpread || false;
            return result;
        }

        /**
         * Returns the effective arguments for an expression that works like a function invocation.
         */
        function getEffectiveCallArguments(node: CallLikeExpression): ReadonlyArray<Expression> {
            if (node.kind === SyntaxKind.TaggedTemplateExpression) {
                const template = node.template;
                const args: Expression[] = [createSyntheticExpression(template, getGlobalTemplateStringsArrayType())];
                if (template.kind === SyntaxKind.TemplateExpression) {
                    forEach(template.templateSpans, span => {
                        args.push(span.expression);
                    });
                }
                return args;
            }
            if (node.kind === SyntaxKind.Decorator) {
                return getEffectiveDecoratorArguments(node);
            }
            if (isJsxOpeningLikeElement(node)) {
                return node.attributes.properties.length > 0 ? [node.attributes] : emptyArray;
            }
            const args = node.arguments || emptyArray;
            const length = args.length;
            if (length && isSpreadArgument(args[length - 1]) && getSpreadArgumentIndex(args) === length - 1) {
                // We have a spread argument in the last position and no other spread arguments. If the type
                // of the argument is a tuple type, spread the tuple elements into the argument list. We can
                // call checkExpressionCached because spread expressions never have a contextual type.
                const spreadArgument = <SpreadElement>args[length - 1];
                const type = checkExpressionCached(spreadArgument.expression);
                if (isTupleType(type)) {
                    const typeArguments = (<TypeReference>type).typeArguments || emptyArray;
                    const restIndex = type.target.hasRestElement ? typeArguments.length - 1 : -1;
                    const syntheticArgs = map(typeArguments, (t, i) => createSyntheticExpression(spreadArgument, t, /*isSpread*/ i === restIndex));
                    return concatenate(args.slice(0, length - 1), syntheticArgs);
                }
            }
            return args;
        }

        /**
         * Returns the synthetic argument list for a decorator invocation.
         */
        function getEffectiveDecoratorArguments(node: Decorator): ReadonlyArray<Expression> {
            const parent = node.parent;
            const expr = node.expression;
            switch (parent.kind) {
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.ClassExpression:
                    // For a class decorator, the `target` is the type of the class (e.g. the
                    // "static" or "constructor" side of the class).
                    return [
                        createSyntheticExpression(expr, getTypeOfSymbol(getSymbolOfNode(parent)))
                    ];
                case SyntaxKind.Parameter:
                    // A parameter declaration decorator will have three arguments (see
                    // `ParameterDecorator` in core.d.ts).
                    const func = <FunctionLikeDeclaration>parent.parent;
                    return [
                        createSyntheticExpression(expr, parent.parent.kind === SyntaxKind.Constructor ? getTypeOfSymbol(getSymbolOfNode(func)) : errorType),
                        createSyntheticExpression(expr, anyType),
                        createSyntheticExpression(expr, numberType)
                    ];
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    // A method or accessor declaration decorator will have two or three arguments (see
                    // `PropertyDecorator` and `MethodDecorator` in core.d.ts). If we are emitting decorators
                    // for ES3, we will only pass two arguments.
                    const hasPropDesc = parent.kind !== SyntaxKind.PropertyDeclaration && languageVersion !== ScriptTarget.ES3;
                    return [
                        createSyntheticExpression(expr, getParentTypeOfClassElement(<ClassElement>parent)),
                        createSyntheticExpression(expr, getClassElementPropertyKeyType(<ClassElement>parent)),
                        createSyntheticExpression(expr, hasPropDesc ? createTypedPropertyDescriptorType(getTypeOfNode(parent)) : anyType)
                    ];
            }
            return Debug.fail();
        }

        /**
         * Returns the argument count for a decorator node that works like a function invocation.
         */
        function getDecoratorArgumentCount(node: Decorator, signature: Signature) {
            switch (node.parent.kind) {
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.ClassExpression:
                    return 1;
                case SyntaxKind.PropertyDeclaration:
                    return 2;
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    // For ES3 or decorators with only two parameters we supply only two arguments
                    return languageVersion === ScriptTarget.ES3 || signature.parameters.length <= 2 ? 2 : 3;
                case SyntaxKind.Parameter:
                    return 3;
                default:
                    return Debug.fail();
            }
        }

        function getArgumentArityError(node: Node, signatures: ReadonlyArray<Signature>, args: ReadonlyArray<Expression>) {
            let min = Number.POSITIVE_INFINITY;
            let max = Number.NEGATIVE_INFINITY;
            let belowArgCount = Number.NEGATIVE_INFINITY;
            let aboveArgCount = Number.POSITIVE_INFINITY;

            let argCount = args.length;
            let closestSignature: Signature | undefined;
            for (const sig of signatures) {
                const minCount = getMinArgumentCount(sig);
                const maxCount = getParameterCount(sig);
                if (minCount < argCount && minCount > belowArgCount) belowArgCount = minCount;
                if (argCount < maxCount && maxCount < aboveArgCount) aboveArgCount = maxCount;
                if (minCount < min) {
                    min = minCount;
                    closestSignature = sig;
                }
                max = Math.max(max, maxCount);
            }

            const hasRestParameter = some(signatures, hasEffectiveRestParameter);
            const paramRange = hasRestParameter ? min :
                min < max ? min + "-" + max :
                min;
            const hasSpreadArgument = getSpreadArgumentIndex(args) > -1;
            if (argCount <= max && hasSpreadArgument) {
                argCount--;
            }

            let related: DiagnosticWithLocation | undefined;
            if (closestSignature && getMinArgumentCount(closestSignature) > argCount && closestSignature.declaration) {
                const paramDecl = closestSignature.declaration.parameters[closestSignature.thisParameter ? argCount + 1 : argCount];
                if (paramDecl) {
                    related = createDiagnosticForNode(
                        paramDecl,
                        isBindingPattern(paramDecl.name) ? Diagnostics.An_argument_matching_this_binding_pattern_was_not_provided : Diagnostics.An_argument_for_0_was_not_provided,
                        !paramDecl.name ? argCount : !isBindingPattern(paramDecl.name) ? idText(getFirstIdentifier(paramDecl.name)) : undefined
                    );
                }
            }
            if (hasRestParameter || hasSpreadArgument) {
                const error = hasRestParameter && hasSpreadArgument ? Diagnostics.Expected_at_least_0_arguments_but_got_1_or_more :
                    hasRestParameter ? Diagnostics.Expected_at_least_0_arguments_but_got_1 :
                    Diagnostics.Expected_0_arguments_but_got_1_or_more;
                const diagnostic = createDiagnosticForNode(node, error, paramRange, argCount);
                return related ? addRelatedInfo(diagnostic, related) : diagnostic;
            }
            if (min < argCount && argCount < max) {
                return createDiagnosticForNode(node, Diagnostics.No_overload_expects_0_arguments_but_overloads_do_exist_that_expect_either_1_or_2_arguments, argCount, belowArgCount, aboveArgCount);
            }
            const diagnostic = createDiagnosticForNode(node, Diagnostics.Expected_0_arguments_but_got_1, paramRange, argCount);
            return related ? addRelatedInfo(diagnostic, related) : diagnostic;
        }

        function getTypeArgumentArityError(node: Node, signatures: ReadonlyArray<Signature>, typeArguments: NodeArray<TypeNode>) {
            let min = Infinity;
            let max = -Infinity;
            for (const sig of signatures) {
                min = Math.min(min, getMinTypeArgumentCount(sig.typeParameters));
                max = Math.max(max, length(sig.typeParameters));
            }
            const paramCount = min === max ? min : min + "-" + max;
            return createDiagnosticForNodeArray(getSourceFileOfNode(node), typeArguments, Diagnostics.Expected_0_type_arguments_but_got_1, paramCount, typeArguments.length);
        }

        function resolveCall(node: CallLikeExpression, signatures: ReadonlyArray<Signature>, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean, fallbackError?: DiagnosticMessage): Signature {
            const isTaggedTemplate = node.kind === SyntaxKind.TaggedTemplateExpression;
            const isDecorator = node.kind === SyntaxKind.Decorator;
            const isJsxOpeningOrSelfClosingElement = isJsxOpeningLikeElement(node);

            let typeArguments: NodeArray<TypeNode> | undefined;

            if (!isDecorator) {
                typeArguments = (<CallExpression>node).typeArguments;

                // We already perform checking on the type arguments on the class declaration itself.
                if (isTaggedTemplate || isJsxOpeningOrSelfClosingElement || (<CallExpression>node).expression.kind !== SyntaxKind.SuperKeyword) {
                    forEach(typeArguments, checkSourceElement);
                }
            }

            const candidates = candidatesOutArray || [];
            // reorderCandidates fills up the candidates array directly
            reorderCandidates(signatures, candidates);
            if (!candidates.length) {
                diagnostics.add(createDiagnosticForNode(node, Diagnostics.Call_target_does_not_contain_any_signatures));
                return resolveErrorCall(node);
            }

            const args = getEffectiveCallArguments(node);

            // The excludeArgument array contains true for each context sensitive argument (an argument
            // is context sensitive it is susceptible to a one-time permanent contextual typing).
            //
            // The idea is that we will perform type argument inference & assignability checking once
            // without using the susceptible parameters that are functions, and once more for those
            // parameters, contextually typing each as we go along.
            //
            // For a tagged template, then the first argument be 'undefined' if necessary because it
            // represents a TemplateStringsArray.
            //
            // For a decorator, no arguments are susceptible to contextual typing due to the fact
            // decorators are applied to a declaration by the emitter, and not to an expression.
            const isSingleNonGenericCandidate = candidates.length === 1 && !candidates[0].typeParameters;
            let excludeArgument = !isDecorator && !isSingleNonGenericCandidate ? getExcludeArgument(args) : undefined;

            // The following variables are captured and modified by calls to chooseOverload.
            // If overload resolution or type argument inference fails, we want to report the
            // best error possible. The best error is one which says that an argument was not
            // assignable to a parameter. This implies that everything else about the overload
            // was fine. So if there is any overload that is only incorrect because of an
            // argument, we will report an error on that one.
            //
            //     function foo(s: string): void;
            //     function foo(n: number): void; // Report argument error on this overload
            //     function foo(): void;
            //     foo(true);
            //
            // If none of the overloads even made it that far, there are two possibilities.
            // There was a problem with type arguments for some overload, in which case
            // report an error on that. Or none of the overloads even had correct arity,
            // in which case give an arity error.
            //
            //     function foo<T extends string>(x: T): void; // Report type argument error
            //     function foo(): void;
            //     foo<number>(0);
            //
            let candidateForArgumentError: Signature | undefined;
            let candidateForArgumentArityError: Signature | undefined;
            let candidateForTypeArgumentError: Signature | undefined;
            let result: Signature | undefined;

            // If we are in signature help, a trailing comma indicates that we intend to provide another argument,
            // so we will only accept overloads with arity at least 1 higher than the current number of provided arguments.
            const signatureHelpTrailingComma =
                isForSignatureHelp && node.kind === SyntaxKind.CallExpression && node.arguments.hasTrailingComma;

            // Section 4.12.1:
            // if the candidate list contains one or more signatures for which the type of each argument
            // expression is a subtype of each corresponding parameter type, the return type of the first
            // of those signatures becomes the return type of the function call.
            // Otherwise, the return type of the first signature in the candidate list becomes the return
            // type of the function call.
            //
            // Whether the call is an error is determined by assignability of the arguments. The subtype pass
            // is just important for choosing the best signature. So in the case where there is only one
            // signature, the subtype pass is useless. So skipping it is an optimization.
            if (candidates.length > 1) {
                result = chooseOverload(candidates, subtypeRelation, signatureHelpTrailingComma);
            }
            if (!result) {
                result = chooseOverload(candidates, assignableRelation, signatureHelpTrailingComma);
            }
            if (result) {
                return result;
            }

            // No signatures were applicable. Now report errors based on the last applicable signature with
            // no arguments excluded from assignability checks.
            // If candidate is undefined, it means that no candidates had a suitable arity. In that case,
            // skip the checkApplicableSignature check.
            if (candidateForArgumentError) {
                if (isJsxOpeningOrSelfClosingElement) {
                    // We do not report any error here because any error will be handled in "resolveCustomJsxElementAttributesType".
                    return candidateForArgumentError;
                }
                // excludeArgument is undefined, in this case also equivalent to [undefined, undefined, ...]
                // The importance of excludeArgument is to prevent us from typing function expression parameters
                // in arguments too early. If possible, we'd like to only type them once we know the correct
                // overload. However, this matters for the case where the call is correct. When the call is
                // an error, we don't need to exclude any arguments, although it would cause no harm to do so.
                checkApplicableSignature(node, args, candidateForArgumentError, assignableRelation, /*excludeArgument*/ undefined, /*reportErrors*/ true);
            }
            else if (candidateForArgumentArityError) {
                diagnostics.add(getArgumentArityError(node, [candidateForArgumentArityError], args));
            }
            else if (candidateForTypeArgumentError) {
                checkTypeArguments(candidateForTypeArgumentError, (node as CallExpression | TaggedTemplateExpression).typeArguments!, /*reportErrors*/ true, fallbackError);
            }
            else {
                const signaturesWithCorrectTypeArgumentArity = filter(signatures, s => hasCorrectTypeArgumentArity(s, typeArguments));
                if (signaturesWithCorrectTypeArgumentArity.length === 0) {
                    diagnostics.add(getTypeArgumentArityError(node, signatures, typeArguments!));
                }
                else if (!isDecorator) {
                    diagnostics.add(getArgumentArityError(node, signaturesWithCorrectTypeArgumentArity, args));
                }
                else if (fallbackError) {
                    diagnostics.add(createDiagnosticForNode(node, fallbackError));
                }
            }

            return produceDiagnostics || !args ? resolveErrorCall(node) : getCandidateForOverloadFailure(node, candidates, args, !!candidatesOutArray);

            function chooseOverload(candidates: Signature[], relation: Map<RelationComparisonResult>, signatureHelpTrailingComma = false) {
                candidateForArgumentError = undefined;
                candidateForArgumentArityError = undefined;
                candidateForTypeArgumentError = undefined;

                if (isSingleNonGenericCandidate) {
                    const candidate = candidates[0];
                    if (typeArguments || !hasCorrectArity(node, args, candidate, signatureHelpTrailingComma)) {
                        return undefined;
                    }
                    if (!checkApplicableSignature(node, args, candidate, relation, excludeArgument, /*reportErrors*/ false)) {
                        candidateForArgumentError = candidate;
                        return undefined;
                    }
                    return candidate;
                }

                for (let candidateIndex = 0; candidateIndex < candidates.length; candidateIndex++) {
                    const candidate = candidates[candidateIndex];
                    if (!hasCorrectTypeArgumentArity(candidate, typeArguments) || !hasCorrectArity(node, args, candidate, signatureHelpTrailingComma)) {
                        continue;
                    }

                    let checkCandidate: Signature;
                    let inferenceContext: InferenceContext | undefined;

                    if (candidate.typeParameters) {
                        let typeArgumentTypes: Type[] | undefined;
                        if (typeArguments) {
                            typeArgumentTypes = checkTypeArguments(candidate, typeArguments, /*reportErrors*/ false);
                            if (!typeArgumentTypes) {
                                candidateForTypeArgumentError = candidate;
                                continue;
                            }
                        }
                        else {
                            inferenceContext = createInferenceContext(candidate.typeParameters, candidate, /*flags*/ isInJSFile(node) ? InferenceFlags.AnyDefault : InferenceFlags.None);
                            typeArgumentTypes = inferTypeArguments(node, candidate, args, excludeArgument, inferenceContext);
                        }
                        checkCandidate = getSignatureInstantiation(candidate, typeArgumentTypes, isInJSFile(candidate.declaration));
                        // If the original signature has a generic rest type, instantiation may produce a
                        // signature with different arity and we need to perform another arity check.
                        if (getNonArrayRestType(candidate) && !hasCorrectArity(node, args, checkCandidate, signatureHelpTrailingComma)) {
                            candidateForArgumentArityError = checkCandidate;
                            continue;
                        }
                    }
                    else {
                        checkCandidate = candidate;
                    }
                    if (!checkApplicableSignature(node, args, checkCandidate, relation, excludeArgument, /*reportErrors*/ false)) {
                        candidateForArgumentError = checkCandidate;
                        continue;
                    }
                    if (excludeArgument) {
                        // If one or more context sensitive arguments were excluded, we start including
                        // them now (and keeping do so for any subsequent candidates) and perform a second
                        // round of type inference and applicability checking for this particular candidate.
                        excludeArgument = undefined;
                        if (inferenceContext) {
                            const typeArgumentTypes = inferTypeArguments(node, candidate, args, excludeArgument, inferenceContext);
                            checkCandidate = getSignatureInstantiation(candidate, typeArgumentTypes, isInJSFile(candidate.declaration));
                        }
                        if (!checkApplicableSignature(node, args, checkCandidate, relation, excludeArgument, /*reportErrors*/ false)) {
                            candidateForArgumentError = checkCandidate;
                            continue;
                        }
                    }
                    candidates[candidateIndex] = checkCandidate;
                    return checkCandidate;
                }

                return undefined;
            }
        }

        function getExcludeArgument(args: ReadonlyArray<Expression>): boolean[] | undefined {
            let excludeArgument: boolean[] | undefined;
            // We do not need to call `getEffectiveArgumentCount` here as it only
            // applies when calculating the number of arguments for a decorator.
            for (let i = 0; i < args.length; i++) {
                if (isContextSensitive(args[i])) {
                    if (!excludeArgument) {
                        excludeArgument = new Array(args.length);
                    }
                    excludeArgument[i] = true;
                }
            }
            return excludeArgument;
        }

        // No signature was applicable. We have already reported the errors for the invalid signature.
        // If this is a type resolution session, e.g. Language Service, try to get better information than anySignature.
        function getCandidateForOverloadFailure(
            node: CallLikeExpression,
            candidates: Signature[],
            args: ReadonlyArray<Expression>,
            hasCandidatesOutArray: boolean,
        ): Signature {
            Debug.assert(candidates.length > 0); // Else should not have called this.
            // Normally we will combine overloads. Skip this if they have type parameters since that's hard to combine.
            // Don't do this if there is a `candidatesOutArray`,
            // because then we want the chosen best candidate to be one of the overloads, not a combination.
            return hasCandidatesOutArray || candidates.length === 1 || candidates.some(c => !!c.typeParameters)
                ? pickLongestCandidateSignature(node, candidates, args)
                : createUnionOfSignaturesForOverloadFailure(candidates);
        }

        function createUnionOfSignaturesForOverloadFailure(candidates: ReadonlyArray<Signature>): Signature {
            const thisParameters = mapDefined(candidates, c => c.thisParameter);
            let thisParameter: Symbol | undefined;
            if (thisParameters.length) {
                thisParameter = createCombinedSymbolFromTypes(thisParameters, thisParameters.map(getTypeOfParameter));
            }
            const { min: minArgumentCount, max: maxNonRestParam } = minAndMax(candidates, getNumNonRestParameters);
            const parameters: Symbol[] = [];
            for (let i = 0; i < maxNonRestParam; i++) {
                const symbols = mapDefined(candidates, ({ parameters, hasRestParameter }) => hasRestParameter ?
                    i < parameters.length - 1 ? parameters[i] : last(parameters) :
                    i < parameters.length ? parameters[i] : undefined);
                Debug.assert(symbols.length !== 0);
                parameters.push(createCombinedSymbolFromTypes(symbols, mapDefined(candidates, candidate => tryGetTypeAtPosition(candidate, i))));
            }
            const restParameterSymbols = mapDefined(candidates, c => c.hasRestParameter ? last(c.parameters) : undefined);
            const hasRestParameter = restParameterSymbols.length !== 0;
            if (hasRestParameter) {
                const type = createArrayType(getUnionType(mapDefined(candidates, tryGetRestTypeOfSignature), UnionReduction.Subtype));
                parameters.push(createCombinedSymbolForOverloadFailure(restParameterSymbols, type));
            }
            return createSignature(
                candidates[0].declaration,
                /*typeParameters*/ undefined, // Before calling this we tested for `!candidates.some(c => !!c.typeParameters)`.
                thisParameter,
                parameters,
                /*resolvedReturnType*/ getIntersectionType(candidates.map(getReturnTypeOfSignature)),
                /*typePredicate*/ undefined,
                minArgumentCount,
                hasRestParameter,
                /*hasLiteralTypes*/ candidates.some(c => c.hasLiteralTypes));
        }

        function getNumNonRestParameters(signature: Signature): number {
            const numParams = signature.parameters.length;
            return signature.hasRestParameter ? numParams - 1 : numParams;
        }

        function createCombinedSymbolFromTypes(sources: ReadonlyArray<Symbol>, types: Type[]): Symbol {
            return createCombinedSymbolForOverloadFailure(sources, getUnionType(types, UnionReduction.Subtype));
        }

        function createCombinedSymbolForOverloadFailure(sources: ReadonlyArray<Symbol>, type: Type): Symbol {
            // This function is currently only used for erroneous overloads, so it's good enough to just use the first source.
            return createSymbolWithType(first(sources), type);
        }

        function pickLongestCandidateSignature(node: CallLikeExpression, candidates: Signature[], args: ReadonlyArray<Expression>): Signature {
            // Pick the longest signature. This way we can get a contextual type for cases like:
            //     declare function f(a: { xa: number; xb: number; }, b: number);
            //     f({ |
            // Also, use explicitly-supplied type arguments if they are provided, so we can get a contextual signature in cases like:
            //     declare function f<T>(k: keyof T);
            //     f<Foo>("
            const bestIndex = getLongestCandidateIndex(candidates, apparentArgumentCount === undefined ? args.length : apparentArgumentCount);
            const candidate = candidates[bestIndex];
            const { typeParameters } = candidate;
            if (!typeParameters) {
                return candidate;
            }

            const typeArgumentNodes: ReadonlyArray<TypeNode> | undefined = callLikeExpressionMayHaveTypeArguments(node) ? node.typeArguments : undefined;
            const instantiated = typeArgumentNodes
                ? createSignatureInstantiation(candidate, getTypeArgumentsFromNodes(typeArgumentNodes, typeParameters, isInJSFile(node)))
                : inferSignatureInstantiationForOverloadFailure(node, typeParameters, candidate, args);
            candidates[bestIndex] = instantiated;
            return instantiated;
        }

        function getTypeArgumentsFromNodes(typeArgumentNodes: ReadonlyArray<TypeNode>, typeParameters: ReadonlyArray<TypeParameter>, isJs: boolean): ReadonlyArray<Type> {
            const typeArguments = typeArgumentNodes.map(getTypeOfNode);
            while (typeArguments.length > typeParameters.length) {
                typeArguments.pop();
            }
            while (typeArguments.length < typeParameters.length) {
                typeArguments.push(getConstraintOfTypeParameter(typeParameters[typeArguments.length]) || getDefaultTypeArgumentType(isJs));
            }
            return typeArguments;
        }

        function inferSignatureInstantiationForOverloadFailure(node: CallLikeExpression, typeParameters: ReadonlyArray<TypeParameter>, candidate: Signature, args: ReadonlyArray<Expression>): Signature {
            const inferenceContext = createInferenceContext(typeParameters, candidate, /*flags*/ isInJSFile(node) ? InferenceFlags.AnyDefault : InferenceFlags.None);
            const typeArgumentTypes = inferTypeArguments(node, candidate, args, getExcludeArgument(args), inferenceContext);
            return createSignatureInstantiation(candidate, typeArgumentTypes);
        }

        function getLongestCandidateIndex(candidates: Signature[], argsCount: number): number {
            let maxParamsIndex = -1;
            let maxParams = -1;

            for (let i = 0; i < candidates.length; i++) {
                const candidate = candidates[i];
                const paramCount = getParameterCount(candidate);
                if (hasEffectiveRestParameter(candidate) || paramCount >= argsCount) {
                    return i;
                }
                if (paramCount > maxParams) {
                    maxParams = paramCount;
                    maxParamsIndex = i;
                }
            }

            return maxParamsIndex;
        }

        function resolveCallExpression(node: CallExpression, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean): Signature {
            if (node.expression.kind === SyntaxKind.SuperKeyword) {
                const superType = checkSuperExpression(node.expression);
                if (isTypeAny(superType)) {
                    for (const arg of node.arguments) {
                        checkExpression(arg); // Still visit arguments so they get marked for visibility, etc
                    }
                    return anySignature;
                }
                if (superType !== errorType) {
                    // In super call, the candidate signatures are the matching arity signatures of the base constructor function instantiated
                    // with the type arguments specified in the extends clause.
                    const baseTypeNode = getEffectiveBaseTypeNode(getContainingClass(node)!);
                    if (baseTypeNode) {
                        const baseConstructors = getInstantiatedConstructorsForTypeArguments(superType, baseTypeNode.typeArguments, baseTypeNode);
                        return resolveCall(node, baseConstructors, candidatesOutArray, isForSignatureHelp);
                    }
                }
                return resolveUntypedCall(node);
            }

            const funcType = checkNonNullExpression(
                node.expression,
                Diagnostics.Cannot_invoke_an_object_which_is_possibly_null,
                Diagnostics.Cannot_invoke_an_object_which_is_possibly_undefined,
                Diagnostics.Cannot_invoke_an_object_which_is_possibly_null_or_undefined
            );

            if (funcType === silentNeverType) {
                return silentNeverSignature;
            }
            const apparentType = getApparentType(funcType);

            if (apparentType === errorType) {
                // Another error has already been reported
                return resolveErrorCall(node);
            }

            // Technically, this signatures list may be incomplete. We are taking the apparent type,
            // but we are not including call signatures that may have been added to the Object or
            // Function interface, since they have none by default. This is a bit of a leap of faith
            // that the user will not add any.
            const callSignatures = getSignaturesOfType(apparentType, SignatureKind.Call);
            const constructSignatures = getSignaturesOfType(apparentType, SignatureKind.Construct);

            // TS 1.0 Spec: 4.12
            // In an untyped function call no TypeArgs are permitted, Args can be any argument list, no contextual
            // types are provided for the argument expressions, and the result is always of type Any.
            if (isUntypedFunctionCall(funcType, apparentType, callSignatures.length, constructSignatures.length)) {
                // The unknownType indicates that an error already occurred (and was reported).  No
                // need to report another error in this case.
                if (funcType !== errorType && node.typeArguments) {
                    error(node, Diagnostics.Untyped_function_calls_may_not_accept_type_arguments);
                }
                return resolveUntypedCall(node);
            }
            // If FuncExpr's apparent type(section 3.8.1) is a function type, the call is a typed function call.
            // TypeScript employs overload resolution in typed function calls in order to support functions
            // with multiple call signatures.
            if (!callSignatures.length) {
                if (constructSignatures.length) {
                    error(node, Diagnostics.Value_of_type_0_is_not_callable_Did_you_mean_to_include_new, typeToString(funcType));
                }
                else {
                    let relatedInformation: DiagnosticRelatedInformation | undefined;
                    if (node.arguments.length === 1 && isTypeAssertion(first(node.arguments))) {
                        const text = getSourceFileOfNode(node).text;
                        if (isLineBreak(text.charCodeAt(skipTrivia(text, node.expression.end, /* stopAfterLineBreak */ true) - 1))) {
                            relatedInformation = createDiagnosticForNode(node.expression, Diagnostics.It_is_highly_likely_that_you_are_missing_a_semicolon);
                        }
                    }
                    invocationError(node, apparentType, SignatureKind.Call, relatedInformation);
                }
                return resolveErrorCall(node);
            }
            // If the function is explicitly marked with `@class`, then it must be constructed.
            if (callSignatures.some(sig => isInJSFile(sig.declaration) && !!getJSDocClassTag(sig.declaration!))) {
                error(node, Diagnostics.Value_of_type_0_is_not_callable_Did_you_mean_to_include_new, typeToString(funcType));
                return resolveErrorCall(node);
            }
            return resolveCall(node, callSignatures, candidatesOutArray, isForSignatureHelp);
        }

        /**
         * TS 1.0 spec: 4.12
         * If FuncExpr is of type Any, or of an object type that has no call or construct signatures
         * but is a subtype of the Function interface, the call is an untyped function call.
         */
        function isUntypedFunctionCall(funcType: Type, apparentFuncType: Type, numCallSignatures: number, numConstructSignatures: number) {
            // We exclude union types because we may have a union of function types that happen to have no common signatures.
            return isTypeAny(funcType) || isTypeAny(apparentFuncType) && funcType.flags & TypeFlags.TypeParameter ||
                !numCallSignatures && !numConstructSignatures && !(apparentFuncType.flags & (TypeFlags.Union | TypeFlags.Never)) && isTypeAssignableTo(funcType, globalFunctionType);
        }

        function resolveNewExpression(node: NewExpression, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean): Signature {
            if (node.arguments && languageVersion < ScriptTarget.ES5) {
                const spreadIndex = getSpreadArgumentIndex(node.arguments);
                if (spreadIndex >= 0) {
                    error(node.arguments[spreadIndex], Diagnostics.Spread_operator_in_new_expressions_is_only_available_when_targeting_ECMAScript_5_and_higher);
                }
            }

            let expressionType = checkNonNullExpression(node.expression);
            if (expressionType === silentNeverType) {
                return silentNeverSignature;
            }

            // If expressionType's apparent type(section 3.8.1) is an object type with one or
            // more construct signatures, the expression is processed in the same manner as a
            // function call, but using the construct signatures as the initial set of candidate
            // signatures for overload resolution. The result type of the function call becomes
            // the result type of the operation.
            expressionType = getApparentType(expressionType);
            if (expressionType === errorType) {
                // Another error has already been reported
                return resolveErrorCall(node);
            }

            // TS 1.0 spec: 4.11
            // If expressionType is of type Any, Args can be any argument
            // list and the result of the operation is of type Any.
            if (isTypeAny(expressionType)) {
                if (node.typeArguments) {
                    error(node, Diagnostics.Untyped_function_calls_may_not_accept_type_arguments);
                }
                return resolveUntypedCall(node);
            }

            // Technically, this signatures list may be incomplete. We are taking the apparent type,
            // but we are not including construct signatures that may have been added to the Object or
            // Function interface, since they have none by default. This is a bit of a leap of faith
            // that the user will not add any.
            const constructSignatures = getSignaturesOfType(expressionType, SignatureKind.Construct);
            if (constructSignatures.length) {
                if (!isConstructorAccessible(node, constructSignatures[0])) {
                    return resolveErrorCall(node);
                }
                // If the expression is a class of abstract type, then it cannot be instantiated.
                // Note, only class declarations can be declared abstract.
                // In the case of a merged class-module or class-interface declaration,
                // only the class declaration node will have the Abstract flag set.
                const valueDecl = expressionType.symbol && getClassLikeDeclarationOfSymbol(expressionType.symbol);
                if (valueDecl && hasModifier(valueDecl, ModifierFlags.Abstract)) {
                    error(node, Diagnostics.Cannot_create_an_instance_of_an_abstract_class);
                    return resolveErrorCall(node);
                }

                return resolveCall(node, constructSignatures, candidatesOutArray, isForSignatureHelp);
            }

            // If expressionType's apparent type is an object type with no construct signatures but
            // one or more call signatures, the expression is processed as a function call. A compile-time
            // error occurs if the result of the function call is not Void. The type of the result of the
            // operation is Any. It is an error to have a Void this type.
            const callSignatures = getSignaturesOfType(expressionType, SignatureKind.Call);
            if (callSignatures.length) {
                const signature = resolveCall(node, callSignatures, candidatesOutArray, isForSignatureHelp);
                if (!noImplicitAny) {
                    if (signature.declaration && !isJavascriptConstructor(signature.declaration) && getReturnTypeOfSignature(signature) !== voidType) {
                        error(node, Diagnostics.Only_a_void_function_can_be_called_with_the_new_keyword);
                    }
                    if (getThisTypeOfSignature(signature) === voidType) {
                        error(node, Diagnostics.A_function_that_is_called_with_the_new_keyword_cannot_have_a_this_type_that_is_void);
                    }
                }
                return signature;
            }

            invocationError(node, expressionType, SignatureKind.Construct);
            return resolveErrorCall(node);
        }

        function typeHasProtectedAccessibleBase(target: Symbol, type: InterfaceType): boolean {
            const baseTypes = getBaseTypes(type);
            if (!length(baseTypes)) {
                return false;
            }
            const firstBase = baseTypes[0];
            if (firstBase.flags & TypeFlags.Intersection) {
                const types = (firstBase as IntersectionType).types;
                const mixinCount = countWhere(types, isMixinConstructorType);
                let i = 0;
                for (const intersectionMember of (firstBase as IntersectionType).types) {
                    i++;
                    // We want to ignore mixin ctors
                    if (mixinCount === 0 || mixinCount === types.length && i === 0 || !isMixinConstructorType(intersectionMember)) {
                        if (getObjectFlags(intersectionMember) & (ObjectFlags.Class | ObjectFlags.Interface)) {
                            if (intersectionMember.symbol === target) {
                                return true;
                            }
                            if (typeHasProtectedAccessibleBase(target, intersectionMember as InterfaceType)) {
                                return true;
                            }
                        }
                    }
                }
                return false;
            }
            if (firstBase.symbol === target) {
                return true;
            }
            return typeHasProtectedAccessibleBase(target, firstBase as InterfaceType);
        }

        function isConstructorAccessible(node: NewExpression, signature: Signature) {
            if (!signature || !signature.declaration) {
                return true;
            }

            const declaration = signature.declaration;
            const modifiers = getSelectedModifierFlags(declaration, ModifierFlags.NonPublicAccessibilityModifier);

            // Public constructor is accessible.
            if (!modifiers) {
                return true;
            }

            const declaringClassDeclaration = getClassLikeDeclarationOfSymbol(declaration.parent.symbol)!;
            const declaringClass = <InterfaceType>getDeclaredTypeOfSymbol(declaration.parent.symbol);

            // A private or protected constructor can only be instantiated within its own class (or a subclass, for protected)
            if (!isNodeWithinClass(node, declaringClassDeclaration)) {
                const containingClass = getContainingClass(node);
                if (containingClass && modifiers & ModifierFlags.Protected) {
                    const containingType = getTypeOfNode(containingClass);
                    if (typeHasProtectedAccessibleBase(declaration.parent.symbol, containingType as InterfaceType)) {
                        return true;
                    }
                }
                if (modifiers & ModifierFlags.Private) {
                    error(node, Diagnostics.Constructor_of_class_0_is_private_and_only_accessible_within_the_class_declaration, typeToString(declaringClass));
                }
                if (modifiers & ModifierFlags.Protected) {
                    error(node, Diagnostics.Constructor_of_class_0_is_protected_and_only_accessible_within_the_class_declaration, typeToString(declaringClass));
                }
                return false;
            }

            return true;
        }

        function invocationError(node: Node, apparentType: Type, kind: SignatureKind, relatedInformation?: DiagnosticRelatedInformation) {
            const diagnostic = error(node, (kind === SignatureKind.Call ?
                Diagnostics.Cannot_invoke_an_expression_whose_type_lacks_a_call_signature_Type_0_has_no_compatible_call_signatures :
                Diagnostics.Cannot_use_new_with_an_expression_whose_type_lacks_a_call_or_construct_signature
            ), typeToString(apparentType));
            invocationErrorRecovery(apparentType, kind, relatedInformation ? addRelatedInfo(diagnostic, relatedInformation) : diagnostic);
        }

        function invocationErrorRecovery(apparentType: Type, kind: SignatureKind, diagnostic: Diagnostic) {
            if (!apparentType.symbol) {
                return;
            }
            const importNode = getSymbolLinks(apparentType.symbol).originatingImport;
            // Create a diagnostic on the originating import if possible onto which we can attach a quickfix
            //  An import call expression cannot be rewritten into another form to correct the error - the only solution is to use `.default` at the use-site
            if (importNode && !isImportCall(importNode)) {
                const sigs = getSignaturesOfType(getTypeOfSymbol(getSymbolLinks(apparentType.symbol).target!), kind);
                if (!sigs || !sigs.length) return;

                addRelatedInfo(diagnostic,
                    createDiagnosticForNode(importNode, Diagnostics.Type_originates_at_this_import_A_namespace_style_import_cannot_be_called_or_constructed_and_will_cause_a_failure_at_runtime_Consider_using_a_default_import_or_import_require_here_instead)
                );
            }
        }

        function resolveTaggedTemplateExpression(node: TaggedTemplateExpression, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean): Signature {
            const tagType = checkExpression(node.tag);
            const apparentType = getApparentType(tagType);

            if (apparentType === errorType) {
                // Another error has already been reported
                return resolveErrorCall(node);
            }

            const callSignatures = getSignaturesOfType(apparentType, SignatureKind.Call);
            const constructSignatures = getSignaturesOfType(apparentType, SignatureKind.Construct);

            if (isUntypedFunctionCall(tagType, apparentType, callSignatures.length, constructSignatures.length)) {
                return resolveUntypedCall(node);
            }

            if (!callSignatures.length) {
                invocationError(node, apparentType, SignatureKind.Call);
                return resolveErrorCall(node);
            }

            return resolveCall(node, callSignatures, candidatesOutArray, isForSignatureHelp);
        }

        /**
         * Gets the localized diagnostic head message to use for errors when resolving a decorator as a call expression.
         */
        function getDiagnosticHeadMessageForDecoratorResolution(node: Decorator) {
            switch (node.parent.kind) {
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.ClassExpression:
                    return Diagnostics.Unable_to_resolve_signature_of_class_decorator_when_called_as_an_expression;

                case SyntaxKind.Parameter:
                    return Diagnostics.Unable_to_resolve_signature_of_parameter_decorator_when_called_as_an_expression;

                case SyntaxKind.PropertyDeclaration:
                    return Diagnostics.Unable_to_resolve_signature_of_property_decorator_when_called_as_an_expression;

                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    return Diagnostics.Unable_to_resolve_signature_of_method_decorator_when_called_as_an_expression;

                default:
                    return Debug.fail();
            }
        }

        /**
         * Resolves a decorator as if it were a call expression.
         */
        function resolveDecorator(node: Decorator, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean): Signature {
            const funcType = checkExpression(node.expression);
            const apparentType = getApparentType(funcType);
            if (apparentType === errorType) {
                return resolveErrorCall(node);
            }

            const callSignatures = getSignaturesOfType(apparentType, SignatureKind.Call);
            const constructSignatures = getSignaturesOfType(apparentType, SignatureKind.Construct);
            if (isUntypedFunctionCall(funcType, apparentType, callSignatures.length, constructSignatures.length)) {
                return resolveUntypedCall(node);
            }

            if (isPotentiallyUncalledDecorator(node, callSignatures)) {
                const nodeStr = getTextOfNode(node.expression, /*includeTrivia*/ false);
                error(node, Diagnostics._0_accepts_too_few_arguments_to_be_used_as_a_decorator_here_Did_you_mean_to_call_it_first_and_write_0, nodeStr);
                return resolveErrorCall(node);
            }

            const headMessage = getDiagnosticHeadMessageForDecoratorResolution(node);
            if (!callSignatures.length) {
                let errorInfo = chainDiagnosticMessages(/*details*/ undefined, Diagnostics.Cannot_invoke_an_expression_whose_type_lacks_a_call_signature_Type_0_has_no_compatible_call_signatures, typeToString(apparentType));
                errorInfo = chainDiagnosticMessages(errorInfo, headMessage);
                const diag = createDiagnosticForNodeFromMessageChain(node, errorInfo);
                diagnostics.add(diag);
                invocationErrorRecovery(apparentType, SignatureKind.Call, diag);
                return resolveErrorCall(node);
            }

            return resolveCall(node, callSignatures, candidatesOutArray, isForSignatureHelp, headMessage);
        }

        /**
         * Sometimes, we have a decorator that could accept zero arguments,
         * but is receiving too many arguments as part of the decorator invocation.
         * In those cases, a user may have meant to *call* the expression before using it as a decorator.
         */
        function isPotentiallyUncalledDecorator(decorator: Decorator, signatures: ReadonlyArray<Signature>) {
            return signatures.length && every(signatures, signature =>
                signature.minArgumentCount === 0 &&
                !signature.hasRestParameter &&
                signature.parameters.length < getDecoratorArgumentCount(decorator, signature));
        }

        /**
         * This function is similar to getResolvedSignature but is exclusively for trying to resolve JSX stateless-function component.
         * The main reason we have to use this function instead of getResolvedSignature because, the caller of this function will already check the type of openingLikeElement's tagName
         * and pass the type as elementType. The elementType can not be a union (as such case should be handled by the caller of this function)
         * Note: at this point, we are still not sure whether the opening-like element is a stateless function component or not.
         * @param openingLikeElement an opening-like JSX element to try to resolve as JSX stateless function
         * @param elementType an element type of the opneing-like element by checking opening-like element's tagname.
         * @param candidatesOutArray an array of signature to be filled in by the function. It is passed by signature help in the language service;
         *                           the function will fill it up with appropriate candidate signatures
         */
        function getResolvedJsxStatelessFunctionSignature(openingLikeElement: JsxOpeningLikeElement, elementType: Type, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean): Signature | undefined {
            Debug.assert(!(elementType.flags & TypeFlags.Union));
            const callSignatures = elementType && getSignaturesOfType(elementType, SignatureKind.Call);
            if (callSignatures && callSignatures.length > 0) {
                return resolveCall(openingLikeElement, callSignatures, candidatesOutArray, isForSignatureHelp);
            }

            return undefined;
        }

        function resolveSignature(node: CallLikeExpression, candidatesOutArray: Signature[] | undefined, isForSignatureHelp: boolean): Signature {
            switch (node.kind) {
                case SyntaxKind.CallExpression:
                    return resolveCallExpression(node, candidatesOutArray, isForSignatureHelp);
                case SyntaxKind.NewExpression:
                    return resolveNewExpression(node, candidatesOutArray, isForSignatureHelp);
                case SyntaxKind.TaggedTemplateExpression:
                    return resolveTaggedTemplateExpression(node, candidatesOutArray, isForSignatureHelp);
                case SyntaxKind.Decorator:
                    return resolveDecorator(node, candidatesOutArray, isForSignatureHelp);
                case SyntaxKind.JsxOpeningElement:
                case SyntaxKind.JsxSelfClosingElement:
                    // This code-path is called by language service
                    const exprTypes = checkExpression(node.tagName);
                    return forEachType(exprTypes, exprType => {
                        const sfcResult = getResolvedJsxStatelessFunctionSignature(node, exprType, candidatesOutArray, isForSignatureHelp);
                        if (sfcResult && sfcResult !== unknownSignature) {
                            return sfcResult;
                        }
                        const sigs = getInstantiatedJsxSignatures(node, exprType);
                        if (candidatesOutArray && length(sigs)) {
                            candidatesOutArray.push(...sigs!);
                        }
                        return length(sigs) ? sigs![0] : unknownSignature;
                    }) || unknownSignature;
            }
            throw Debug.assertNever(node, "Branch in 'resolveSignature' should be unreachable.");
        }

        /**
         * Resolve a signature of a given call-like expression.
         * @param node a call-like expression to try resolve a signature for
         * @param candidatesOutArray an array of signature to be filled in by the function. It is passed by signature help in the language service;
         *                           the function will fill it up with appropriate candidate signatures
         * @return a signature of the call-like expression or undefined if one can't be found
         */
        function getResolvedSignature(node: CallLikeExpression, candidatesOutArray?: Signature[] | undefined, isForSignatureHelp = false): Signature {
            const links = getNodeLinks(node);
            // If getResolvedSignature has already been called, we will have cached the resolvedSignature.
            // However, it is possible that either candidatesOutArray was not passed in the first time,
            // or that a different candidatesOutArray was passed in. Therefore, we need to redo the work
            // to correctly fill the candidatesOutArray.
            const cached = links.resolvedSignature;
            if (cached && cached !== resolvingSignature && !candidatesOutArray) {
                return cached;
            }
            links.resolvedSignature = resolvingSignature;
            const result = resolveSignature(node, candidatesOutArray, isForSignatureHelp);
            // If signature resolution originated in control flow type analysis (for example to compute the
            // assigned type in a flow assignment) we don't cache the result as it may be based on temporary
            // types from the control flow analysis.
            links.resolvedSignature = flowLoopStart === flowLoopCount ? result : cached;
            return result;
        }

        /**
         * Indicates whether a declaration can be treated as a constructor in a JavaScript
         * file.
         */
        function isJavascriptConstructor(node: Declaration | undefined): boolean {
            if (node && isInJSFile(node)) {
                // If the node has a @class tag, treat it like a constructor.
                if (getJSDocClassTag(node)) return true;

                // If the symbol of the node has members, treat it like a constructor.
                const symbol = isFunctionDeclaration(node) || isFunctionExpression(node) ? getSymbolOfNode(node) :
                     isVariableDeclaration(node) && node.initializer && isFunctionExpression(node.initializer) ? getSymbolOfNode(node.initializer) :
                     undefined;

                return !!symbol && symbol.members !== undefined;
            }

            return false;
        }

        function isJavascriptConstructorType(type: Type) {
            if (type.flags & TypeFlags.Object) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                return resolved.callSignatures.length === 1 && isJavascriptConstructor(resolved.callSignatures[0].declaration);
            }
            return false;
        }

        function getJavascriptClassType(symbol: Symbol): Type | undefined {
            let inferred: Type | undefined;
            if (isJavascriptConstructor(symbol.valueDeclaration)) {
                inferred = getInferredClassType(symbol);
            }
            const assigned = getAssignedClassType(symbol);
            const valueType = getTypeOfSymbol(symbol);
            if (valueType.symbol && !isInferredClassType(valueType) && isJavascriptConstructor(valueType.symbol.valueDeclaration)) {
                inferred = getInferredClassType(valueType.symbol);
            }
            return assigned && inferred ?
                getIntersectionType([inferred, assigned]) :
                assigned || inferred;
        }

        function getAssignedClassType(symbol: Symbol) {
            const decl = symbol.valueDeclaration;
            const assignmentSymbol = decl && decl.parent &&
                (isFunctionDeclaration(decl) && getSymbolOfNode(decl) ||
                 isBinaryExpression(decl.parent) && getSymbolOfNode(decl.parent.left) ||
                 isVariableDeclaration(decl.parent) && getSymbolOfNode(decl.parent));
            if (assignmentSymbol) {
                const prototype = forEach(assignmentSymbol.declarations, getAssignedJavascriptPrototype);
                if (prototype) {
                    return checkExpression(prototype);
                }
            }
        }

        function getAssignedJavascriptPrototype(node: Node) {
            if (!node.parent) {
                return false;
            }
            let parent: Node = node.parent;
            while (parent && parent.kind === SyntaxKind.PropertyAccessExpression) {
                parent = parent.parent;
            }
            if (parent && isBinaryExpression(parent) && isPrototypeAccess(parent.left) && parent.operatorToken.kind === SyntaxKind.EqualsToken) {
                const right = getInitializerOfBinaryExpression(parent);
                return isObjectLiteralExpression(right) && right;
            }
        }


        function getInferredClassType(symbol: Symbol) {
            const links = getSymbolLinks(symbol);
            if (!links.inferredClassType) {
                links.inferredClassType = createAnonymousType(symbol, getMembersOfSymbol(symbol) || emptySymbols, emptyArray, emptyArray, /*stringIndexType*/ undefined, /*numberIndexType*/ undefined);
            }
            return links.inferredClassType;
        }

        function isInferredClassType(type: Type) {
            return type.symbol
                && getObjectFlags(type) & ObjectFlags.Anonymous
                && getSymbolLinks(type.symbol).inferredClassType === type;
        }

        /**
         * Syntactically and semantically checks a call or new expression.
         * @param node The call/new expression to be checked.
         * @returns On success, the expression's signature's return type. On failure, anyType.
         */
        function checkCallExpression(node: CallExpression | NewExpression): Type {
            if (!checkGrammarTypeArguments(node, node.typeArguments)) checkGrammarArguments(node.arguments);

            const signature = getResolvedSignature(node);

            if (node.expression.kind === SyntaxKind.SuperKeyword) {
                return voidType;
            }

            if (node.kind === SyntaxKind.NewExpression) {
                const declaration = signature.declaration;

                if (declaration &&
                    declaration.kind !== SyntaxKind.Constructor &&
                    declaration.kind !== SyntaxKind.ConstructSignature &&
                    declaration.kind !== SyntaxKind.ConstructorType &&
                    !isJSDocConstructSignature(declaration)) {

                    // When resolved signature is a call signature (and not a construct signature) the result type is any, unless
                    // the declaring function had members created through 'x.prototype.y = expr' or 'this.y = expr' psuedodeclarations
                    // in a JS file
                    // Note:JS inferred classes might come from a variable declaration instead of a function declaration.
                    // In this case, using getResolvedSymbol directly is required to avoid losing the members from the declaration.
                    let funcSymbol = checkExpression(node.expression).symbol;
                    if (!funcSymbol && node.expression.kind === SyntaxKind.Identifier) {
                        funcSymbol = getResolvedSymbol(node.expression as Identifier);
                    }
                    const type = funcSymbol && getJavascriptClassType(funcSymbol);
                    if (type) {
                        return signature.target ? instantiateType(type, signature.mapper) : type;
                    }
                    if (noImplicitAny) {
                        error(node, Diagnostics.new_expression_whose_target_lacks_a_construct_signature_implicitly_has_an_any_type);
                    }
                    return anyType;
                }
            }

            // In JavaScript files, calls to any identifier 'require' are treated as external module imports
            if (isInJSFile(node) && isCommonJsRequire(node)) {
                return resolveExternalModuleTypeByLiteral(node.arguments![0] as StringLiteral);
            }

            const returnType = getReturnTypeOfSignature(signature);
            // Treat any call to the global 'Symbol' function that is part of a const variable or readonly property
            // as a fresh unique symbol literal type.
            if (returnType.flags & TypeFlags.ESSymbolLike && isSymbolOrSymbolForCall(node)) {
                return getESSymbolLikeTypeForNode(walkUpParenthesizedExpressions(node.parent));
            }
            let jsAssignmentType: Type | undefined;
            if (isInJSFile(node)) {
                const decl = getDeclarationOfExpando(node);
                if (decl) {
                    const jsSymbol = getSymbolOfNode(decl);
                    if (jsSymbol && hasEntries(jsSymbol.exports)) {
                        jsAssignmentType = createAnonymousType(jsSymbol, jsSymbol.exports, emptyArray, emptyArray, undefined, undefined);
                        (jsAssignmentType as ObjectType).objectFlags |= ObjectFlags.JSLiteral;
                    }
                }
            }
            return jsAssignmentType ? getIntersectionType([returnType, jsAssignmentType]) : returnType;
        }

        function isSymbolOrSymbolForCall(node: Node) {
            if (!isCallExpression(node)) return false;
            let left = node.expression;
            if (isPropertyAccessExpression(left) && left.name.escapedText === "for") {
                left = left.expression;
            }
            if (!isIdentifier(left) || left.escapedText !== "Symbol") {
                return false;
            }

            // make sure `Symbol` is the global symbol
            const globalESSymbol = getGlobalESSymbolConstructorSymbol(/*reportErrors*/ false);
            if (!globalESSymbol) {
                return false;
            }

            return globalESSymbol === resolveName(left, "Symbol" as __String, SymbolFlags.Value, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ false);
        }

        function checkImportCallExpression(node: ImportCall): Type {
            // Check grammar of dynamic import
            if (!checkGrammarArguments(node.arguments)) checkGrammarImportCallExpression(node);

            if (node.arguments.length === 0) {
                return createPromiseReturnType(node, anyType);
            }
            const specifier = node.arguments[0];
            const specifierType = checkExpressionCached(specifier);
            // Even though multiple arguments is grammatically incorrect, type-check extra arguments for completion
            for (let i = 1; i < node.arguments.length; ++i) {
                checkExpressionCached(node.arguments[i]);
            }

            if (specifierType.flags & TypeFlags.Undefined || specifierType.flags & TypeFlags.Null || !isTypeAssignableTo(specifierType, stringType)) {
                error(specifier, Diagnostics.Dynamic_import_s_specifier_must_be_of_type_string_but_here_has_type_0, typeToString(specifierType));
            }

            // resolveExternalModuleName will return undefined if the moduleReferenceExpression is not a string literal
            const moduleSymbol = resolveExternalModuleName(node, specifier);
            if (moduleSymbol) {
                const esModuleSymbol = resolveESModuleSymbol(moduleSymbol, specifier, /*dontRecursivelyResolve*/ true);
                if (esModuleSymbol) {
                    return createPromiseReturnType(node, getTypeWithSyntheticDefaultImportType(getTypeOfSymbol(esModuleSymbol), esModuleSymbol, moduleSymbol));
                }
            }
            return createPromiseReturnType(node, anyType);
        }

        function getTypeWithSyntheticDefaultImportType(type: Type, symbol: Symbol, originalSymbol: Symbol): Type {
            if (allowSyntheticDefaultImports && type && type !== errorType) {
                const synthType = type as SyntheticDefaultModuleType;
                if (!synthType.syntheticType) {
                    const file = find(originalSymbol.declarations, isSourceFile);
                    const hasSyntheticDefault = canHaveSyntheticDefault(file, originalSymbol, /*dontResolveAlias*/ false);
                    if (hasSyntheticDefault) {
                        const memberTable = createSymbolTable();
                        const newSymbol = createSymbol(SymbolFlags.Alias, InternalSymbolName.Default);
                        newSymbol.target = resolveSymbol(symbol);
                        memberTable.set(InternalSymbolName.Default, newSymbol);
                        const anonymousSymbol = createSymbol(SymbolFlags.TypeLiteral, InternalSymbolName.Type);
                        const defaultContainingObject = createAnonymousType(anonymousSymbol, memberTable, emptyArray, emptyArray, /*stringIndexInfo*/ undefined, /*numberIndexInfo*/ undefined);
                        anonymousSymbol.type = defaultContainingObject;
                        synthType.syntheticType = isValidSpreadType(type) ? getSpreadType(type, defaultContainingObject, anonymousSymbol, /*typeFLags*/ 0, /*objectFlags*/ 0) : defaultContainingObject;
                    }
                    else {
                        synthType.syntheticType = type;
                    }
                }
                return synthType.syntheticType;
            }
            return type;
        }

        function isCommonJsRequire(node: Node): boolean {
            if (!isRequireCall(node, /*checkArgumentIsStringLiteralLike*/ true)) {
                return false;
            }

            // Make sure require is not a local function
            if (!isIdentifier(node.expression)) return Debug.fail();
            const resolvedRequire = resolveName(node.expression, node.expression.escapedText, SymbolFlags.Value, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ true)!; // TODO: GH#18217
            if (resolvedRequire === requireSymbol) {
                return true;
            }
            // project includes symbol named 'require' - make sure that it is ambient and local non-alias
            if (resolvedRequire.flags & SymbolFlags.Alias) {
                return false;
            }

            const targetDeclarationKind = resolvedRequire.flags & SymbolFlags.Function
                ? SyntaxKind.FunctionDeclaration
                : resolvedRequire.flags & SymbolFlags.Variable
                    ? SyntaxKind.VariableDeclaration
                    : SyntaxKind.Unknown;
            if (targetDeclarationKind !== SyntaxKind.Unknown) {
                const decl = getDeclarationOfKind(resolvedRequire, targetDeclarationKind)!;
                // function/variable declaration should be ambient
                return !!decl && !!(decl.flags & NodeFlags.Ambient);
            }
            return false;
        }

        function checkTaggedTemplateExpression(node: TaggedTemplateExpression): Type {
            checkGrammarTypeArguments(node, node.typeArguments);
            if (languageVersion < ScriptTarget.ES2015) {
                checkExternalEmitHelpers(node, ExternalEmitHelpers.MakeTemplateObject);
            }
            return getReturnTypeOfSignature(getResolvedSignature(node));
        }

        function checkAssertion(node: AssertionExpression) {
            return checkAssertionWorker(node, node.type, node.expression);
        }

        function checkAssertionWorker(errNode: Node, type: TypeNode, expression: UnaryExpression | Expression, checkMode?: CheckMode) {
            const exprType = getRegularTypeOfObjectLiteral(getBaseTypeOfLiteralType(checkExpression(expression, checkMode)));

            checkSourceElement(type);
            const targetType = getTypeFromTypeNode(type);

            if (produceDiagnostics && targetType !== errorType) {
                const widenedType = getWidenedType(exprType);
                if (!isTypeComparableTo(targetType, widenedType)) {
                    checkTypeComparableTo(exprType, targetType, errNode,
                        Diagnostics.Conversion_of_type_0_to_type_1_may_be_a_mistake_because_neither_type_sufficiently_overlaps_with_the_other_If_this_was_intentional_convert_the_expression_to_unknown_first);
                }
            }
            return targetType;
        }

        function checkNonNullAssertion(node: NonNullExpression) {
            return getNonNullableType(checkExpression(node.expression));
        }

        function checkMetaProperty(node: MetaProperty): Type {
            checkGrammarMetaProperty(node);

            if (node.keywordToken === SyntaxKind.NewKeyword) {
                return checkNewTargetMetaProperty(node);
            }

            if (node.keywordToken === SyntaxKind.ImportKeyword) {
                return checkImportMetaProperty(node);
            }

            return Debug.assertNever(node.keywordToken);
        }

        function checkNewTargetMetaProperty(node: MetaProperty) {
            const container = getNewTargetContainer(node);
            if (!container) {
                error(node, Diagnostics.Meta_property_0_is_only_allowed_in_the_body_of_a_function_declaration_function_expression_or_constructor, "new.target");
                return errorType;
            }
            else if (container.kind === SyntaxKind.Constructor) {
                const symbol = getSymbolOfNode(container.parent as ClassLikeDeclaration);
                return getTypeOfSymbol(symbol);
            }
            else {
                const symbol = getSymbolOfNode(container)!;
                return getTypeOfSymbol(symbol);
            }
        }

        function checkImportMetaProperty(node: MetaProperty) {
            if (languageVersion < ScriptTarget.ESNext || moduleKind < ModuleKind.ESNext) {
                error(node, Diagnostics.The_import_meta_meta_property_is_only_allowed_using_ESNext_for_the_target_and_module_compiler_options);
            }
            const file = getSourceFileOfNode(node);
            Debug.assert(!!(file.flags & NodeFlags.PossiblyContainsImportMeta), "Containing file is missing import meta node flag.");
            Debug.assert(!!file.externalModuleIndicator, "Containing file should be a module.");
            return node.name.escapedText === "meta" ? getGlobalImportMetaType() : errorType;
        }

        function getTypeOfParameter(symbol: Symbol) {
            const type = getTypeOfSymbol(symbol);
            if (strictNullChecks) {
                const declaration = symbol.valueDeclaration;
                if (declaration && hasInitializer(declaration)) {
                    return getOptionalType(type);
                }
            }
            return type;
        }

        function getParameterNameAtPosition(signature: Signature, pos: number) {
            const paramCount = signature.parameters.length - (signature.hasRestParameter ? 1 : 0);
            if (pos < paramCount) {
                return signature.parameters[pos].escapedName;
            }
            const restParameter = signature.parameters[paramCount] || unknownSymbol;
            const restType = getTypeOfSymbol(restParameter);
            if (isTupleType(restType)) {
                const associatedNames = (<TupleType>(<TypeReference>restType).target).associatedNames;
                const index = pos - paramCount;
                return associatedNames ? associatedNames[index] : restParameter.escapedName + "_" + index as __String;
            }
            return restParameter.escapedName;
        }

        function getTypeAtPosition(signature: Signature, pos: number): Type {
            return tryGetTypeAtPosition(signature, pos) || anyType;
        }

        function tryGetTypeAtPosition(signature: Signature, pos: number): Type | undefined {
            const paramCount = signature.parameters.length - (signature.hasRestParameter ? 1 : 0);
            if (pos < paramCount) {
                return getTypeOfParameter(signature.parameters[pos]);
            }
            if (signature.hasRestParameter) {
                const restType = getTypeOfSymbol(signature.parameters[paramCount]);
                if (isTupleType(restType)) {
                    if (pos - paramCount < getLengthOfTupleType(restType)) {
                        return restType.typeArguments![pos - paramCount];
                    }
                    return getRestTypeOfTupleType(restType);
                }
                return getIndexTypeOfType(restType, IndexKind.Number);
            }
            return undefined;
        }

        function getRestTypeAtPosition(source: Signature, pos: number): Type {
            const paramCount = getParameterCount(source);
            const restType = getEffectiveRestType(source);
            if (restType && pos === paramCount - 1) {
                return restType;
            }
            const start = restType ? Math.min(pos, paramCount - 1) : pos;
            const types = [];
            const names = [];
            for (let i = start; i < paramCount; i++) {
                types.push(getTypeAtPosition(source, i));
                names.push(getParameterNameAtPosition(source, i));
            }
            const minArgumentCount = getMinArgumentCount(source);
            const minLength = minArgumentCount < start ? 0 : minArgumentCount - start;
            return createTupleType(types, minLength, !!restType, names);
        }

        function getParameterCount(signature: Signature) {
            const length = signature.parameters.length;
            if (signature.hasRestParameter) {
                const restType = getTypeOfSymbol(signature.parameters[length - 1]);
                if (isTupleType(restType)) {
                    return length + (restType.typeArguments || emptyArray).length - 1;
                }
            }
            return length;
        }

        function getMinArgumentCount(signature: Signature) {
            if (signature.hasRestParameter) {
                const restType = getTypeOfSymbol(signature.parameters[signature.parameters.length - 1]);
                if (isTupleType(restType)) {
                    const minLength = restType.target.minLength;
                    if (minLength > 0) {
                        return signature.parameters.length - 1 + minLength;
                    }
                }
            }
            return signature.minArgumentCount;
        }

        function hasEffectiveRestParameter(signature: Signature) {
            if (signature.hasRestParameter) {
                const restType = getTypeOfSymbol(signature.parameters[signature.parameters.length - 1]);
                return !isTupleType(restType) || restType.target.hasRestElement;
            }
            return false;
        }

        function getEffectiveRestType(signature: Signature) {
            if (signature.hasRestParameter) {
                const restType = getTypeOfSymbol(signature.parameters[signature.parameters.length - 1]);
                return isTupleType(restType) ? getRestArrayTypeOfTupleType(restType) : restType;
            }
            return undefined;
        }

        function getNonArrayRestType(signature: Signature) {
            const restType = getEffectiveRestType(signature);
            return restType && !isArrayType(restType) && !isTypeAny(restType) ? restType : undefined;
        }

        function getTypeOfFirstParameterOfSignature(signature: Signature) {
            return getTypeOfFirstParameterOfSignatureWithFallback(signature, neverType);
        }

        function getTypeOfFirstParameterOfSignatureWithFallback(signature: Signature, fallbackType: Type) {
            return signature.parameters.length > 0 ? getTypeAtPosition(signature, 0) : fallbackType;
        }

        function inferFromAnnotatedParameters(signature: Signature, context: Signature, mapper: TypeMapper) {
            const len = signature.parameters.length - (signature.hasRestParameter ? 1 : 0);
            for (let i = 0; i < len; i++) {
                const declaration = <ParameterDeclaration>signature.parameters[i].valueDeclaration;
                if (declaration.type) {
                    const typeNode = getEffectiveTypeAnnotationNode(declaration);
                    if (typeNode) {
                        inferTypes((<InferenceContext>mapper).inferences, getTypeFromTypeNode(typeNode), getTypeAtPosition(context, i));
                    }
                }
            }
        }

        function assignContextualParameterTypes(signature: Signature, context: Signature) {
            signature.typeParameters = context.typeParameters;
            if (context.thisParameter) {
                const parameter = signature.thisParameter;
                if (!parameter || parameter.valueDeclaration && !(<ParameterDeclaration>parameter.valueDeclaration).type) {
                    if (!parameter) {
                        signature.thisParameter = createSymbolWithType(context.thisParameter, /*type*/ undefined);
                    }
                    assignTypeToParameterAndFixTypeParameters(signature.thisParameter!, getTypeOfSymbol(context.thisParameter));
                }
            }
            const len = signature.parameters.length - (signature.hasRestParameter ? 1 : 0);
            for (let i = 0; i < len; i++) {
                const parameter = signature.parameters[i];
                if (!getEffectiveTypeAnnotationNode(<ParameterDeclaration>parameter.valueDeclaration)) {
                    const contextualParameterType = getTypeAtPosition(context, i);
                    assignTypeToParameterAndFixTypeParameters(parameter, contextualParameterType);
                }
            }
            if (signature.hasRestParameter) {
                // parameter might be a transient symbol generated by use of `arguments` in the function body.
                const parameter = last(signature.parameters);
                if (isTransientSymbol(parameter) || !getEffectiveTypeAnnotationNode(<ParameterDeclaration>parameter.valueDeclaration)) {
                    const contextualParameterType = getRestTypeAtPosition(context, len);
                    assignTypeToParameterAndFixTypeParameters(parameter, contextualParameterType);
                }
            }
        }

        // When contextual typing assigns a type to a parameter that contains a binding pattern, we also need to push
        // the destructured type into the contained binding elements.
        function assignBindingElementTypes(pattern: BindingPattern) {
            for (const element of pattern.elements) {
                if (!isOmittedExpression(element)) {
                    if (element.name.kind === SyntaxKind.Identifier) {
                        getSymbolLinks(getSymbolOfNode(element)).type = getTypeForBindingElement(element);
                    }
                    else {
                        assignBindingElementTypes(element.name);
                    }
                }
            }
        }

        function assignTypeToParameterAndFixTypeParameters(parameter: Symbol, contextualType: Type) {
            const links = getSymbolLinks(parameter);
            if (!links.type) {
                links.type = contextualType;
                const decl = parameter.valueDeclaration as ParameterDeclaration;
                if (decl.name.kind !== SyntaxKind.Identifier) {
                    // if inference didn't come up with anything but {}, fall back to the binding pattern if present.
                    if (links.type === emptyObjectType) {
                        links.type = getTypeFromBindingPattern(decl.name);
                    }
                    assignBindingElementTypes(decl.name);
                }
            }
        }

        function createPromiseType(promisedType: Type): Type {
            // creates a `Promise<T>` type where `T` is the promisedType argument
            const globalPromiseType = getGlobalPromiseType(/*reportErrors*/ true);
            if (globalPromiseType !== emptyGenericType) {
                // if the promised type is itself a promise, get the underlying type; otherwise, fallback to the promised type
                promisedType = getAwaitedType(promisedType) || emptyObjectType;
                return createTypeReference(globalPromiseType, [promisedType]);
            }

            return emptyObjectType;
        }

        function createPromiseReturnType(func: FunctionLikeDeclaration | ImportCall, promisedType: Type) {
            const promiseType = createPromiseType(promisedType);
            if (promiseType === emptyObjectType) {
                error(func, isImportCall(func) ?
                    Diagnostics.A_dynamic_import_call_returns_a_Promise_Make_sure_you_have_a_declaration_for_Promise_or_include_ES2015_in_your_lib_option :
                    Diagnostics.An_async_function_or_method_must_return_a_Promise_Make_sure_you_have_a_declaration_for_Promise_or_include_ES2015_in_your_lib_option);
                return errorType;
            }
            else if (!getGlobalPromiseConstructorSymbol(/*reportErrors*/ true)) {
                error(func, isImportCall(func) ?
                    Diagnostics.A_dynamic_import_call_in_ES5_SlashES3_requires_the_Promise_constructor_Make_sure_you_have_a_declaration_for_the_Promise_constructor_or_include_ES2015_in_your_lib_option :
                    Diagnostics.An_async_function_or_method_in_ES5_SlashES3_requires_the_Promise_constructor_Make_sure_you_have_a_declaration_for_the_Promise_constructor_or_include_ES2015_in_your_lib_option);
            }

            return promiseType;
        }

        function getReturnTypeFromBody(func: FunctionLikeDeclaration, checkMode?: CheckMode): Type {
            if (!func.body) {
                return errorType;
            }

            const functionFlags = getFunctionFlags(func);
            let type: Type;
            if (func.body.kind !== SyntaxKind.Block) {
                type = checkExpressionCached(func.body, checkMode);
                if (functionFlags & FunctionFlags.Async) {
                    // From within an async function you can return either a non-promise value or a promise. Any
                    // Promise/A+ compatible implementation will always assimilate any foreign promise, so the
                    // return type of the body should be unwrapped to its awaited type, which we will wrap in
                    // the native Promise<T> type later in this function.
                    type = checkAwaitedType(type, /*errorNode*/ func, Diagnostics.The_return_type_of_an_async_function_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
                }
            }
            else {
                let types = checkAndAggregateReturnExpressionTypes(func, checkMode);
                if (functionFlags & FunctionFlags.Generator) { // Generator or AsyncGenerator function
                    types = concatenate(checkAndAggregateYieldOperandTypes(func, checkMode), types);
                    if (!types || types.length === 0) {
                        const iterableIteratorAny = functionFlags & FunctionFlags.Async
                            ? createAsyncIterableIteratorType(anyType) // AsyncGenerator function
                            : createIterableIteratorType(anyType); // Generator function
                        if (noImplicitAny) {
                            error(func.asteriskToken,
                                Diagnostics.Generator_implicitly_has_type_0_because_it_does_not_yield_any_values_Consider_supplying_a_return_type, typeToString(iterableIteratorAny));
                        }
                        return iterableIteratorAny;
                    }
                }
                else {
                    if (!types) {
                        // For an async function, the return type will not be never, but rather a Promise for never.
                        return functionFlags & FunctionFlags.Async
                            ? createPromiseReturnType(func, neverType) // Async function
                            : neverType; // Normal function
                    }
                    if (types.length === 0) {
                        // For an async function, the return type will not be void, but rather a Promise for void.
                        return functionFlags & FunctionFlags.Async
                            ? createPromiseReturnType(func, voidType) // Async function
                            : voidType; // Normal function
                    }
                }

                // Return a union of the return expression types.
                type = getUnionType(types, UnionReduction.Subtype);
            }

            const contextualSignature = getContextualSignatureForFunctionLikeDeclaration(func);
            if (!contextualSignature) {
                reportErrorsFromWidening(func, type);
            }

            if (isUnitType(type)) {
                let contextualType = !contextualSignature ? undefined :
                    contextualSignature === getSignatureFromDeclaration(func) ? type :
                    getReturnTypeOfSignature(contextualSignature);
                if (contextualType) {
                    switch (functionFlags & FunctionFlags.AsyncGenerator) {
                        case FunctionFlags.AsyncGenerator:
                            contextualType = getIteratedTypeOfGenerator(contextualType, /*isAsyncGenerator*/ true);
                            break;
                        case FunctionFlags.Generator:
                            contextualType = getIteratedTypeOfGenerator(contextualType, /*isAsyncGenerator*/ false);
                            break;
                        case FunctionFlags.Async:
                            contextualType = getPromisedTypeOfPromise(contextualType);
                            break;
                    }
                }
                type = getWidenedLiteralLikeTypeForContextualType(type, contextualType);
            }

            const widenedType = getWidenedType(type);
            switch (functionFlags & FunctionFlags.AsyncGenerator) {
                case FunctionFlags.AsyncGenerator:
                    return createAsyncIterableIteratorType(widenedType);
                case FunctionFlags.Generator:
                    return createIterableIteratorType(widenedType);
                case FunctionFlags.Async:
                    // From within an async function you can return either a non-promise value or a promise. Any
                    // Promise/A+ compatible implementation will always assimilate any foreign promise, so the
                    // return type of the body is awaited type of the body, wrapped in a native Promise<T> type.
                    return createPromiseType(widenedType);
                default:
                    return widenedType;
            }
        }

        function checkAndAggregateYieldOperandTypes(func: FunctionLikeDeclaration, checkMode: CheckMode | undefined): Type[] {
            const aggregatedTypes: Type[] = [];
            const isAsync = (getFunctionFlags(func) & FunctionFlags.Async) !== 0;
            forEachYieldExpression(<Block>func.body, yieldExpression => {
                pushIfUnique(aggregatedTypes, getYieldedTypeOfYieldExpression(yieldExpression, isAsync, checkMode));
            });
            return aggregatedTypes;
        }

        function getYieldedTypeOfYieldExpression(node: YieldExpression, isAsync: boolean, checkMode?: CheckMode): Type | undefined {
            const errorNode = node.expression || node;
            const expressionType = node.expression ? checkExpression(node.expression, checkMode) : undefinedWideningType;
            // A `yield*` expression effectively yields everything that its operand yields
            const yieldedType = node.asteriskToken ? checkIteratedTypeOrElementType(expressionType, errorNode, /*allowStringInput*/ false, isAsync) : expressionType;
            return !isAsync ? yieldedType : getAwaitedType(yieldedType, errorNode, node.asteriskToken
                ? Diagnostics.Type_of_iterated_elements_of_a_yield_Asterisk_operand_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member
                : Diagnostics.Type_of_yield_operand_in_an_async_generator_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
        }

        /**
         * Collect the TypeFacts learned from a typeof switch with
         * total clauses `witnesses`, and the active clause ranging
         * from `start` to `end`. Parameter `hasDefault` denotes
         * whether the active clause contains a default clause.
         */
        function getFactsFromTypeofSwitch(start: number, end: number, witnesses: string[], hasDefault: boolean): TypeFacts {
            let facts: TypeFacts = TypeFacts.None;
            // When in the default we only collect inequality facts
            // because default is 'in theory' a set of infinite
            // equalities.
            if (hasDefault) {
                // Value is not equal to any types after the active clause.
                for (let i = end; i < witnesses.length; i++) {
                    facts |= typeofNEFacts.get(witnesses[i]) || TypeFacts.TypeofNEHostObject;
                }
                // Remove inequalities for types that appear in the
                // active clause because they appear before other
                // types collected so far.
                for (let i = start; i < end; i++) {
                    facts &= ~(typeofNEFacts.get(witnesses[i]) || 0);
                }
                // Add inequalities for types before the active clause unconditionally.
                for (let i = 0; i < start; i++) {
                    facts |= typeofNEFacts.get(witnesses[i]) || TypeFacts.TypeofNEHostObject;
                }
            }
            // When in an active clause without default the set of
            // equalities is finite.
            else {
                // Add equalities for all types in the active clause.
                for (let i = start; i < end; i++) {
                    facts |= typeofEQFacts.get(witnesses[i]) || TypeFacts.TypeofEQHostObject;
                }
                // Remove equalities for types that appear before the
                // active clause.
                for (let i = 0; i < start; i++) {
                    facts &= ~(typeofEQFacts.get(witnesses[i]) || 0);
                }
            }
            return facts;
        }

        function isExhaustiveSwitchStatement(node: SwitchStatement): boolean {
            if (!node.possiblyExhaustive) {
                return false;
            }
            if (node.expression.kind === SyntaxKind.TypeOfExpression) {
                const operandType = getTypeOfExpression((node.expression as TypeOfExpression).expression);
                // This cast is safe because the switch is possibly exhaustive and does not contain a default case, so there can be no undefined.
                const witnesses = <string[]>getSwitchClauseTypeOfWitnesses(node);
                // notEqualFacts states that the type of the switched value is not equal to every type in the switch.
                const notEqualFacts = getFactsFromTypeofSwitch(0, 0, witnesses, /*hasDefault*/ true);
                const type = getBaseConstraintOfType(operandType) || operandType;
                return !!(filterType(type, t => (getTypeFacts(t) & notEqualFacts) === notEqualFacts).flags & TypeFlags.Never);
            }
            const type = getTypeOfExpression(node.expression);
            if (!isLiteralType(type)) {
                return false;
            }
            const switchTypes = getSwitchClauseTypes(node);
            if (!switchTypes.length || some(switchTypes, isNeitherUnitTypeNorNever)) {
                return false;
            }
            return eachTypeContainedIn(mapType(type, getRegularTypeOfLiteralType), switchTypes);
        }

        function functionHasImplicitReturn(func: FunctionLikeDeclaration) {
            if (!(func.flags & NodeFlags.HasImplicitReturn)) {
                return false;
            }

            if (some((<Block>func.body).statements, statement => statement.kind === SyntaxKind.SwitchStatement && isExhaustiveSwitchStatement(<SwitchStatement>statement))) {
                return false;
            }
            return true;
        }

        /** NOTE: Return value of `[]` means a different thing than `undefined`. `[]` means func returns `void`, `undefined` means it returns `never`. */
        function checkAndAggregateReturnExpressionTypes(func: FunctionLikeDeclaration, checkMode: CheckMode | undefined): Type[] | undefined {
            const functionFlags = getFunctionFlags(func);
            const aggregatedTypes: Type[] = [];
            let hasReturnWithNoExpression = functionHasImplicitReturn(func);
            let hasReturnOfTypeNever = false;
            forEachReturnStatement(<Block>func.body, returnStatement => {
                const expr = returnStatement.expression;
                if (expr) {
                    let type = checkExpressionCached(expr, checkMode);
                    if (functionFlags & FunctionFlags.Async) {
                        // From within an async function you can return either a non-promise value or a promise. Any
                        // Promise/A+ compatible implementation will always assimilate any foreign promise, so the
                        // return type of the body should be unwrapped to its awaited type, which should be wrapped in
                        // the native Promise<T> type by the caller.
                        type = checkAwaitedType(type, func, Diagnostics.The_return_type_of_an_async_function_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
                    }
                    if (type.flags & TypeFlags.Never) {
                        hasReturnOfTypeNever = true;
                    }
                    pushIfUnique(aggregatedTypes, type);
                }
                else {
                    hasReturnWithNoExpression = true;
                }
            });
            if (aggregatedTypes.length === 0 && !hasReturnWithNoExpression && (hasReturnOfTypeNever || mayReturnNever(func))) {
                return undefined;
            }
            if (strictNullChecks && aggregatedTypes.length && hasReturnWithNoExpression &&
                !(isJavascriptConstructor(func) && aggregatedTypes.some(t => t.symbol === func.symbol))) {
                // Javascript "callable constructors", containing eg `if (!(this instanceof A)) return new A()` should not add undefined
                pushIfUnique(aggregatedTypes, undefinedType);
            }
            return aggregatedTypes;
        }
        function mayReturnNever(func: FunctionLikeDeclaration): boolean {
            switch (func.kind) {
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    return true;
                case SyntaxKind.MethodDeclaration:
                    return func.parent.kind === SyntaxKind.ObjectLiteralExpression;
                default:
                    return false;
            }
        }

        /**
         * TypeScript Specification 1.0 (6.3) - July 2014
         *   An explicitly typed function whose return type isn't the Void type,
         *   the Any type, or a union type containing the Void or Any type as a constituent
         *   must have at least one return statement somewhere in its body.
         *   An exception to this rule is if the function implementation consists of a single 'throw' statement.
         *
         * @param returnType - return type of the function, can be undefined if return type is not explicitly specified
         */
        function checkAllCodePathsInNonVoidFunctionReturnOrThrow(func: FunctionLikeDeclaration | MethodSignature, returnType: Type | undefined): void {
            if (!produceDiagnostics) {
                return;
            }

            // Functions with with an explicitly specified 'void' or 'any' return type don't need any return expressions.
            if (returnType && maybeTypeOfKind(returnType, TypeFlags.Any | TypeFlags.Void)) {
                return;
            }

            // If all we have is a function signature, or an arrow function with an expression body, then there is nothing to check.
            // also if HasImplicitReturn flag is not set this means that all codepaths in function body end with return or throw
            if (func.kind === SyntaxKind.MethodSignature || nodeIsMissing(func.body) || func.body!.kind !== SyntaxKind.Block || !functionHasImplicitReturn(func)) {
                return;
            }

            const hasExplicitReturn = func.flags & NodeFlags.HasExplicitReturn;

            if (returnType && returnType.flags & TypeFlags.Never) {
                error(getEffectiveReturnTypeNode(func), Diagnostics.A_function_returning_never_cannot_have_a_reachable_end_point);
            }
            else if (returnType && !hasExplicitReturn) {
                // minimal check: function has syntactic return type annotation and no explicit return statements in the body
                // this function does not conform to the specification.
                // NOTE: having returnType !== undefined is a precondition for entering this branch so func.type will always be present
                error(getEffectiveReturnTypeNode(func), Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value);
            }
            else if (returnType && strictNullChecks && !isTypeAssignableTo(undefinedType, returnType)) {
                error(getEffectiveReturnTypeNode(func), Diagnostics.Function_lacks_ending_return_statement_and_return_type_does_not_include_undefined);
            }
            else if (compilerOptions.noImplicitReturns) {
                if (!returnType) {
                    // If return type annotation is omitted check if function has any explicit return statements.
                    // If it does not have any - its inferred return type is void - don't do any checks.
                    // Otherwise get inferred return type from function body and report error only if it is not void / anytype
                    if (!hasExplicitReturn) {
                        return;
                    }
                    const inferredReturnType = getReturnTypeOfSignature(getSignatureFromDeclaration(func));
                    if (isUnwrappedReturnTypeVoidOrAny(func, inferredReturnType)) {
                        return;
                    }
                }
                error(getEffectiveReturnTypeNode(func) || func, Diagnostics.Not_all_code_paths_return_a_value);
            }
        }

        function checkFunctionExpressionOrObjectLiteralMethod(node: FunctionExpression | MethodDeclaration, checkMode?: CheckMode): Type {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));

            // The identityMapper object is used to indicate that function expressions are wildcards
            if (checkMode === CheckMode.SkipContextSensitive && isContextSensitive(node)) {
                // Skip parameters, return signature with return type that retains noncontextual parts so inferences can still be drawn in an early stage
                if (!getEffectiveReturnTypeNode(node) && hasContextSensitiveReturnExpression(node)) {
                    const links = getNodeLinks(node);
                    if (links.contextFreeType) {
                        return links.contextFreeType;
                    }
                    const returnType = getReturnTypeFromBody(node, checkMode);
                    const returnOnlySignature = createSignature(undefined, undefined, undefined, emptyArray, returnType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
                    const returnOnlyType = createAnonymousType(node.symbol, emptySymbols, [returnOnlySignature], emptyArray, undefined, undefined);
                    returnOnlyType.flags |= TypeFlags.ContainsAnyFunctionType;
                    return links.contextFreeType = returnOnlyType;
                }
                return anyFunctionType;
            }

            // Grammar checking
            const hasGrammarError = checkGrammarFunctionLikeDeclaration(node);
            if (!hasGrammarError && node.kind === SyntaxKind.FunctionExpression) {
                checkGrammarForGenerator(node);
            }

            const links = getNodeLinks(node);
            const type = getTypeOfSymbol(getMergedSymbol(node.symbol));
            if (isTypeAny(type)) {
                return type;
            }

            // Check if function expression is contextually typed and assign parameter types if so.
            if (!(links.flags & NodeCheckFlags.ContextChecked)) {
                const contextualSignature = getContextualSignature(node);
                // If a type check is started at a function expression that is an argument of a function call, obtaining the
                // contextual type may recursively get back to here during overload resolution of the call. If so, we will have
                // already assigned contextual types.
                if (!(links.flags & NodeCheckFlags.ContextChecked)) {
                    links.flags |= NodeCheckFlags.ContextChecked;
                    if (contextualSignature) {
                        const signature = getSignaturesOfType(type, SignatureKind.Call)[0];
                        if (isContextSensitive(node)) {
                            const contextualMapper = getContextualMapper(node);
                            if (checkMode === CheckMode.Inferential) {
                                inferFromAnnotatedParameters(signature, contextualSignature, contextualMapper);
                            }
                            const instantiatedContextualSignature = contextualMapper === identityMapper ?
                                contextualSignature : instantiateSignature(contextualSignature, contextualMapper);
                            assignContextualParameterTypes(signature, instantiatedContextualSignature);
                        }
                        if (!getReturnTypeFromAnnotation(node) && !signature.resolvedReturnType) {
                            const returnType = getReturnTypeFromBody(node, checkMode);
                            if (!signature.resolvedReturnType) {
                                signature.resolvedReturnType = returnType;
                            }
                        }
                    }
                    checkSignatureDeclaration(node);
                    checkNodeDeferred(node);
                }
            }

            return type;
        }

        function getReturnOrPromisedType(node: FunctionLikeDeclaration | MethodSignature, functionFlags: FunctionFlags) {
            const type = getReturnTypeFromAnnotation(node);
            return type && ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.Async) ?
                getAwaitedType(type) || errorType : type;
        }

        function checkFunctionExpressionOrObjectLiteralMethodDeferred(node: ArrowFunction | FunctionExpression | MethodDeclaration) {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));

            const functionFlags = getFunctionFlags(node);
            const returnOrPromisedType = getReturnOrPromisedType(node, functionFlags);

            if ((functionFlags & FunctionFlags.Generator) === 0) { // Async function or normal function
                // return is not necessary in the body of generators
                checkAllCodePathsInNonVoidFunctionReturnOrThrow(node, returnOrPromisedType);
            }

            if (node.body) {
                if (!getEffectiveReturnTypeNode(node)) {
                    // There are some checks that are only performed in getReturnTypeFromBody, that may produce errors
                    // we need. An example is the noImplicitAny errors resulting from widening the return expression
                    // of a function. Because checking of function expression bodies is deferred, there was never an
                    // appropriate time to do this during the main walk of the file (see the comment at the top of
                    // checkFunctionExpressionBodies). So it must be done now.
                    getReturnTypeOfSignature(getSignatureFromDeclaration(node));
                }

                if (node.body.kind === SyntaxKind.Block) {
                    checkSourceElement(node.body);
                }
                else {
                    // From within an async function you can return either a non-promise value or a promise. Any
                    // Promise/A+ compatible implementation will always assimilate any foreign promise, so we
                    // should not be checking assignability of a promise to the return type. Instead, we need to
                    // check assignability of the awaited type of the expression body against the promised type of
                    // its return type annotation.
                    const exprType = checkExpression(node.body);
                    if (returnOrPromisedType) {
                        if ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.Async) { // Async function
                            const awaitedType = checkAwaitedType(exprType, node.body, Diagnostics.The_return_type_of_an_async_function_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
                            checkTypeAssignableToAndOptionallyElaborate(awaitedType, returnOrPromisedType, node.body, node.body);
                        }
                        else { // Normal function
                            checkTypeAssignableToAndOptionallyElaborate(exprType, returnOrPromisedType, node.body, node.body);
                        }
                    }
                }
            }
        }

        function checkArithmeticOperandType(operand: Node, type: Type, diagnostic: DiagnosticMessage): boolean {
            if (!isTypeAssignableToKind(type, TypeFlags.NumberLike)) {
                error(operand, diagnostic);
                return false;
            }
            return true;
        }

        function isReadonlySymbol(symbol: Symbol): boolean {
            // The following symbols are considered read-only:
            // Properties with a 'readonly' modifier
            // Variables declared with 'const'
            // Get accessors without matching set accessors
            // Enum members
            // Unions and intersections of the above (unions and intersections eagerly set isReadonly on creation)
            return !!(getCheckFlags(symbol) & CheckFlags.Readonly ||
                symbol.flags & SymbolFlags.Property && getDeclarationModifierFlagsFromSymbol(symbol) & ModifierFlags.Readonly ||
                symbol.flags & SymbolFlags.Variable && getDeclarationNodeFlagsFromSymbol(symbol) & NodeFlags.Const ||
                symbol.flags & SymbolFlags.Accessor && !(symbol.flags & SymbolFlags.SetAccessor) ||
                symbol.flags & SymbolFlags.EnumMember);
        }

        function isReferenceToReadonlyEntity(expr: Expression, symbol: Symbol): boolean {
            if (isReadonlySymbol(symbol)) {
                // Allow assignments to readonly properties within constructors of the same class declaration.
                if (symbol.flags & SymbolFlags.Property &&
                    (expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) &&
                    (expr as PropertyAccessExpression | ElementAccessExpression).expression.kind === SyntaxKind.ThisKeyword) {
                    // Look for if this is the constructor for the class that `symbol` is a property of.
                    const func = getContainingFunction(expr);
                    if (!(func && func.kind === SyntaxKind.Constructor)) {
                        return true;
                    }
                    // If func.parent is a class and symbol is a (readonly) property of that class, or
                    // if func is a constructor and symbol is a (readonly) parameter property declared in it,
                    // then symbol is writeable here.
                    return !symbol.valueDeclaration || !(func.parent === symbol.valueDeclaration.parent || func === symbol.valueDeclaration.parent);
                }
                return true;
            }
            return false;
        }

        function isReferenceThroughNamespaceImport(expr: Expression): boolean {
            if (expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) {
                const node = skipParentheses((expr as PropertyAccessExpression | ElementAccessExpression).expression);
                if (node.kind === SyntaxKind.Identifier) {
                    const symbol = getNodeLinks(node).resolvedSymbol!;
                    if (symbol.flags & SymbolFlags.Alias) {
                        const declaration = getDeclarationOfAliasSymbol(symbol);
                        return !!declaration && declaration.kind === SyntaxKind.NamespaceImport;
                    }
                }
            }
            return false;
        }

        function checkReferenceExpression(expr: Expression, invalidReferenceMessage: DiagnosticMessage): boolean {
            // References are combinations of identifiers, parentheses, and property accesses.
            const node = skipOuterExpressions(expr, OuterExpressionKinds.Assertions | OuterExpressionKinds.Parentheses);
            if (node.kind !== SyntaxKind.Identifier && node.kind !== SyntaxKind.PropertyAccessExpression && node.kind !== SyntaxKind.ElementAccessExpression) {
                error(expr, invalidReferenceMessage);
                return false;
            }
            return true;
        }

        function checkDeleteExpression(node: DeleteExpression): Type {
            checkExpression(node.expression);
            const expr = skipParentheses(node.expression);
            if (expr.kind !== SyntaxKind.PropertyAccessExpression && expr.kind !== SyntaxKind.ElementAccessExpression) {
                error(expr, Diagnostics.The_operand_of_a_delete_operator_must_be_a_property_reference);
                return booleanType;
            }
            const links = getNodeLinks(expr);
            const symbol = getExportSymbolOfValueSymbolIfExported(links.resolvedSymbol);
            if (symbol && isReadonlySymbol(symbol)) {
                error(expr, Diagnostics.The_operand_of_a_delete_operator_cannot_be_a_read_only_property);
            }
            return booleanType;
        }

        function checkTypeOfExpression(node: TypeOfExpression): Type {
            checkExpression(node.expression);
            return typeofType;
        }

        function checkVoidExpression(node: VoidExpression): Type {
            checkExpression(node.expression);
            return undefinedWideningType;
        }

        function checkAwaitExpression(node: AwaitExpression): Type {
            // Grammar checking
            if (produceDiagnostics) {
                if (!(node.flags & NodeFlags.AwaitContext)) {
                    grammarErrorOnFirstToken(node, Diagnostics.await_expression_is_only_allowed_within_an_async_function);
                }

                if (isInParameterInitializerBeforeContainingFunction(node)) {
                    error(node, Diagnostics.await_expressions_cannot_be_used_in_a_parameter_initializer);
                }
            }

            const operandType = checkExpression(node.expression);
            return checkAwaitedType(operandType, node, Diagnostics.Type_of_await_operand_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
        }

        function checkPrefixUnaryExpression(node: PrefixUnaryExpression): Type {
            const operandType = checkExpression(node.operand);
            if (operandType === silentNeverType) {
                return silentNeverType;
            }
            if (node.operand.kind === SyntaxKind.NumericLiteral) {
                if (node.operator === SyntaxKind.MinusToken) {
                    return getFreshTypeOfLiteralType(getLiteralType(-(<LiteralExpression>node.operand).text));
                }
                else if (node.operator === SyntaxKind.PlusToken) {
                    return getFreshTypeOfLiteralType(getLiteralType(+(<LiteralExpression>node.operand).text));
                }
            }
            switch (node.operator) {
                case SyntaxKind.PlusToken:
                case SyntaxKind.MinusToken:
                case SyntaxKind.TildeToken:
                    checkNonNullType(operandType, node.operand);
                    if (maybeTypeOfKind(operandType, TypeFlags.ESSymbolLike)) {
                        error(node.operand, Diagnostics.The_0_operator_cannot_be_applied_to_type_symbol, tokenToString(node.operator));
                    }
                    return numberType;
                case SyntaxKind.ExclamationToken:
                    checkTruthinessExpression(node.operand);
                    const facts = getTypeFacts(operandType) & (TypeFacts.Truthy | TypeFacts.Falsy);
                    return facts === TypeFacts.Truthy ? falseType :
                        facts === TypeFacts.Falsy ? trueType :
                        booleanType;
                case SyntaxKind.PlusPlusToken:
                case SyntaxKind.MinusMinusToken:
                    const ok = checkArithmeticOperandType(node.operand, checkNonNullType(operandType, node.operand),
                        Diagnostics.An_arithmetic_operand_must_be_of_type_any_number_or_an_enum_type);
                    if (ok) {
                        // run check only if former checks succeeded to avoid reporting cascading errors
                        checkReferenceExpression(node.operand, Diagnostics.The_operand_of_an_increment_or_decrement_operator_must_be_a_variable_or_a_property_access);
                    }
                    return numberType;
            }
            return errorType;
        }

        function checkPostfixUnaryExpression(node: PostfixUnaryExpression): Type {
            const operandType = checkExpression(node.operand);
            if (operandType === silentNeverType) {
                return silentNeverType;
            }
            const ok = checkArithmeticOperandType(
                node.operand,
                checkNonNullType(operandType, node.operand),
                Diagnostics.An_arithmetic_operand_must_be_of_type_any_number_or_an_enum_type);
            if (ok) {
                // run check only if former checks succeeded to avoid reporting cascading errors
                checkReferenceExpression(node.operand, Diagnostics.The_operand_of_an_increment_or_decrement_operator_must_be_a_variable_or_a_property_access);
            }
            return numberType;
        }

        // Return true if type might be of the given kind. A union or intersection type might be of a given
        // kind if at least one constituent type is of the given kind.
        function maybeTypeOfKind(type: Type, kind: TypeFlags): boolean {
            if (type.flags & kind & ~TypeFlags.GenericMappedType || kind & TypeFlags.GenericMappedType && isGenericMappedType(type)) {
                return true;
            }
            if (type.flags & TypeFlags.UnionOrIntersection) {
                const types = (<UnionOrIntersectionType>type).types;
                for (const t of types) {
                    if (maybeTypeOfKind(t, kind)) {
                        return true;
                    }
                }
            }
            return false;
        }

        function isTypeAssignableToKind(source: Type, kind: TypeFlags, strict?: boolean): boolean {
            if (source.flags & kind) {
                return true;
            }
            if (strict && source.flags & (TypeFlags.AnyOrUnknown | TypeFlags.Void | TypeFlags.Undefined | TypeFlags.Null)) {
                return false;
            }
            return !!(kind & TypeFlags.NumberLike) && isTypeAssignableTo(source, numberType) ||
                !!(kind & TypeFlags.StringLike) && isTypeAssignableTo(source, stringType) ||
                !!(kind & TypeFlags.BooleanLike) && isTypeAssignableTo(source, booleanType) ||
                !!(kind & TypeFlags.Void) && isTypeAssignableTo(source, voidType) ||
                !!(kind & TypeFlags.Never) && isTypeAssignableTo(source, neverType) ||
                !!(kind & TypeFlags.Null) && isTypeAssignableTo(source, nullType) ||
                !!(kind & TypeFlags.Undefined) && isTypeAssignableTo(source, undefinedType) ||
                !!(kind & TypeFlags.ESSymbol) && isTypeAssignableTo(source, esSymbolType) ||
                !!(kind & TypeFlags.NonPrimitive) && isTypeAssignableTo(source, nonPrimitiveType);
        }

        function allTypesAssignableToKind(source: Type, kind: TypeFlags, strict?: boolean): boolean {
            return source.flags & TypeFlags.Union ?
                every((source as UnionType).types, subType => allTypesAssignableToKind(subType, kind, strict)) :
                isTypeAssignableToKind(source, kind, strict);
        }

        function isConstEnumObjectType(type: Type): boolean {
            return !!(getObjectFlags(type) & ObjectFlags.Anonymous) && !!type.symbol && isConstEnumSymbol(type.symbol);
        }

        function isConstEnumSymbol(symbol: Symbol): boolean {
            return (symbol.flags & SymbolFlags.ConstEnum) !== 0;
        }

        function checkInstanceOfExpression(left: Expression, right: Expression, leftType: Type, rightType: Type): Type {
            if (leftType === silentNeverType || rightType === silentNeverType) {
                return silentNeverType;
            }
            // TypeScript 1.0 spec (April 2014): 4.15.4
            // The instanceof operator requires the left operand to be of type Any, an object type, or a type parameter type,
            // and the right operand to be of type Any, a subtype of the 'Function' interface type, or have a call or construct signature.
            // The result is always of the Boolean primitive type.
            // NOTE: do not raise error if leftType is unknown as related error was already reported
            if (!isTypeAny(leftType) &&
                allTypesAssignableToKind(leftType, TypeFlags.Primitive)) {
                error(left, Diagnostics.The_left_hand_side_of_an_instanceof_expression_must_be_of_type_any_an_object_type_or_a_type_parameter);
            }
            // NOTE: do not raise error if right is unknown as related error was already reported
            if (!(isTypeAny(rightType) || typeHasCallOrConstructSignatures(rightType) || isTypeSubtypeOf(rightType, globalFunctionType))) {
                error(right, Diagnostics.The_right_hand_side_of_an_instanceof_expression_must_be_of_type_any_or_of_a_type_assignable_to_the_Function_interface_type);
            }
            return booleanType;
        }

        function checkInExpression(left: Expression, right: Expression, leftType: Type, rightType: Type): Type {
            if (leftType === silentNeverType || rightType === silentNeverType) {
                return silentNeverType;
            }
            leftType = checkNonNullType(leftType, left);
            rightType = checkNonNullType(rightType, right);
            // TypeScript 1.0 spec (April 2014): 4.15.5
            // The in operator requires the left operand to be of type Any, the String primitive type, or the Number primitive type,
            // and the right operand to be of type Any, an object type, or a type parameter type.
            // The result is always of the Boolean primitive type.
            if (!(isTypeComparableTo(leftType, stringType) || isTypeAssignableToKind(leftType, TypeFlags.NumberLike | TypeFlags.ESSymbolLike))) {
                error(left, Diagnostics.The_left_hand_side_of_an_in_expression_must_be_of_type_any_string_number_or_symbol);
            }
            if (!isTypeAssignableToKind(rightType, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive)) {
                error(right, Diagnostics.The_right_hand_side_of_an_in_expression_must_be_of_type_any_an_object_type_or_a_type_parameter);
            }
            return booleanType;
        }

        function checkObjectLiteralAssignment(node: ObjectLiteralExpression, sourceType: Type, rightIsThis?: boolean): Type {
            const properties = node.properties;
            if (strictNullChecks && properties.length === 0) {
                return checkNonNullType(sourceType, node);
            }
            for (const p of properties) {
                checkObjectLiteralDestructuringPropertyAssignment(sourceType, p, properties, rightIsThis);
            }
            return sourceType;
        }

        /** Note: If property cannot be a SpreadAssignment, then allProperties does not need to be provided */
        function checkObjectLiteralDestructuringPropertyAssignment(objectLiteralType: Type, property: ObjectLiteralElementLike, allProperties?: NodeArray<ObjectLiteralElementLike>, rightIsThis = false) {
            if (property.kind === SyntaxKind.PropertyAssignment || property.kind === SyntaxKind.ShorthandPropertyAssignment) {
                const name = property.name;
                if (name.kind === SyntaxKind.ComputedPropertyName) {
                    checkComputedPropertyName(name);
                }
                if (isComputedNonLiteralName(name)) {
                    return undefined;
                }

                const type = getTypeOfObjectLiteralDestructuringProperty(objectLiteralType, name, property, rightIsThis);
                if (type) {
                    // non-shorthand property assignments should always have initializers
                    return checkDestructuringAssignment(property.kind === SyntaxKind.ShorthandPropertyAssignment ? property : property.initializer, type);
                }
                else {
                    error(name, Diagnostics.Type_0_has_no_property_1_and_no_string_index_signature, typeToString(objectLiteralType), declarationNameToString(name));
                }
            }
            else if (property.kind === SyntaxKind.SpreadAssignment) {
                if (languageVersion < ScriptTarget.ESNext) {
                    checkExternalEmitHelpers(property, ExternalEmitHelpers.Rest);
                }
                const nonRestNames: PropertyName[] = [];
                if (allProperties) {
                    for (let i = 0; i < allProperties.length - 1; i++) {
                        nonRestNames.push(allProperties[i].name!);
                    }
                }
                const type = getRestType(objectLiteralType, nonRestNames, objectLiteralType.symbol);
                checkGrammarForDisallowedTrailingComma(allProperties, Diagnostics.A_rest_parameter_or_binding_pattern_may_not_have_a_trailing_comma);
                return checkDestructuringAssignment(property.expression, type);
            }
            else {
                error(property, Diagnostics.Property_assignment_expected);
            }
        }

        function getTypeOfObjectLiteralDestructuringProperty(objectLiteralType: Type, name: PropertyName, property: PropertyAssignment | ShorthandPropertyAssignment, rightIsThis: boolean) {
            if (isTypeAny(objectLiteralType)) {
                return objectLiteralType;
            }

            let type: Type | undefined;
            const text = getTextOfPropertyName(name);
            if (text) { // TODO: GH#26379
                const prop = getPropertyOfType(objectLiteralType, text);
                if (prop) {
                    markPropertyAsReferenced(prop, property, rightIsThis);
                    checkPropertyAccessibility(property, /*isSuper*/ false, objectLiteralType, prop);
                    type = getTypeOfSymbol(prop);
                }
                type = type || (isNumericLiteralName(text) ? getIndexTypeOfType(objectLiteralType, IndexKind.Number) : undefined);
            }
            return type || getIndexTypeOfType(objectLiteralType, IndexKind.String);
        }

        function checkArrayLiteralAssignment(node: ArrayLiteralExpression, sourceType: Type, checkMode?: CheckMode): Type {
            const elements = node.elements;
            if (languageVersion < ScriptTarget.ES2015 && compilerOptions.downlevelIteration) {
                checkExternalEmitHelpers(node, ExternalEmitHelpers.Read);
            }

            // This elementType will be used if the specific property corresponding to this index is not
            // present (aka the tuple element property). This call also checks that the parentType is in
            // fact an iterable or array (depending on target language).
            const elementType = checkIteratedTypeOrElementType(sourceType, node, /*allowStringInput*/ false, /*allowAsyncIterables*/ false) || errorType;
            for (let i = 0; i < elements.length; i++) {
                checkArrayLiteralDestructuringElementAssignment(node, sourceType, i, elementType, checkMode);
            }
            return sourceType;
        }

        function checkArrayLiteralDestructuringElementAssignment(node: ArrayLiteralExpression, sourceType: Type,
            elementIndex: number, elementType: Type, checkMode?: CheckMode) {
            const elements = node.elements;
            const element = elements[elementIndex];
            if (element.kind !== SyntaxKind.OmittedExpression) {
                if (element.kind !== SyntaxKind.SpreadElement) {
                    const propName = "" + elementIndex as __String;
                    const type = isTypeAny(sourceType) ? sourceType :
                        isTupleLikeType(sourceType) ? getTupleElementType(sourceType, elementIndex) :
                        elementType;
                    if (type) {
                        return checkDestructuringAssignment(element, type, checkMode);
                    }
                    // We still need to check element expression here because we may need to set appropriate flag on the expression
                    // such as NodeCheckFlags.LexicalThis on "this"expression.
                    checkExpression(element);
                    if (isTupleType(sourceType)) {
                        error(element, Diagnostics.Tuple_type_0_with_length_1_cannot_be_assigned_to_tuple_with_length_2, typeToString(sourceType), getTypeReferenceArity(<TypeReference>sourceType), elements.length);
                    }
                    else {
                        error(element, Diagnostics.Type_0_has_no_property_1, typeToString(sourceType), propName as string);
                    }
                }
                else {
                    if (elementIndex < elements.length - 1) {
                        error(element, Diagnostics.A_rest_element_must_be_last_in_a_destructuring_pattern);
                    }
                    else {
                        const restExpression = (<SpreadElement>element).expression;
                        if (restExpression.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>restExpression).operatorToken.kind === SyntaxKind.EqualsToken) {
                            error((<BinaryExpression>restExpression).operatorToken, Diagnostics.A_rest_element_cannot_have_an_initializer);
                        }
                        else {
                            checkGrammarForDisallowedTrailingComma(node.elements, Diagnostics.A_rest_parameter_or_binding_pattern_may_not_have_a_trailing_comma);
                            const type = isTupleType(sourceType) ?
                                getArrayLiteralType((sourceType.typeArguments || emptyArray).slice(elementIndex, getTypeReferenceArity(sourceType))) :
                                createArrayType(elementType);
                            return checkDestructuringAssignment(restExpression, type, checkMode);
                        }
                    }
                }
            }
            return undefined;
        }

        function checkDestructuringAssignment(exprOrAssignment: Expression | ShorthandPropertyAssignment, sourceType: Type, checkMode?: CheckMode, rightIsThis?: boolean): Type {
            let target: Expression;
            if (exprOrAssignment.kind === SyntaxKind.ShorthandPropertyAssignment) {
                const prop = <ShorthandPropertyAssignment>exprOrAssignment;
                if (prop.objectAssignmentInitializer) {
                    // In strict null checking mode, if a default value of a non-undefined type is specified, remove
                    // undefined from the final type.
                    if (strictNullChecks &&
                        !(getFalsyFlags(checkExpression(prop.objectAssignmentInitializer)) & TypeFlags.Undefined)) {
                        sourceType = getTypeWithFacts(sourceType, TypeFacts.NEUndefined);
                    }
                    checkBinaryLikeExpression(prop.name, prop.equalsToken!, prop.objectAssignmentInitializer, checkMode);
                }
                target = (<ShorthandPropertyAssignment>exprOrAssignment).name;
            }
            else {
                target = exprOrAssignment;
            }

            if (target.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>target).operatorToken.kind === SyntaxKind.EqualsToken) {
                checkBinaryExpression(<BinaryExpression>target, checkMode);
                target = (<BinaryExpression>target).left;
            }
            if (target.kind === SyntaxKind.ObjectLiteralExpression) {
                return checkObjectLiteralAssignment(<ObjectLiteralExpression>target, sourceType, rightIsThis);
            }
            if (target.kind === SyntaxKind.ArrayLiteralExpression) {
                return checkArrayLiteralAssignment(<ArrayLiteralExpression>target, sourceType, checkMode);
            }
            return checkReferenceAssignment(target, sourceType, checkMode);
        }

        function checkReferenceAssignment(target: Expression, sourceType: Type, checkMode?: CheckMode): Type {
            const targetType = checkExpression(target, checkMode);
            const error = target.parent.kind === SyntaxKind.SpreadAssignment ?
                Diagnostics.The_target_of_an_object_rest_assignment_must_be_a_variable_or_a_property_access :
                Diagnostics.The_left_hand_side_of_an_assignment_expression_must_be_a_variable_or_a_property_access;
            if (checkReferenceExpression(target, error)) {
                checkTypeAssignableToAndOptionallyElaborate(sourceType, targetType, target, target);
            }
            return sourceType;
        }

        /**
         * This is a *shallow* check: An expression is side-effect-free if the
         * evaluation of the expression *itself* cannot produce side effects.
         * For example, x++ / 3 is side-effect free because the / operator
         * does not have side effects.
         * The intent is to "smell test" an expression for correctness in positions where
         * its value is discarded (e.g. the left side of the comma operator).
         */
        function isSideEffectFree(node: Node): boolean {
            node = skipParentheses(node);
            switch (node.kind) {
                case SyntaxKind.Identifier:
                case SyntaxKind.StringLiteral:
                case SyntaxKind.RegularExpressionLiteral:
                case SyntaxKind.TaggedTemplateExpression:
                case SyntaxKind.TemplateExpression:
                case SyntaxKind.NoSubstitutionTemplateLiteral:
                case SyntaxKind.NumericLiteral:
                case SyntaxKind.TrueKeyword:
                case SyntaxKind.FalseKeyword:
                case SyntaxKind.NullKeyword:
                case SyntaxKind.UndefinedKeyword:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ClassExpression:
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.ArrayLiteralExpression:
                case SyntaxKind.ObjectLiteralExpression:
                case SyntaxKind.TypeOfExpression:
                case SyntaxKind.NonNullExpression:
                case SyntaxKind.JsxSelfClosingElement:
                case SyntaxKind.JsxElement:
                    return true;

                case SyntaxKind.ConditionalExpression:
                    return isSideEffectFree((node as ConditionalExpression).whenTrue) &&
                        isSideEffectFree((node as ConditionalExpression).whenFalse);

                case SyntaxKind.BinaryExpression:
                    if (isAssignmentOperator((node as BinaryExpression).operatorToken.kind)) {
                        return false;
                    }
                    return isSideEffectFree((node as BinaryExpression).left) &&
                            isSideEffectFree((node as BinaryExpression).right);

                case SyntaxKind.PrefixUnaryExpression:
                case SyntaxKind.PostfixUnaryExpression:
                    // Unary operators ~, !, +, and - have no side effects.
                    // The rest do.
                    switch ((node as PrefixUnaryExpression).operator) {
                        case SyntaxKind.ExclamationToken:
                        case SyntaxKind.PlusToken:
                        case SyntaxKind.MinusToken:
                        case SyntaxKind.TildeToken:
                            return true;
                    }
                    return false;

                // Some forms listed here for clarity
                case SyntaxKind.VoidExpression: // Explicit opt-out
                case SyntaxKind.TypeAssertionExpression: // Not SEF, but can produce useful type warnings
                case SyntaxKind.AsExpression: // Not SEF, but can produce useful type warnings
                default:
                    return false;
            }
        }

        function isTypeEqualityComparableTo(source: Type, target: Type) {
            return (target.flags & TypeFlags.Nullable) !== 0 || isTypeComparableTo(source, target);
        }

        function checkBinaryExpression(node: BinaryExpression, checkMode?: CheckMode) {
            if (isInJSFile(node) && getAssignedExpandoInitializer(node)) {
                return checkExpression(node.right, checkMode);
            }
            return checkBinaryLikeExpression(node.left, node.operatorToken, node.right, checkMode, node);
        }

        function checkBinaryLikeExpression(left: Expression, operatorToken: Node, right: Expression, checkMode?: CheckMode, errorNode?: Node): Type {
            const operator = operatorToken.kind;
            if (operator === SyntaxKind.EqualsToken && (left.kind === SyntaxKind.ObjectLiteralExpression || left.kind === SyntaxKind.ArrayLiteralExpression)) {
                return checkDestructuringAssignment(left, checkExpression(right, checkMode), checkMode, right.kind === SyntaxKind.ThisKeyword);
            }
            let leftType: Type;
            if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.BarBarToken) {
                leftType = checkTruthinessExpression(left, checkMode);
            }
            else {
                leftType = checkExpression(left, checkMode);
            }

            let rightType = checkExpression(right, checkMode);
            switch (operator) {
                case SyntaxKind.AsteriskToken:
                case SyntaxKind.AsteriskAsteriskToken:
                case SyntaxKind.AsteriskEqualsToken:
                case SyntaxKind.AsteriskAsteriskEqualsToken:
                case SyntaxKind.SlashToken:
                case SyntaxKind.SlashEqualsToken:
                case SyntaxKind.PercentToken:
                case SyntaxKind.PercentEqualsToken:
                case SyntaxKind.MinusToken:
                case SyntaxKind.MinusEqualsToken:
                case SyntaxKind.LessThanLessThanToken:
                case SyntaxKind.LessThanLessThanEqualsToken:
                case SyntaxKind.GreaterThanGreaterThanToken:
                case SyntaxKind.GreaterThanGreaterThanEqualsToken:
                case SyntaxKind.GreaterThanGreaterThanGreaterThanToken:
                case SyntaxKind.GreaterThanGreaterThanGreaterThanEqualsToken:
                case SyntaxKind.BarToken:
                case SyntaxKind.BarEqualsToken:
                case SyntaxKind.CaretToken:
                case SyntaxKind.CaretEqualsToken:
                case SyntaxKind.AmpersandToken:
                case SyntaxKind.AmpersandEqualsToken:
                    if (leftType === silentNeverType || rightType === silentNeverType) {
                        return silentNeverType;
                    }

                    leftType = checkNonNullType(leftType, left);
                    rightType = checkNonNullType(rightType, right);

                    let suggestedOperator: SyntaxKind | undefined;
                    // if a user tries to apply a bitwise operator to 2 boolean operands
                    // try and return them a helpful suggestion
                    if ((leftType.flags & TypeFlags.BooleanLike) &&
                        (rightType.flags & TypeFlags.BooleanLike) &&
                        (suggestedOperator = getSuggestedBooleanOperator(operatorToken.kind)) !== undefined) {
                        error(errorNode || operatorToken, Diagnostics.The_0_operator_is_not_allowed_for_boolean_types_Consider_using_1_instead, tokenToString(operatorToken.kind), tokenToString(suggestedOperator));
                    }
                    else {
                        // otherwise just check each operand separately and report errors as normal
                        const leftOk = checkArithmeticOperandType(left, leftType, Diagnostics.The_left_hand_side_of_an_arithmetic_operation_must_be_of_type_any_number_or_an_enum_type);
                        const rightOk = checkArithmeticOperandType(right, rightType, Diagnostics.The_right_hand_side_of_an_arithmetic_operation_must_be_of_type_any_number_or_an_enum_type);
                        if (leftOk && rightOk) {
                            checkAssignmentOperator(numberType);
                        }
                    }

                    return numberType;
                case SyntaxKind.PlusToken:
                case SyntaxKind.PlusEqualsToken:
                    if (leftType === silentNeverType || rightType === silentNeverType) {
                        return silentNeverType;
                    }

                    if (!isTypeAssignableToKind(leftType, TypeFlags.StringLike) && !isTypeAssignableToKind(rightType, TypeFlags.StringLike)) {
                        leftType = checkNonNullType(leftType, left);
                        rightType = checkNonNullType(rightType, right);
                    }

                    let resultType: Type | undefined;
                    if (isTypeAssignableToKind(leftType, TypeFlags.NumberLike, /*strict*/ true) && isTypeAssignableToKind(rightType, TypeFlags.NumberLike, /*strict*/ true)) {
                        // Operands of an enum type are treated as having the primitive type Number.
                        // If both operands are of the Number primitive type, the result is of the Number primitive type.
                        resultType = numberType;
                    }
                    else if (isTypeAssignableToKind(leftType, TypeFlags.StringLike, /*strict*/ true) || isTypeAssignableToKind(rightType, TypeFlags.StringLike, /*strict*/ true)) {
                            // If one or both operands are of the String primitive type, the result is of the String primitive type.
                            resultType = stringType;
                    }
                    else if (isTypeAny(leftType) || isTypeAny(rightType)) {
                        // Otherwise, the result is of type Any.
                        // NOTE: unknown type here denotes error type. Old compiler treated this case as any type so do we.
                        resultType = leftType === errorType || rightType === errorType ? errorType : anyType;
                    }

                    // Symbols are not allowed at all in arithmetic expressions
                    if (resultType && !checkForDisallowedESSymbolOperand(operator)) {
                        return resultType;
                    }

                    if (!resultType) {
                        reportOperatorError();
                        return anyType;
                    }

                    if (operator === SyntaxKind.PlusEqualsToken) {
                        checkAssignmentOperator(resultType);
                    }
                    return resultType;
                case SyntaxKind.LessThanToken:
                case SyntaxKind.GreaterThanToken:
                case SyntaxKind.LessThanEqualsToken:
                case SyntaxKind.GreaterThanEqualsToken:
                    if (checkForDisallowedESSymbolOperand(operator)) {
                        leftType = getBaseTypeOfLiteralType(checkNonNullType(leftType, left));
                        rightType = getBaseTypeOfLiteralType(checkNonNullType(rightType, right));
                        if (!isTypeComparableTo(leftType, rightType) && !isTypeComparableTo(rightType, leftType)) {
                            reportOperatorError();
                        }
                    }
                    return booleanType;
                case SyntaxKind.EqualsEqualsToken:
                case SyntaxKind.ExclamationEqualsToken:
                case SyntaxKind.EqualsEqualsEqualsToken:
                case SyntaxKind.ExclamationEqualsEqualsToken:
                    const leftIsLiteral = isLiteralType(leftType);
                    const rightIsLiteral = isLiteralType(rightType);
                    if (!leftIsLiteral || !rightIsLiteral) {
                        leftType = leftIsLiteral ? getBaseTypeOfLiteralType(leftType) : leftType;
                        rightType = rightIsLiteral ? getBaseTypeOfLiteralType(rightType) : rightType;
                    }
                    if (!isTypeEqualityComparableTo(leftType, rightType) && !isTypeEqualityComparableTo(rightType, leftType)) {
                        reportOperatorError();
                    }
                    return booleanType;
                case SyntaxKind.InstanceOfKeyword:
                    return checkInstanceOfExpression(left, right, leftType, rightType);
                case SyntaxKind.InKeyword:
                    return checkInExpression(left, right, leftType, rightType);
                case SyntaxKind.AmpersandAmpersandToken:
                    return getTypeFacts(leftType) & TypeFacts.Truthy ?
                        getUnionType([extractDefinitelyFalsyTypes(strictNullChecks ? leftType : getBaseTypeOfLiteralType(rightType)), rightType]) :
                        leftType;
                case SyntaxKind.BarBarToken:
                    return getTypeFacts(leftType) & TypeFacts.Falsy ?
                        getUnionType([removeDefinitelyFalsyTypes(leftType), rightType], UnionReduction.Subtype) :
                        leftType;
                case SyntaxKind.EqualsToken:
                    const declKind = isBinaryExpression(left.parent) ? getAssignmentDeclarationKind(left.parent) : AssignmentDeclarationKind.None;
                    checkAssignmentDeclaration(declKind, right);
                    if (isAssignmentDeclaration(declKind)) {
                        return leftType;
                    }
                    else {
                        checkAssignmentOperator(rightType);
                        return getRegularTypeOfObjectLiteral(rightType);
                    }
                case SyntaxKind.CommaToken:
                    if (!compilerOptions.allowUnreachableCode && isSideEffectFree(left) && !isEvalNode(right)) {
                        error(left, Diagnostics.Left_side_of_comma_operator_is_unused_and_has_no_side_effects);
                    }
                    return rightType;

                default:
                    return Debug.fail();
            }

            function checkAssignmentDeclaration(kind: AssignmentDeclarationKind, right: Expression) {
                if (kind === AssignmentDeclarationKind.ModuleExports) {
                    const rightType = checkExpression(right, checkMode);
                    for (const prop of getPropertiesOfObjectType(rightType)) {
                        const propType = getTypeOfSymbol(prop);
                        if (propType.symbol && propType.symbol.flags & SymbolFlags.Class) {
                            const name = prop.escapedName;
                            const symbol = resolveName(prop.valueDeclaration, name, SymbolFlags.Type, undefined, name, /*isUse*/ false);
                            if (symbol && symbol.declarations.some(isJSDocTypedefTag)) {
                                grammarErrorOnNode(symbol.declarations[0], Diagnostics.Duplicate_identifier_0, unescapeLeadingUnderscores(name));
                                return grammarErrorOnNode(prop.valueDeclaration, Diagnostics.Duplicate_identifier_0, unescapeLeadingUnderscores(name));
                            }
                        }
                    }
                }
            }

            function isEvalNode(node: Expression) {
                return node.kind === SyntaxKind.Identifier && (node as Identifier).escapedText === "eval";
            }

            // Return true if there was no error, false if there was an error.
            function checkForDisallowedESSymbolOperand(operator: SyntaxKind): boolean {
                const offendingSymbolOperand =
                    maybeTypeOfKind(leftType, TypeFlags.ESSymbolLike) ? left :
                        maybeTypeOfKind(rightType, TypeFlags.ESSymbolLike) ? right :
                            undefined;
                if (offendingSymbolOperand) {
                    error(offendingSymbolOperand, Diagnostics.The_0_operator_cannot_be_applied_to_type_symbol, tokenToString(operator));
                    return false;
                }

                return true;
            }

            function getSuggestedBooleanOperator(operator: SyntaxKind): SyntaxKind | undefined {
                switch (operator) {
                    case SyntaxKind.BarToken:
                    case SyntaxKind.BarEqualsToken:
                        return SyntaxKind.BarBarToken;
                    case SyntaxKind.CaretToken:
                    case SyntaxKind.CaretEqualsToken:
                        return SyntaxKind.ExclamationEqualsEqualsToken;
                    case SyntaxKind.AmpersandToken:
                    case SyntaxKind.AmpersandEqualsToken:
                        return SyntaxKind.AmpersandAmpersandToken;
                    default:
                        return undefined;
                }
            }

            function checkAssignmentOperator(valueType: Type): void {
                if (produceDiagnostics && isAssignmentOperator(operator)) {
                    // TypeScript 1.0 spec (April 2014): 4.17
                    // An assignment of the form
                    //    VarExpr = ValueExpr
                    // requires VarExpr to be classified as a reference
                    // A compound assignment furthermore requires VarExpr to be classified as a reference (section 4.1)
                    // and the type of the non-compound operation to be assignable to the type of VarExpr.

                    if (checkReferenceExpression(left, Diagnostics.The_left_hand_side_of_an_assignment_expression_must_be_a_variable_or_a_property_access)
                        && (!isIdentifier(left) || unescapeLeadingUnderscores(left.escapedText) !== "exports")) {
                        // to avoid cascading errors check assignability only if 'isReference' check succeeded and no errors were reported
                        checkTypeAssignableToAndOptionallyElaborate(valueType, leftType, left, right);
                    }
                }
            }

            function isAssignmentDeclaration(kind: AssignmentDeclarationKind) {
                switch (kind) {
                    case AssignmentDeclarationKind.ModuleExports:
                        return true;
                    case AssignmentDeclarationKind.ExportsProperty:
                    case AssignmentDeclarationKind.Property:
                    case AssignmentDeclarationKind.Prototype:
                    case AssignmentDeclarationKind.PrototypeProperty:
                    case AssignmentDeclarationKind.ThisProperty:
                        const symbol = getSymbolOfNode(left);
                        const init = getAssignedExpandoInitializer(right);
                        return init && isObjectLiteralExpression(init) &&
                            symbol && hasEntries(symbol.exports);
                    default:
                        return false;
                }
            }

            function reportOperatorError() {
                const leftStr = typeToString(leftType);
                const rightStr = typeToString(rightType);
                const errNode = errorNode || operatorToken;
                if (!tryGiveBetterPrimaryError(errNode, leftStr, rightStr)) {
                    error(
                        errNode,
                        Diagnostics.Operator_0_cannot_be_applied_to_types_1_and_2,
                        tokenToString(operatorToken.kind),
                        leftStr,
                        rightStr,
                    );
                }
            }

            function tryGiveBetterPrimaryError(errNode: Node, leftStr: string, rightStr: string) {
                switch (operatorToken.kind) {
                    case SyntaxKind.EqualsEqualsEqualsToken:
                    case SyntaxKind.EqualsEqualsToken:
                        return error(errNode, Diagnostics.This_condition_will_always_return_0_since_the_types_1_and_2_have_no_overlap, "false", leftStr, rightStr);
                    case SyntaxKind.ExclamationEqualsEqualsToken:
                    case SyntaxKind.ExclamationEqualsToken:
                        return error(errNode, Diagnostics.This_condition_will_always_return_0_since_the_types_1_and_2_have_no_overlap, "true", leftStr, rightStr);
                    }
                return undefined;
            }
        }

        function isYieldExpressionInClass(node: YieldExpression): boolean {
            let current: Node = node;
            let parent = node.parent;
            while (parent) {
                if (isFunctionLike(parent) && current === (<FunctionLikeDeclaration>parent).body) {
                    return false;
                }
                else if (isClassLike(current)) {
                    return true;
                }

                current = parent;
                parent = parent.parent;
            }

            return false;
        }

        function checkYieldExpression(node: YieldExpression): Type {
            // Grammar checking
            if (produceDiagnostics) {
                if (!(node.flags & NodeFlags.YieldContext) || isYieldExpressionInClass(node)) {
                    grammarErrorOnFirstToken(node, Diagnostics.A_yield_expression_is_only_allowed_in_a_generator_body);
                }

                if (isInParameterInitializerBeforeContainingFunction(node)) {
                    error(node, Diagnostics.yield_expressions_cannot_be_used_in_a_parameter_initializer);
                }
            }

            const func = getContainingFunction(node);
            if (!func) return anyType;
            const functionFlags = getFunctionFlags(func);

            if (!(functionFlags & FunctionFlags.Generator)) {
                // If the user's code is syntactically correct, the func should always have a star. After all, we are in a yield context.
                return anyType;
            }

            if (node.asteriskToken) {
                // Async generator functions prior to ESNext require the __await, __asyncDelegator,
                // and __asyncValues helpers
                if ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.AsyncGenerator &&
                    languageVersion < ScriptTarget.ESNext) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.AsyncDelegatorIncludes);
                }

                // Generator functions prior to ES2015 require the __values helper
                if ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.Generator &&
                    languageVersion < ScriptTarget.ES2015 && compilerOptions.downlevelIteration) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.Values);
                }
            }

            const isAsync = (functionFlags & FunctionFlags.Async) !== 0;
            const yieldedType = getYieldedTypeOfYieldExpression(node, isAsync)!; // TODO: GH#18217
            // There is no point in doing an assignability check if the function
            // has no explicit return type because the return type is directly computed
            // from the yield expressions.
            const returnType = getReturnTypeFromAnnotation(func);
            if (returnType) {
                const signatureElementType = getIteratedTypeOfGenerator(returnType, isAsync) || anyType;
                checkTypeAssignableToAndOptionallyElaborate(yieldedType, signatureElementType, node.expression || node, node.expression);
            }

            // Both yield and yield* expressions have type 'any'
            return anyType;
        }

        function checkConditionalExpression(node: ConditionalExpression, checkMode?: CheckMode): Type {
            checkTruthinessExpression(node.condition);
            const type1 = checkExpression(node.whenTrue, checkMode);
            const type2 = checkExpression(node.whenFalse, checkMode);
            return getUnionType([type1, type2], UnionReduction.Subtype);
        }

        function checkTemplateExpression(node: TemplateExpression): Type {
            // We just want to check each expressions, but we are unconcerned with
            // the type of each expression, as any value may be coerced into a string.
            // It is worth asking whether this is what we really want though.
            // A place where we actually *are* concerned with the expressions' types are
            // in tagged templates.
            forEach(node.templateSpans, templateSpan => {
                if (maybeTypeOfKind(checkExpression(templateSpan.expression), TypeFlags.ESSymbolLike)) {
                    error(templateSpan.expression, Diagnostics.Implicit_conversion_of_a_symbol_to_a_string_will_fail_at_runtime_Consider_wrapping_this_expression_in_String);
                }
            });

            return stringType;
        }

        function getContextNode(node: Expression): Node {
            if (node.kind === SyntaxKind.JsxAttributes) {
                return node.parent.parent; // Needs to be the root JsxElement, so it encompasses the attributes _and_ the children (which are essentially part of the attributes)
            }
            return node;
        }

        function checkExpressionWithContextualType(node: Expression, contextualType: Type, contextualMapper: TypeMapper | undefined): Type {
            const context = getContextNode(node);
            const saveContextualType = context.contextualType;
            const saveContextualMapper = context.contextualMapper;
            context.contextualType = contextualType;
            context.contextualMapper = contextualMapper;
            const checkMode = contextualMapper === identityMapper ? CheckMode.SkipContextSensitive :
                contextualMapper ? CheckMode.Inferential : CheckMode.Contextual;
            const result = checkExpression(node, checkMode);
            context.contextualType = saveContextualType;
            context.contextualMapper = saveContextualMapper;
            return result;
        }

        function checkExpressionCached(node: Expression, checkMode?: CheckMode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                if (checkMode) {
                    return checkExpression(node, checkMode);
                }
                // When computing a type that we're going to cache, we need to ignore any ongoing control flow
                // analysis because variables may have transient types in indeterminable states. Moving flowLoopStart
                // to the top of the stack ensures all transient types are computed from a known point.
                const saveFlowLoopStart = flowLoopStart;
                flowLoopStart = flowLoopCount;
                links.resolvedType = checkExpression(node, checkMode);
                flowLoopStart = saveFlowLoopStart;
            }
            return links.resolvedType;
        }

        function isTypeAssertion(node: Expression) {
            node = skipParentheses(node);
            return node.kind === SyntaxKind.TypeAssertionExpression || node.kind === SyntaxKind.AsExpression;
        }

        function checkDeclarationInitializer(declaration: HasExpressionInitializer) {
            const initializer = getEffectiveInitializer(declaration)!;
            const type = getTypeOfExpression(initializer, /*cache*/ true);
            const widened = getCombinedNodeFlags(declaration) & NodeFlags.Const ||
                isDeclarationReadonly(declaration) ||
                isTypeAssertion(initializer) ? type : getWidenedLiteralType(type);
            if (isInJSFile(declaration)) {
                if (widened.flags & TypeFlags.Nullable) {
                    if (noImplicitAny) {
                        reportImplicitAnyError(declaration, anyType);
                    }
                    return anyType;
                }
                else if (isEmptyArrayLiteralType(widened)) {
                    if (noImplicitAny) {
                        reportImplicitAnyError(declaration, anyArrayType);
                    }
                    return anyArrayType;
                }
            }
            return widened;
        }

        function isLiteralOfContextualType(candidateType: Type, contextualType: Type | undefined): boolean {
            if (contextualType) {
                if (contextualType.flags & TypeFlags.UnionOrIntersection) {
                    const types = (<UnionType>contextualType).types;
                    return some(types, t => isLiteralOfContextualType(candidateType, t));
                }
                if (contextualType.flags & TypeFlags.InstantiableNonPrimitive) {
                    // If the contextual type is a type variable constrained to a primitive type, consider
                    // this a literal context for literals of that primitive type. For example, given a
                    // type parameter 'T extends string', infer string literal types for T.
                    const constraint = getBaseConstraintOfType(contextualType) || emptyObjectType;
                    return maybeTypeOfKind(constraint, TypeFlags.String) && maybeTypeOfKind(candidateType, TypeFlags.StringLiteral) ||
                        maybeTypeOfKind(constraint, TypeFlags.Number) && maybeTypeOfKind(candidateType, TypeFlags.NumberLiteral) ||
                        maybeTypeOfKind(constraint, TypeFlags.ESSymbol) && maybeTypeOfKind(candidateType, TypeFlags.UniqueESSymbol) ||
                        isLiteralOfContextualType(candidateType, constraint);
                }
                // If the contextual type is a literal of a particular primitive type, we consider this a
                // literal context for all literals of that primitive type.
                return !!(contextualType.flags & (TypeFlags.StringLiteral | TypeFlags.Index) && maybeTypeOfKind(candidateType, TypeFlags.StringLiteral) ||
                    contextualType.flags & TypeFlags.NumberLiteral && maybeTypeOfKind(candidateType, TypeFlags.NumberLiteral) ||
                    contextualType.flags & TypeFlags.BooleanLiteral && maybeTypeOfKind(candidateType, TypeFlags.BooleanLiteral) ||
                    contextualType.flags & TypeFlags.UniqueESSymbol && maybeTypeOfKind(candidateType, TypeFlags.UniqueESSymbol));
            }
            return false;
        }

        function checkExpressionForMutableLocation(node: Expression, checkMode: CheckMode | undefined, contextualType?: Type): Type {
            if (arguments.length === 2) {
                contextualType = getContextualType(node);
            }
            const type = checkExpression(node, checkMode);
            return isTypeAssertion(node) ? type :
                getWidenedLiteralLikeTypeForContextualType(type, contextualType);
        }

        function checkPropertyAssignment(node: PropertyAssignment, checkMode?: CheckMode): Type {
            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                checkComputedPropertyName(node.name);
            }

            return checkExpressionForMutableLocation(node.initializer, checkMode);
        }

        function checkObjectLiteralMethod(node: MethodDeclaration, checkMode?: CheckMode): Type {
            // Grammar checking
            checkGrammarMethod(node);

            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                checkComputedPropertyName(node.name);
            }

            const uninstantiatedType = checkFunctionExpressionOrObjectLiteralMethod(node, checkMode);
            return instantiateTypeWithSingleGenericCallSignature(node, uninstantiatedType, checkMode);
        }

        function instantiateTypeWithSingleGenericCallSignature(node: Expression | MethodDeclaration, type: Type, checkMode?: CheckMode) {
            if (checkMode === CheckMode.Inferential) {
                const signature = getSingleCallSignature(type);
                if (signature && signature.typeParameters) {
                    const contextualType = getApparentTypeOfContextualType(<Expression>node);
                    if (contextualType) {
                        const contextualSignature = getSingleCallSignature(getNonNullableType(contextualType));
                        if (contextualSignature && !contextualSignature.typeParameters) {
                            return getOrCreateTypeFromSignature(instantiateSignatureInContextOf(signature, contextualSignature, getContextualMapper(node)));
                        }
                    }
                }
            }

            return type;
        }

        /**
         * Returns the type of an expression. Unlike checkExpression, this function is simply concerned
         * with computing the type and may not fully check all contained sub-expressions for errors.
         * A cache argument of true indicates that if the function performs a full type check, it is ok
         * to cache the result.
         */
        function getTypeOfExpression(node: Expression, cache?: boolean) {
            const expr = skipParentheses(node);
            // Optimize for the common case of a call to a function with a single non-generic call
            // signature where we can just fetch the return type without checking the arguments.
            if (expr.kind === SyntaxKind.CallExpression && (<CallExpression>expr).expression.kind !== SyntaxKind.SuperKeyword && !isRequireCall(expr, /*checkArgumentIsStringLiteralLike*/ true) && !isSymbolOrSymbolForCall(expr)) {
                const funcType = checkNonNullExpression((<CallExpression>expr).expression);
                const signature = getSingleCallSignature(funcType);
                if (signature && !signature.typeParameters) {
                    return getReturnTypeOfSignature(signature);
                }
            }
            else if (expr.kind === SyntaxKind.TypeAssertionExpression || expr.kind === SyntaxKind.AsExpression) {
                return getTypeFromTypeNode((<TypeAssertion>expr).type);
            }
            // Otherwise simply call checkExpression. Ideally, the entire family of checkXXX functions
            // should have a parameter that indicates whether full error checking is required such that
            // we can perform the optimizations locally.
            return cache ? checkExpressionCached(node) : checkExpression(node);
        }

        /**
         * Returns the type of an expression. Unlike checkExpression, this function is simply concerned
         * with computing the type and may not fully check all contained sub-expressions for errors.
         * It is intended for uses where you know there is no contextual type,
         * and requesting the contextual type might cause a circularity or other bad behaviour.
         * It sets the contextual type of the node to any before calling getTypeOfExpression.
         */
        function getContextFreeTypeOfExpression(node: Expression) {
            const links = getNodeLinks(node);
            if (links.contextFreeType) {
                return links.contextFreeType;
            }
            const saveContextualType = node.contextualType;
            node.contextualType = anyType;
            const type = links.contextFreeType = checkExpression(node, CheckMode.SkipContextSensitive);
            node.contextualType = saveContextualType;
            return type;
        }

        // Checks an expression and returns its type. The contextualMapper parameter serves two purposes: When
        // contextualMapper is not undefined and not equal to the identityMapper function object it indicates that the
        // expression is being inferentially typed (section 4.15.2 in spec) and provides the type mapper to use in
        // conjunction with the generic contextual type. When contextualMapper is equal to the identityMapper function
        // object, it serves as an indicator that all contained function and arrow expressions should be considered to
        // have the wildcard function type; this form of type check is used during overload resolution to exclude
        // contextually typed function and arrow expressions in the initial phase.
        function checkExpression(node: Expression | QualifiedName, checkMode?: CheckMode): Type {
            let type: Type;
            if (node.kind === SyntaxKind.QualifiedName) {
                type = checkQualifiedName(<QualifiedName>node);
            }
            else {
                const uninstantiatedType = checkExpressionWorker(node, checkMode);
                type = instantiateTypeWithSingleGenericCallSignature(node, uninstantiatedType, checkMode);
            }

            if (isConstEnumObjectType(type)) {
                // enum object type for const enums are only permitted in:
                // - 'left' in property access
                // - 'object' in indexed access
                // - target in rhs of import statement
                const ok =
                    (node.parent.kind === SyntaxKind.PropertyAccessExpression && (<PropertyAccessExpression>node.parent).expression === node) ||
                    (node.parent.kind === SyntaxKind.ElementAccessExpression && (<ElementAccessExpression>node.parent).expression === node) ||
                    ((node.kind === SyntaxKind.Identifier || node.kind === SyntaxKind.QualifiedName) && isInRightSideOfImportOrExportAssignment(<Identifier>node) ||
                    (node.parent.kind === SyntaxKind.TypeQuery && (<TypeQueryNode>node.parent).exprName === node));

                if (!ok) {
                    error(node, Diagnostics.const_enums_can_only_be_used_in_property_or_index_access_expressions_or_the_right_hand_side_of_an_import_declaration_or_export_assignment_or_type_query);
                }
            }
            return type;
        }

        function checkParenthesizedExpression(node: ParenthesizedExpression, checkMode?: CheckMode): Type {
            const tag = isInJSFile(node) ? getJSDocTypeTag(node) : undefined;
            if (tag) {
                return checkAssertionWorker(tag, tag.typeExpression!.type, node.expression, checkMode);
            }
            return checkExpression(node.expression, checkMode);
        }

        function checkExpressionWorker(node: Expression, checkMode: CheckMode | undefined): Type {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                    return checkIdentifier(<Identifier>node);
                case SyntaxKind.ThisKeyword:
                    return checkThisExpression(node);
                case SyntaxKind.SuperKeyword:
                    return checkSuperExpression(node);
                case SyntaxKind.NullKeyword:
                    return nullWideningType;
                case SyntaxKind.NoSubstitutionTemplateLiteral:
                case SyntaxKind.StringLiteral:
                    return getFreshTypeOfLiteralType(getLiteralType((node as StringLiteralLike).text));
                case SyntaxKind.NumericLiteral:
                    checkGrammarNumericLiteral(node as NumericLiteral);
                    return getFreshTypeOfLiteralType(getLiteralType(+(node as NumericLiteral).text));
                case SyntaxKind.TrueKeyword:
                    return trueType;
                case SyntaxKind.FalseKeyword:
                    return falseType;
                case SyntaxKind.TemplateExpression:
                    return checkTemplateExpression(<TemplateExpression>node);
                case SyntaxKind.RegularExpressionLiteral:
                    return globalRegExpType;
                case SyntaxKind.ArrayLiteralExpression:
                    return checkArrayLiteral(<ArrayLiteralExpression>node, checkMode);
                case SyntaxKind.ObjectLiteralExpression:
                    return checkObjectLiteral(<ObjectLiteralExpression>node, checkMode);
                case SyntaxKind.PropertyAccessExpression:
                    return checkPropertyAccessExpression(<PropertyAccessExpression>node);
                case SyntaxKind.ElementAccessExpression:
                    return checkIndexedAccess(<ElementAccessExpression>node);
                case SyntaxKind.CallExpression:
                    if ((<CallExpression>node).expression.kind === SyntaxKind.ImportKeyword) {
                        return checkImportCallExpression(<ImportCall>node);
                    }
                    /* falls through */
                case SyntaxKind.NewExpression:
                    return checkCallExpression(<CallExpression>node);
                case SyntaxKind.TaggedTemplateExpression:
                    return checkTaggedTemplateExpression(<TaggedTemplateExpression>node);
                case SyntaxKind.ParenthesizedExpression:
                    return checkParenthesizedExpression(<ParenthesizedExpression>node, checkMode);
                case SyntaxKind.ClassExpression:
                    return checkClassExpression(<ClassExpression>node);
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    return checkFunctionExpressionOrObjectLiteralMethod(<FunctionExpression>node, checkMode);
                case SyntaxKind.TypeOfExpression:
                    return checkTypeOfExpression(<TypeOfExpression>node);
                case SyntaxKind.TypeAssertionExpression:
                case SyntaxKind.AsExpression:
                    return checkAssertion(<AssertionExpression>node);
                case SyntaxKind.NonNullExpression:
                    return checkNonNullAssertion(<NonNullExpression>node);
                case SyntaxKind.MetaProperty:
                    return checkMetaProperty(<MetaProperty>node);
                case SyntaxKind.DeleteExpression:
                    return checkDeleteExpression(<DeleteExpression>node);
                case SyntaxKind.VoidExpression:
                    return checkVoidExpression(<VoidExpression>node);
                case SyntaxKind.AwaitExpression:
                    return checkAwaitExpression(<AwaitExpression>node);
                case SyntaxKind.PrefixUnaryExpression:
                    return checkPrefixUnaryExpression(<PrefixUnaryExpression>node);
                case SyntaxKind.PostfixUnaryExpression:
                    return checkPostfixUnaryExpression(<PostfixUnaryExpression>node);
                case SyntaxKind.BinaryExpression:
                    return checkBinaryExpression(<BinaryExpression>node, checkMode);
                case SyntaxKind.ConditionalExpression:
                    return checkConditionalExpression(<ConditionalExpression>node, checkMode);
                case SyntaxKind.SpreadElement:
                    return checkSpreadExpression(<SpreadElement>node, checkMode);
                case SyntaxKind.OmittedExpression:
                    return undefinedWideningType;
                case SyntaxKind.YieldExpression:
                    return checkYieldExpression(<YieldExpression>node);
                case SyntaxKind.SyntheticExpression:
                    return (<SyntheticExpression>node).type;
                case SyntaxKind.JsxExpression:
                    return checkJsxExpression(<JsxExpression>node, checkMode);
                case SyntaxKind.JsxElement:
                    return checkJsxElement(<JsxElement>node, checkMode);
                case SyntaxKind.JsxSelfClosingElement:
                    return checkJsxSelfClosingElement(<JsxSelfClosingElement>node, checkMode);
                case SyntaxKind.JsxFragment:
                    return checkJsxFragment(<JsxFragment>node, checkMode);
                case SyntaxKind.JsxAttributes:
                    return checkJsxAttributes(<JsxAttributes>node, checkMode);
                case SyntaxKind.JsxOpeningElement:
                    Debug.fail("Shouldn't ever directly check a JsxOpeningElement");
            }
            return errorType;
        }

        // DECLARATION AND STATEMENT TYPE CHECKING

        function checkTypeParameter(node: TypeParameterDeclaration) {
            // Grammar Checking
            if (node.expression) {
                grammarErrorOnFirstToken(node.expression, Diagnostics.Type_expected);
            }

            checkSourceElement(node.constraint);
            checkSourceElement(node.default);
            const typeParameter = getDeclaredTypeOfTypeParameter(getSymbolOfNode(node));
            if (!hasNonCircularBaseConstraint(typeParameter)) {
                error(getEffectiveConstraintOfTypeParameter(node), Diagnostics.Type_parameter_0_has_a_circular_constraint, typeToString(typeParameter));
            }
            if (!hasNonCircularTypeParameterDefault(typeParameter)) {
                error(node.default, Diagnostics.Type_parameter_0_has_a_circular_default, typeToString(typeParameter));
            }
            const constraintType = getConstraintOfTypeParameter(typeParameter);
            const defaultType = getDefaultFromTypeParameter(typeParameter);
            if (constraintType && defaultType) {
                checkTypeAssignableTo(defaultType, getTypeWithThisArgument(constraintType, defaultType), node.default, Diagnostics.Type_0_does_not_satisfy_the_constraint_1);
            }
            if (produceDiagnostics) {
                checkTypeNameIsReserved(node.name, Diagnostics.Type_parameter_name_cannot_be_0);
            }
        }

        function checkParameter(node: ParameterDeclaration) {
            // Grammar checking
            // It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the
            // Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict code
            // or if its FunctionBody is strict code(11.1.5).
            checkGrammarDecoratorsAndModifiers(node);

            checkVariableLikeDeclaration(node);
            const func = getContainingFunction(node)!;
            if (hasModifier(node, ModifierFlags.ParameterPropertyModifier)) {
                if (!(func.kind === SyntaxKind.Constructor && nodeIsPresent(func.body))) {
                    error(node, Diagnostics.A_parameter_property_is_only_allowed_in_a_constructor_implementation);
                }
            }
            if (node.questionToken && isBindingPattern(node.name) && (func as FunctionLikeDeclaration).body) {
                error(node, Diagnostics.A_binding_pattern_parameter_cannot_be_optional_in_an_implementation_signature);
            }
            if (node.name && isIdentifier(node.name) && (node.name.escapedText === "this" || node.name.escapedText === "new")) {
                if (func.parameters.indexOf(node) !== 0) {
                    error(node, Diagnostics.A_0_parameter_must_be_the_first_parameter, node.name.escapedText as string);
                }
                if (func.kind === SyntaxKind.Constructor || func.kind === SyntaxKind.ConstructSignature || func.kind === SyntaxKind.ConstructorType) {
                    error(node, Diagnostics.A_constructor_cannot_have_a_this_parameter);
                }
                if (func.kind === SyntaxKind.ArrowFunction) {
                    error(node, Diagnostics.An_arrow_function_cannot_have_a_this_parameter);
                }
            }

            // Only check rest parameter type if it's not a binding pattern. Since binding patterns are
            // not allowed in a rest parameter, we already have an error from checkGrammarParameterList.
            if (node.dotDotDotToken && !isBindingPattern(node.name) && !isTypeAssignableTo(getTypeOfSymbol(node.symbol), anyArrayType)) {
                error(node, Diagnostics.A_rest_parameter_must_be_of_an_array_type);
            }
        }

        function checkTypePredicate(node: TypePredicateNode): void {
            const parent = getTypePredicateParent(node);
            if (!parent) {
                // The parent must not be valid.
                error(node, Diagnostics.A_type_predicate_is_only_allowed_in_return_type_position_for_functions_and_methods);
                return;
            }

            const typePredicate = getTypePredicateOfSignature(getSignatureFromDeclaration(parent));
            if (!typePredicate) {
                return;
            }

            checkSourceElement(node.type);

            const { parameterName } = node;
            if (isThisTypePredicate(typePredicate)) {
                getTypeFromThisTypeNode(parameterName as ThisTypeNode);
            }
            else {
                if (typePredicate.parameterIndex >= 0) {
                    if (parent.parameters[typePredicate.parameterIndex].dotDotDotToken) {
                        error(parameterName,
                            Diagnostics.A_type_predicate_cannot_reference_a_rest_parameter);
                    }
                    else {
                        const leadingError = () => chainDiagnosticMessages(/*details*/ undefined, Diagnostics.A_type_predicate_s_type_must_be_assignable_to_its_parameter_s_type);
                        checkTypeAssignableTo(typePredicate.type,
                            getTypeOfNode(parent.parameters[typePredicate.parameterIndex]),
                            node.type,
                            /*headMessage*/ undefined,
                            leadingError);
                    }
                }
                else if (parameterName) {
                    let hasReportedError = false;
                    for (const { name } of parent.parameters) {
                        if (isBindingPattern(name) &&
                                checkIfTypePredicateVariableIsDeclaredInBindingPattern(name, parameterName, typePredicate.parameterName)) {
                            hasReportedError = true;
                            break;
                        }
                    }
                    if (!hasReportedError) {
                        error(node.parameterName, Diagnostics.Cannot_find_parameter_0, typePredicate.parameterName);
                    }
                }
            }
        }

        function getTypePredicateParent(node: Node): SignatureDeclaration | undefined {
            switch (node.parent.kind) {
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.CallSignature:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.FunctionType:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                    const parent = <SignatureDeclaration>node.parent;
                    if (node === parent.type) {
                        return parent;
                    }
            }
        }

        function checkIfTypePredicateVariableIsDeclaredInBindingPattern(
            pattern: BindingPattern,
            predicateVariableNode: Node,
            predicateVariableName: string) {
            for (const element of pattern.elements) {
                if (isOmittedExpression(element)) {
                    continue;
                }

                const name = element.name;
                if (name.kind === SyntaxKind.Identifier && name.escapedText === predicateVariableName) {
                    error(predicateVariableNode,
                        Diagnostics.A_type_predicate_cannot_reference_element_0_in_a_binding_pattern,
                        predicateVariableName);
                    return true;
                }
                else if (name.kind === SyntaxKind.ArrayBindingPattern || name.kind === SyntaxKind.ObjectBindingPattern) {
                    if (checkIfTypePredicateVariableIsDeclaredInBindingPattern(
                        name,
                        predicateVariableNode,
                        predicateVariableName)) {
                        return true;
                    }
                }
            }
        }

        function checkSignatureDeclaration(node: SignatureDeclaration) {
            // Grammar checking
            if (node.kind === SyntaxKind.IndexSignature) {
                checkGrammarIndexSignature(<SignatureDeclaration>node);
            }
            // TODO (yuisu): Remove this check in else-if when SyntaxKind.Construct is moved and ambient context is handled
            else if (node.kind === SyntaxKind.FunctionType || node.kind === SyntaxKind.FunctionDeclaration || node.kind === SyntaxKind.ConstructorType ||
                node.kind === SyntaxKind.CallSignature || node.kind === SyntaxKind.Constructor ||
                node.kind === SyntaxKind.ConstructSignature) {
                checkGrammarFunctionLikeDeclaration(<FunctionLikeDeclaration>node);
            }

            const functionFlags = getFunctionFlags(<FunctionLikeDeclaration>node);
            if (!(functionFlags & FunctionFlags.Invalid)) {
                // Async generators prior to ESNext require the __await and __asyncGenerator helpers
                if ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.AsyncGenerator && languageVersion < ScriptTarget.ESNext) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.AsyncGeneratorIncludes);
                }

                // Async functions prior to ES2017 require the __awaiter helper
                if ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.Async && languageVersion < ScriptTarget.ES2017) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.Awaiter);
                }

                // Generator functions, Async functions, and Async Generator functions prior to
                // ES2015 require the __generator helper
                if ((functionFlags & FunctionFlags.AsyncGenerator) !== FunctionFlags.Normal && languageVersion < ScriptTarget.ES2015) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.Generator);
                }
            }

            checkTypeParameters(node.typeParameters);

            forEach(node.parameters, checkParameter);

            // TODO(rbuckton): Should we start checking JSDoc types?
            if (node.type) {
                checkSourceElement(node.type);
            }

            if (produceDiagnostics) {
                checkCollisionWithArgumentsInGeneratedCode(node);
                const returnTypeNode = getEffectiveReturnTypeNode(node);
                if (noImplicitAny && !returnTypeNode) {
                    switch (node.kind) {
                        case SyntaxKind.ConstructSignature:
                            error(node, Diagnostics.Construct_signature_which_lacks_return_type_annotation_implicitly_has_an_any_return_type);
                            break;
                        case SyntaxKind.CallSignature:
                            error(node, Diagnostics.Call_signature_which_lacks_return_type_annotation_implicitly_has_an_any_return_type);
                            break;
                    }
                }

                if (returnTypeNode) {
                    const functionFlags = getFunctionFlags(<FunctionDeclaration>node);
                    if ((functionFlags & (FunctionFlags.Invalid | FunctionFlags.Generator)) === FunctionFlags.Generator) {
                        const returnType = getTypeFromTypeNode(returnTypeNode);
                        if (returnType === voidType) {
                            error(returnTypeNode, Diagnostics.A_generator_cannot_have_a_void_type_annotation);
                        }
                        else {
                            const generatorElementType = getIteratedTypeOfGenerator(returnType, (functionFlags & FunctionFlags.Async) !== 0) || anyType;
                            const iterableIteratorInstantiation = functionFlags & FunctionFlags.Async
                                ? createAsyncIterableIteratorType(generatorElementType) // AsyncGenerator function
                                : createIterableIteratorType(generatorElementType); // Generator function

                            // Naively, one could check that IterableIterator<any> is assignable to the return type annotation.
                            // However, that would not catch the error in the following case.
                            //
                            //    interface BadGenerator extends Iterable<number>, Iterator<string> { }
                            //    function* g(): BadGenerator { } // Iterable and Iterator have different types!
                            //
                            checkTypeAssignableTo(iterableIteratorInstantiation, returnType, returnTypeNode);
                        }
                    }
                    else if ((functionFlags & FunctionFlags.AsyncGenerator) === FunctionFlags.Async) {
                        checkAsyncFunctionReturnType(<FunctionLikeDeclaration>node, returnTypeNode);
                    }
                }
                if (node.kind !== SyntaxKind.IndexSignature && node.kind !== SyntaxKind.JSDocFunctionType) {
                    registerForUnusedIdentifiersCheck(node);
                }
            }
        }

        function checkClassForDuplicateDeclarations(node: ClassLikeDeclaration) {
            const enum Declaration {
                Getter = 1,
                Setter = 2,
                Method = 4,
                Property = Getter | Setter
            }

            const instanceNames = createUnderscoreEscapedMap<Declaration>();
            const staticNames = createUnderscoreEscapedMap<Declaration>();
            for (const member of node.members) {
                if (member.kind === SyntaxKind.Constructor) {
                    for (const param of (member as ConstructorDeclaration).parameters) {
                        if (isParameterPropertyDeclaration(param) && !isBindingPattern(param.name)) {
                            addName(instanceNames, param.name, param.name.escapedText, Declaration.Property);
                        }
                    }
                }
                else {
                    const isStatic = hasModifier(member, ModifierFlags.Static);
                    const names = isStatic ? staticNames : instanceNames;

                    const name = member.name;
                    const memberName = name && getPropertyNameForPropertyNameNode(name);
                    if (name && memberName) {
                        switch (member.kind) {
                            case SyntaxKind.GetAccessor:
                                addName(names, name, memberName, Declaration.Getter);
                                break;

                            case SyntaxKind.SetAccessor:
                                addName(names, name, memberName, Declaration.Setter);
                                break;

                            case SyntaxKind.PropertyDeclaration:
                                addName(names, name, memberName, Declaration.Property);
                                break;

                            case SyntaxKind.MethodDeclaration:
                                addName(names, name, memberName, Declaration.Method);
                                break;
                        }
                    }
                }
            }

            function addName(names: UnderscoreEscapedMap<Declaration>, location: Node, name: __String, meaning: Declaration) {
                const prev = names.get(name);
                if (prev) {
                    if (prev & Declaration.Method) {
                        if (meaning !== Declaration.Method) {
                            error(location, Diagnostics.Duplicate_identifier_0, getTextOfNode(location));
                        }
                    }
                    else if (prev & meaning) {
                        error(location, Diagnostics.Duplicate_identifier_0, getTextOfNode(location));
                    }
                    else {
                        names.set(name, prev | meaning);
                    }
                }
                else {
                    names.set(name, meaning);
                }
            }
        }

        /**
         * Static members being set on a constructor function may conflict with built-in properties
         * of Function. Esp. in ECMAScript 5 there are non-configurable and non-writable
         * built-in properties. This check issues a transpile error when a class has a static
         * member with the same name as a non-writable built-in property.
         *
         * @see http://www.ecma-international.org/ecma-262/5.1/#sec-15.3.3
         * @see http://www.ecma-international.org/ecma-262/5.1/#sec-15.3.5
         * @see http://www.ecma-international.org/ecma-262/6.0/#sec-properties-of-the-function-constructor
         * @see http://www.ecma-international.org/ecma-262/6.0/#sec-function-instances
         */
        function checkClassForStaticPropertyNameConflicts(node: ClassLikeDeclaration) {
            for (const member of node.members) {
                const memberNameNode = member.name;
                const isStatic = hasModifier(member, ModifierFlags.Static);
                if (isStatic && memberNameNode) {
                    const memberName = getPropertyNameForPropertyNameNode(memberNameNode);
                    switch (memberName) {
                        case "name":
                        case "length":
                        case "caller":
                        case "arguments":
                        case "prototype":
                            const message = Diagnostics.Static_property_0_conflicts_with_built_in_property_Function_0_of_constructor_function_1;
                            const className = getNameOfSymbolAsWritten(getSymbolOfNode(node));
                            error(memberNameNode, message, memberName, className);
                            break;
                    }
                }
            }
        }

        function checkObjectTypeForDuplicateDeclarations(node: TypeLiteralNode | InterfaceDeclaration) {
            const names = createMap<boolean>();
            for (const member of node.members) {
                if (member.kind === SyntaxKind.PropertySignature) {
                    let memberName: string;
                    const name = member.name!;
                    switch (name.kind) {
                        case SyntaxKind.StringLiteral:
                        case SyntaxKind.NumericLiteral:
                            memberName = name.text;
                            break;
                        case SyntaxKind.Identifier:
                            memberName = idText(name);
                            break;
                        default:
                            continue;
                    }

                    if (names.get(memberName)) {
                        error(getNameOfDeclaration(member.symbol.valueDeclaration), Diagnostics.Duplicate_identifier_0, memberName);
                        error(member.name, Diagnostics.Duplicate_identifier_0, memberName);
                    }
                    else {
                        names.set(memberName, true);
                    }
                }
            }
        }

        function checkTypeForDuplicateIndexSignatures(node: Node) {
            if (node.kind === SyntaxKind.InterfaceDeclaration) {
                const nodeSymbol = getSymbolOfNode(node as InterfaceDeclaration);
                // in case of merging interface declaration it is possible that we'll enter this check procedure several times for every declaration
                // to prevent this run check only for the first declaration of a given kind
                if (nodeSymbol.declarations.length > 0 && nodeSymbol.declarations[0] !== node) {
                    return;
                }
            }

            // TypeScript 1.0 spec (April 2014)
            // 3.7.4: An object type can contain at most one string index signature and one numeric index signature.
            // 8.5: A class declaration can have at most one string index member declaration and one numeric index member declaration
            const indexSymbol = getIndexSymbol(getSymbolOfNode(node)!);
            if (indexSymbol) {
                let seenNumericIndexer = false;
                let seenStringIndexer = false;
                for (const decl of indexSymbol.declarations) {
                    const declaration = <SignatureDeclaration>decl;
                    if (declaration.parameters.length === 1 && declaration.parameters[0].type) {
                        switch (declaration.parameters[0].type.kind) {
                            case SyntaxKind.StringKeyword:
                                if (!seenStringIndexer) {
                                    seenStringIndexer = true;
                                }
                                else {
                                    error(declaration, Diagnostics.Duplicate_string_index_signature);
                                }
                                break;
                            case SyntaxKind.NumberKeyword:
                                if (!seenNumericIndexer) {
                                    seenNumericIndexer = true;
                                }
                                else {
                                    error(declaration, Diagnostics.Duplicate_number_index_signature);
                                }
                                break;
                        }
                    }
                }
            }
        }

        function checkPropertyDeclaration(node: PropertyDeclaration | PropertySignature) {
            // Grammar checking
            if (!checkGrammarDecoratorsAndModifiers(node) && !checkGrammarProperty(node)) checkGrammarComputedPropertyName(node.name);
            checkVariableLikeDeclaration(node);
        }

        function checkMethodDeclaration(node: MethodDeclaration | MethodSignature) {
            // Grammar checking
            if (!checkGrammarMethod(node)) checkGrammarComputedPropertyName(node.name);

            // Grammar checking for modifiers is done inside the function checkGrammarFunctionLikeDeclaration
            checkFunctionOrMethodDeclaration(node);

            // Abstract methods cannot have an implementation.
            // Extra checks are to avoid reporting multiple errors relating to the "abstractness" of the node.
            if (hasModifier(node, ModifierFlags.Abstract) && node.kind === SyntaxKind.MethodDeclaration && node.body) {
                error(node, Diagnostics.Method_0_cannot_have_an_implementation_because_it_is_marked_abstract, declarationNameToString(node.name));
            }
        }

        function checkConstructorDeclaration(node: ConstructorDeclaration) {
            // Grammar check on signature of constructor and modifier of the constructor is done in checkSignatureDeclaration function.
            checkSignatureDeclaration(node);
            // Grammar check for checking only related to constructorDeclaration
            if (!checkGrammarConstructorTypeParameters(node)) checkGrammarConstructorTypeAnnotation(node);

            checkSourceElement(node.body);

            const symbol = getSymbolOfNode(node);
            const firstDeclaration = getDeclarationOfKind(symbol, node.kind);

            // Only type check the symbol once
            if (node === firstDeclaration) {
                checkFunctionOrConstructorSymbol(symbol);
            }

            // exit early in the case of signature - super checks are not relevant to them
            if (nodeIsMissing(node.body)) {
                return;
            }

            if (!produceDiagnostics) {
                return;
            }

            function isInstancePropertyWithInitializer(n: Node): boolean {
                return n.kind === SyntaxKind.PropertyDeclaration &&
                    !hasModifier(n, ModifierFlags.Static) &&
                    !!(<PropertyDeclaration>n).initializer;
            }

            // TS 1.0 spec (April 2014): 8.3.2
            // Constructors of classes with no extends clause may not contain super calls, whereas
            // constructors of derived classes must contain at least one super call somewhere in their function body.
            const containingClassDecl = <ClassDeclaration>node.parent;
            if (getEffectiveBaseTypeNode(containingClassDecl)) {
                captureLexicalThis(node.parent, containingClassDecl);
                const classExtendsNull = classDeclarationExtendsNull(containingClassDecl);
                const superCall = getSuperCallInConstructor(node);
                if (superCall) {
                    if (classExtendsNull) {
                        error(superCall, Diagnostics.A_constructor_cannot_contain_a_super_call_when_its_class_extends_null);
                    }

                    // The first statement in the body of a constructor (excluding prologue directives) must be a super call
                    // if both of the following are true:
                    // - The containing class is a derived class.
                    // - The constructor declares parameter properties
                    //   or the containing class declares instance member variables with initializers.
                    const superCallShouldBeFirst =
                        some((<ClassDeclaration>node.parent).members, isInstancePropertyWithInitializer) ||
                        some(node.parameters, p => hasModifier(p, ModifierFlags.ParameterPropertyModifier));

                    // Skip past any prologue directives to find the first statement
                    // to ensure that it was a super call.
                    if (superCallShouldBeFirst) {
                        const statements = node.body!.statements;
                        let superCallStatement: ExpressionStatement | undefined;

                        for (const statement of statements) {
                            if (statement.kind === SyntaxKind.ExpressionStatement && isSuperCall((<ExpressionStatement>statement).expression)) {
                                superCallStatement = <ExpressionStatement>statement;
                                break;
                            }
                            if (!isPrologueDirective(statement)) {
                                break;
                            }
                        }
                        if (!superCallStatement) {
                            error(node, Diagnostics.A_super_call_must_be_the_first_statement_in_the_constructor_when_a_class_contains_initialized_properties_or_has_parameter_properties);
                        }
                    }
                }
                else if (!classExtendsNull) {
                    error(node, Diagnostics.Constructors_for_derived_classes_must_contain_a_super_call);
                }
            }
        }

        function checkAccessorDeclaration(node: AccessorDeclaration) {
            if (produceDiagnostics) {
                // Grammar checking accessors
                if (!checkGrammarFunctionLikeDeclaration(node) && !checkGrammarAccessor(node)) checkGrammarComputedPropertyName(node.name);

                checkDecorators(node);
                checkSignatureDeclaration(node);
                if (node.kind === SyntaxKind.GetAccessor) {
                    if (!(node.flags & NodeFlags.Ambient) && nodeIsPresent(node.body) && (node.flags & NodeFlags.HasImplicitReturn)) {
                        if (!(node.flags & NodeFlags.HasExplicitReturn)) {
                            error(node.name, Diagnostics.A_get_accessor_must_return_a_value);
                        }
                    }
                }
                // Do not use hasDynamicName here, because that returns false for well known symbols.
                // We want to perform checkComputedPropertyName for all computed properties, including
                // well known symbols.
                if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                    checkComputedPropertyName(node.name);
                }
                if (!hasNonBindableDynamicName(node)) {
                    // TypeScript 1.0 spec (April 2014): 8.4.3
                    // Accessors for the same member name must specify the same accessibility.
                    const otherKind = node.kind === SyntaxKind.GetAccessor ? SyntaxKind.SetAccessor : SyntaxKind.GetAccessor;
                    const otherAccessor = getDeclarationOfKind<AccessorDeclaration>(getSymbolOfNode(node), otherKind);
                    if (otherAccessor) {
                        const nodeFlags = getModifierFlags(node);
                        const otherFlags = getModifierFlags(otherAccessor);
                        if ((nodeFlags & ModifierFlags.AccessibilityModifier) !== (otherFlags & ModifierFlags.AccessibilityModifier)) {
                            error(node.name, Diagnostics.Getter_and_setter_accessors_do_not_agree_in_visibility);
                        }
                        if ((nodeFlags & ModifierFlags.Abstract) !== (otherFlags & ModifierFlags.Abstract)) {
                            error(node.name, Diagnostics.Accessors_must_both_be_abstract_or_non_abstract);
                        }

                        // TypeScript 1.0 spec (April 2014): 4.5
                        // If both accessors include type annotations, the specified types must be identical.
                        checkAccessorDeclarationTypesIdentical(node, otherAccessor, getAnnotatedAccessorType, Diagnostics.get_and_set_accessor_must_have_the_same_type);
                        checkAccessorDeclarationTypesIdentical(node, otherAccessor, getThisTypeOfDeclaration, Diagnostics.get_and_set_accessor_must_have_the_same_this_type);
                    }
                }
                const returnType = getTypeOfAccessors(getSymbolOfNode(node));
                if (node.kind === SyntaxKind.GetAccessor) {
                    checkAllCodePathsInNonVoidFunctionReturnOrThrow(node, returnType);
                }
            }
            checkSourceElement(node.body);
        }

        function checkAccessorDeclarationTypesIdentical(first: AccessorDeclaration, second: AccessorDeclaration, getAnnotatedType: (a: AccessorDeclaration) => Type | undefined, message: DiagnosticMessage) {
            const firstType = getAnnotatedType(first);
            const secondType = getAnnotatedType(second);
            if (firstType && secondType && !isTypeIdenticalTo(firstType, secondType)) {
                error(first, message);
            }
        }

        function checkMissingDeclaration(node: Node) {
            checkDecorators(node);
        }

        function getEffectiveTypeArguments(node: TypeReferenceNode | ExpressionWithTypeArguments, typeParameters: ReadonlyArray<TypeParameter>): Type[] {
            return fillMissingTypeArguments(map(node.typeArguments!, getTypeFromTypeNode), typeParameters,
                getMinTypeArgumentCount(typeParameters), isInJSFile(node));
        }

        function checkTypeArgumentConstraints(node: TypeReferenceNode | ExpressionWithTypeArguments, typeParameters: ReadonlyArray<TypeParameter>): boolean {
            let typeArguments: Type[] | undefined;
            let mapper: TypeMapper | undefined;
            let result = true;
            for (let i = 0; i < typeParameters.length; i++) {
                const constraint = getConstraintOfTypeParameter(typeParameters[i]);
                if (constraint) {
                    if (!typeArguments) {
                        typeArguments = getEffectiveTypeArguments(node, typeParameters);
                        mapper = createTypeMapper(typeParameters, typeArguments);
                    }
                    result = result && checkTypeAssignableTo(
                        typeArguments[i],
                        instantiateType(constraint, mapper!),
                        node.typeArguments![i],
                        Diagnostics.Type_0_does_not_satisfy_the_constraint_1);
                }
            }
            return result;
        }

        function getTypeParametersForTypeReference(node: TypeReferenceNode | ExpressionWithTypeArguments) {
            const type = getTypeFromTypeReference(node);
            if (type !== errorType) {
                const symbol = getNodeLinks(node).resolvedSymbol;
                if (symbol) {
                    return symbol.flags & SymbolFlags.TypeAlias && getSymbolLinks(symbol).typeParameters ||
                        (getObjectFlags(type) & ObjectFlags.Reference ? (<TypeReference>type).target.localTypeParameters : undefined);
                }
            }
            return undefined;
        }

        function checkTypeReferenceNode(node: TypeReferenceNode | ExpressionWithTypeArguments) {
            checkGrammarTypeArguments(node, node.typeArguments);
            if (node.kind === SyntaxKind.TypeReference && node.typeName.jsdocDotPos !== undefined && !isInJSFile(node) && !isInJSDoc(node)) {
                grammarErrorAtPos(node, node.typeName.jsdocDotPos, 1, Diagnostics.JSDoc_types_can_only_be_used_inside_documentation_comments);
            }
            const type = getTypeFromTypeReference(node);
            if (type !== errorType) {
                if (node.typeArguments) {
                    // Do type argument local checks only if referenced type is successfully resolved
                    forEach(node.typeArguments, checkSourceElement);
                    if (produceDiagnostics) {
                        const typeParameters = getTypeParametersForTypeReference(node);
                        if (typeParameters) {
                            checkTypeArgumentConstraints(node, typeParameters);
                        }
                    }
                }
                if (type.flags & TypeFlags.Enum && getNodeLinks(node).resolvedSymbol!.flags & SymbolFlags.EnumMember) {
                    error(node, Diagnostics.Enum_type_0_has_members_with_initializers_that_are_not_literals, typeToString(type));
                }
            }
        }

        function getTypeArgumentConstraint(node: TypeNode): Type | undefined {
            const typeReferenceNode = tryCast(node.parent, isTypeReferenceType);
            if (!typeReferenceNode) return undefined;
            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode)!; // TODO: GH#18217
            const constraint = getConstraintOfTypeParameter(typeParameters[typeReferenceNode.typeArguments!.indexOf(node)]);
            return constraint && instantiateType(constraint, createTypeMapper(typeParameters, getEffectiveTypeArguments(typeReferenceNode, typeParameters)));
        }

        function checkTypeQuery(node: TypeQueryNode) {
            getTypeFromTypeQueryNode(node);
        }

        function checkTypeLiteral(node: TypeLiteralNode) {
            forEach(node.members, checkSourceElement);
            if (produceDiagnostics) {
                const type = getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode(node);
                checkIndexConstraints(type);
                checkTypeForDuplicateIndexSignatures(node);
                checkObjectTypeForDuplicateDeclarations(node);
            }
        }

        function checkArrayType(node: ArrayTypeNode) {
            checkSourceElement(node.elementType);
        }

        function checkTupleType(node: TupleTypeNode) {
            const elementTypes = node.elementTypes;
            let seenOptionalElement = false;
            for (let i = 0; i < elementTypes.length; i++) {
                const e = elementTypes[i];
                if (e.kind === SyntaxKind.RestType) {
                    if (i !== elementTypes.length - 1) {
                        grammarErrorOnNode(e, Diagnostics.A_rest_element_must_be_last_in_a_tuple_type);
                        break;
                    }
                    if (!isArrayType(getTypeFromTypeNode(e))) {
                        error(e, Diagnostics.A_rest_element_type_must_be_an_array_type);
                    }
                }
                else if (e.kind === SyntaxKind.OptionalType) {
                    seenOptionalElement = true;
                }
                else if (seenOptionalElement) {
                    grammarErrorOnNode(e, Diagnostics.A_required_element_cannot_follow_an_optional_element);
                    break;
                }
            }
            checkGrammarForDisallowedTrailingComma(node.elementTypes);
            forEach(node.elementTypes, checkSourceElement);
        }

        function checkUnionOrIntersectionType(node: UnionOrIntersectionTypeNode) {
            forEach(node.types, checkSourceElement);
        }

        function checkIndexedAccessIndexType(type: Type, accessNode: ElementAccessExpression | IndexedAccessTypeNode) {
            if (!(type.flags & TypeFlags.IndexedAccess)) {
                return type;
            }
            // Check if the index type is assignable to 'keyof T' for the object type.
            const objectType = (<IndexedAccessType>type).objectType;
            const indexType = (<IndexedAccessType>type).indexType;
            if (isTypeAssignableTo(indexType, getIndexType(objectType, /*stringsOnly*/ false))) {
                if (accessNode.kind === SyntaxKind.ElementAccessExpression && isAssignmentTarget(accessNode) &&
                    getObjectFlags(objectType) & ObjectFlags.Mapped && getMappedTypeModifiers(<MappedType>objectType) & MappedTypeModifiers.IncludeReadonly) {
                    error(accessNode, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(objectType));
                }
                return type;
            }
            // Check if we're indexing with a numeric type and if either object or index types
            // is a generic type with a constraint that has a numeric index signature.
            if (getIndexInfoOfType(getApparentType(objectType), IndexKind.Number) && isTypeAssignableToKind(indexType, TypeFlags.NumberLike)) {
                return type;
            }
            error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
            return type;
        }

        function checkIndexedAccessType(node: IndexedAccessTypeNode) {
            checkSourceElement(node.objectType);
            checkSourceElement(node.indexType);
            checkIndexedAccessIndexType(getTypeFromIndexedAccessTypeNode(node), node);
        }

        function checkMappedType(node: MappedTypeNode) {
            checkSourceElement(node.typeParameter);
            checkSourceElement(node.type);

            if (noImplicitAny && !node.type) {
                reportImplicitAnyError(node, anyType);
            }

            const type = <MappedType>getTypeFromMappedTypeNode(node);
            const constraintType = getConstraintTypeFromMappedType(type);
            checkTypeAssignableTo(constraintType, keyofConstraintType, getEffectiveConstraintOfTypeParameter(node.typeParameter));
        }

        function checkThisType(node: ThisTypeNode) {
            getTypeFromThisTypeNode(node);
        }

        function checkTypeOperator(node: TypeOperatorNode) {
            checkGrammarTypeOperatorNode(node);
            checkSourceElement(node.type);
        }

        function checkConditionalType(node: ConditionalTypeNode) {
            forEachChild(node, checkSourceElement);
        }

        function checkInferType(node: InferTypeNode) {
            if (!findAncestor(node, n => n.parent && n.parent.kind === SyntaxKind.ConditionalType && (<ConditionalTypeNode>n.parent).extendsType === n)) {
                grammarErrorOnNode(node, Diagnostics.infer_declarations_are_only_permitted_in_the_extends_clause_of_a_conditional_type);
            }
            checkSourceElement(node.typeParameter);
            registerForUnusedIdentifiersCheck(node);
        }

        function checkImportType(node: ImportTypeNode) {
            checkSourceElement(node.argument);
            getTypeFromTypeNode(node);
        }

        function isPrivateWithinAmbient(node: Node): boolean {
            return hasModifier(node, ModifierFlags.Private) && !!(node.flags & NodeFlags.Ambient);
        }

        function getEffectiveDeclarationFlags(n: Declaration, flagsToCheck: ModifierFlags): ModifierFlags {
            let flags = getCombinedModifierFlags(n);

            // children of classes (even ambient classes) should not be marked as ambient or export
            // because those flags have no useful semantics there.
            if (n.parent.kind !== SyntaxKind.InterfaceDeclaration &&
                n.parent.kind !== SyntaxKind.ClassDeclaration &&
                n.parent.kind !== SyntaxKind.ClassExpression &&
                n.flags & NodeFlags.Ambient) {
                if (!(flags & ModifierFlags.Ambient) && !(isModuleBlock(n.parent) && isModuleDeclaration(n.parent.parent) && isGlobalScopeAugmentation(n.parent.parent))) {
                    // It is nested in an ambient context, which means it is automatically exported
                    flags |= ModifierFlags.Export;
                }
                flags |= ModifierFlags.Ambient;
            }

            return flags & flagsToCheck;
        }

        function checkFunctionOrConstructorSymbol(symbol: Symbol): void {
            if (!produceDiagnostics) {
                return;
            }

            function getCanonicalOverload(overloads: Declaration[], implementation: FunctionLikeDeclaration | undefined): Declaration {
                // Consider the canonical set of flags to be the flags of the bodyDeclaration or the first declaration
                // Error on all deviations from this canonical set of flags
                // The caveat is that if some overloads are defined in lib.d.ts, we don't want to
                // report the errors on those. To achieve this, we will say that the implementation is
                // the canonical signature only if it is in the same container as the first overload
                const implementationSharesContainerWithFirstOverload = implementation !== undefined && implementation.parent === overloads[0].parent;
                return implementationSharesContainerWithFirstOverload ? implementation! : overloads[0];
            }

            function checkFlagAgreementBetweenOverloads(overloads: Declaration[], implementation: FunctionLikeDeclaration | undefined, flagsToCheck: ModifierFlags, someOverloadFlags: ModifierFlags, allOverloadFlags: ModifierFlags): void {
                // Error if some overloads have a flag that is not shared by all overloads. To find the
                // deviations, we XOR someOverloadFlags with allOverloadFlags
                const someButNotAllOverloadFlags = someOverloadFlags ^ allOverloadFlags;
                if (someButNotAllOverloadFlags !== 0) {
                    const canonicalFlags = getEffectiveDeclarationFlags(getCanonicalOverload(overloads, implementation), flagsToCheck);

                    forEach(overloads, o => {
                        const deviation = getEffectiveDeclarationFlags(o, flagsToCheck) ^ canonicalFlags;
                        if (deviation & ModifierFlags.Export) {
                            error(getNameOfDeclaration(o), Diagnostics.Overload_signatures_must_all_be_exported_or_non_exported);
                        }
                        else if (deviation & ModifierFlags.Ambient) {
                            error(getNameOfDeclaration(o), Diagnostics.Overload_signatures_must_all_be_ambient_or_non_ambient);
                        }
                        else if (deviation & (ModifierFlags.Private | ModifierFlags.Protected)) {
                            error(getNameOfDeclaration(o) || o, Diagnostics.Overload_signatures_must_all_be_public_private_or_protected);
                        }
                        else if (deviation & ModifierFlags.Abstract) {
                            error(getNameOfDeclaration(o), Diagnostics.Overload_signatures_must_all_be_abstract_or_non_abstract);
                        }
                    });
                }
            }

            function checkQuestionTokenAgreementBetweenOverloads(overloads: Declaration[], implementation: FunctionLikeDeclaration | undefined, someHaveQuestionToken: boolean, allHaveQuestionToken: boolean): void {
                if (someHaveQuestionToken !== allHaveQuestionToken) {
                    const canonicalHasQuestionToken = hasQuestionToken(getCanonicalOverload(overloads, implementation));
                    forEach(overloads, o => {
                        const deviation = hasQuestionToken(o) !== canonicalHasQuestionToken;
                        if (deviation) {
                            error(getNameOfDeclaration(o), Diagnostics.Overload_signatures_must_all_be_optional_or_required);
                        }
                    });
                }
            }

            const flagsToCheck: ModifierFlags = ModifierFlags.Export | ModifierFlags.Ambient | ModifierFlags.Private | ModifierFlags.Protected | ModifierFlags.Abstract;
            let someNodeFlags: ModifierFlags = ModifierFlags.None;
            let allNodeFlags = flagsToCheck;
            let someHaveQuestionToken = false;
            let allHaveQuestionToken = true;
            let hasOverloads = false;
            let bodyDeclaration: FunctionLikeDeclaration | undefined;
            let lastSeenNonAmbientDeclaration: FunctionLikeDeclaration | undefined;
            let previousDeclaration: SignatureDeclaration | undefined;

            const declarations = symbol.declarations;
            const isConstructor = (symbol.flags & SymbolFlags.Constructor) !== 0;

            function reportImplementationExpectedError(node: SignatureDeclaration): void {
                if (node.name && nodeIsMissing(node.name)) {
                    return;
                }

                let seen = false;
                const subsequentNode = forEachChild(node.parent, c => {
                    if (seen) {
                        return c;
                    }
                    else {
                        seen = c === node;
                    }
                });
                // We may be here because of some extra nodes between overloads that could not be parsed into a valid node.
                // In this case the subsequent node is not really consecutive (.pos !== node.end), and we must ignore it here.
                if (subsequentNode && subsequentNode.pos === node.end) {
                    if (subsequentNode.kind === node.kind) {
                        const errorNode: Node = (<FunctionLikeDeclaration>subsequentNode).name || subsequentNode;
                        // TODO: GH#17345: These are methods, so handle computed name case. (`Always allowing computed property names is *not* the correct behavior!)
                        const subsequentName = (<FunctionLikeDeclaration>subsequentNode).name;
                        if (node.name && subsequentName &&
                            (isComputedPropertyName(node.name) && isComputedPropertyName(subsequentName) ||
                                !isComputedPropertyName(node.name) && !isComputedPropertyName(subsequentName) && getEscapedTextOfIdentifierOrLiteral(node.name) === getEscapedTextOfIdentifierOrLiteral(subsequentName))) {
                            const reportError =
                                (node.kind === SyntaxKind.MethodDeclaration || node.kind === SyntaxKind.MethodSignature) &&
                                hasModifier(node, ModifierFlags.Static) !== hasModifier(subsequentNode, ModifierFlags.Static);
                            // we can get here in two cases
                            // 1. mixed static and instance class members
                            // 2. something with the same name was defined before the set of overloads that prevents them from merging
                            // here we'll report error only for the first case since for second we should already report error in binder
                            if (reportError) {
                                const diagnostic = hasModifier(node, ModifierFlags.Static) ? Diagnostics.Function_overload_must_be_static : Diagnostics.Function_overload_must_not_be_static;
                                error(errorNode, diagnostic);
                            }
                            return;
                        }
                        else if (nodeIsPresent((<FunctionLikeDeclaration>subsequentNode).body)) {
                            error(errorNode, Diagnostics.Function_implementation_name_must_be_0, declarationNameToString(node.name!));
                            return;
                        }
                    }
                }
                const errorNode: Node = node.name || node;
                if (isConstructor) {
                    error(errorNode, Diagnostics.Constructor_implementation_is_missing);
                }
                else {
                    // Report different errors regarding non-consecutive blocks of declarations depending on whether
                    // the node in question is abstract.
                    if (hasModifier(node, ModifierFlags.Abstract)) {
                        error(errorNode, Diagnostics.All_declarations_of_an_abstract_method_must_be_consecutive);
                    }
                    else {
                        error(errorNode, Diagnostics.Function_implementation_is_missing_or_not_immediately_following_the_declaration);
                    }
                }
            }

            let duplicateFunctionDeclaration = false;
            let multipleConstructorImplementation = false;
            for (const current of declarations) {
                const node = <SignatureDeclaration>current;
                const inAmbientContext = node.flags & NodeFlags.Ambient;
                const inAmbientContextOrInterface = node.parent.kind === SyntaxKind.InterfaceDeclaration || node.parent.kind === SyntaxKind.TypeLiteral || inAmbientContext;
                if (inAmbientContextOrInterface) {
                    // check if declarations are consecutive only if they are non-ambient
                    // 1. ambient declarations can be interleaved
                    // i.e. this is legal
                    //     declare function foo();
                    //     declare function bar();
                    //     declare function foo();
                    // 2. mixing ambient and non-ambient declarations is a separate error that will be reported - do not want to report an extra one
                    previousDeclaration = undefined;
                }

                if (node.kind === SyntaxKind.FunctionDeclaration || node.kind === SyntaxKind.MethodDeclaration || node.kind === SyntaxKind.MethodSignature || node.kind === SyntaxKind.Constructor) {
                    const currentNodeFlags = getEffectiveDeclarationFlags(node, flagsToCheck);
                    someNodeFlags |= currentNodeFlags;
                    allNodeFlags &= currentNodeFlags;
                    someHaveQuestionToken = someHaveQuestionToken || hasQuestionToken(node);
                    allHaveQuestionToken = allHaveQuestionToken && hasQuestionToken(node);

                    if (nodeIsPresent((node as FunctionLikeDeclaration).body) && bodyDeclaration) {
                        if (isConstructor) {
                            multipleConstructorImplementation = true;
                        }
                        else {
                            duplicateFunctionDeclaration = true;
                        }
                    }
                    else if (previousDeclaration && previousDeclaration.parent === node.parent && previousDeclaration.end !== node.pos) {
                        reportImplementationExpectedError(previousDeclaration);
                    }

                    if (nodeIsPresent((node as FunctionLikeDeclaration).body)) {
                        if (!bodyDeclaration) {
                            bodyDeclaration = node as FunctionLikeDeclaration;
                        }
                    }
                    else {
                        hasOverloads = true;
                    }

                    previousDeclaration = node;

                    if (!inAmbientContextOrInterface) {
                        lastSeenNonAmbientDeclaration = node as FunctionLikeDeclaration;
                    }
                }
            }

            if (multipleConstructorImplementation) {
                forEach(declarations, declaration => {
                    error(declaration, Diagnostics.Multiple_constructor_implementations_are_not_allowed);
                });
            }

            if (duplicateFunctionDeclaration) {
                forEach(declarations, declaration => {
                    error(getNameOfDeclaration(declaration), Diagnostics.Duplicate_function_implementation);
                });
            }

            // Abstract methods can't have an implementation -- in particular, they don't need one.
            if (lastSeenNonAmbientDeclaration && !lastSeenNonAmbientDeclaration.body &&
                !hasModifier(lastSeenNonAmbientDeclaration, ModifierFlags.Abstract) && !lastSeenNonAmbientDeclaration.questionToken) {
                reportImplementationExpectedError(lastSeenNonAmbientDeclaration);
            }

            if (hasOverloads) {
                checkFlagAgreementBetweenOverloads(declarations, bodyDeclaration, flagsToCheck, someNodeFlags, allNodeFlags);
                checkQuestionTokenAgreementBetweenOverloads(declarations, bodyDeclaration, someHaveQuestionToken, allHaveQuestionToken);

                if (bodyDeclaration) {
                    const signatures = getSignaturesOfSymbol(symbol);
                    const bodySignature = getSignatureFromDeclaration(bodyDeclaration);
                    for (const signature of signatures) {
                        if (!isImplementationCompatibleWithOverload(bodySignature, signature)) {
                            error(signature.declaration, Diagnostics.Overload_signature_is_not_compatible_with_function_implementation);
                            break;
                        }
                    }
                }
            }
        }

        const enum DeclarationSpaces {
            None = 0,
            ExportValue = 1 << 0,
            ExportType = 1 << 1,
            ExportNamespace = 1 << 2,
        }
        function checkExportsOnMergedDeclarations(node: Node): void {
            if (!produceDiagnostics) {
                return;
            }

            // if localSymbol is defined on node then node itself is exported - check is required
            let symbol = node.localSymbol;
            if (!symbol) {
                // local symbol is undefined => this declaration is non-exported.
                // however symbol might contain other declarations that are exported
                symbol = getSymbolOfNode(node)!;
                if (!symbol.exportSymbol) {
                    // this is a pure local symbol (all declarations are non-exported) - no need to check anything
                    return;
                }
            }

            // run the check only for the first declaration in the list
            if (getDeclarationOfKind(symbol, node.kind) !== node) {
                return;
            }

            let exportedDeclarationSpaces = DeclarationSpaces.None;
            let nonExportedDeclarationSpaces = DeclarationSpaces.None;
            let defaultExportedDeclarationSpaces = DeclarationSpaces.None;
            for (const d of symbol.declarations) {
                const declarationSpaces = getDeclarationSpaces(d);
                const effectiveDeclarationFlags = getEffectiveDeclarationFlags(d, ModifierFlags.Export | ModifierFlags.Default);

                if (effectiveDeclarationFlags & ModifierFlags.Export) {
                    if (effectiveDeclarationFlags & ModifierFlags.Default) {
                        defaultExportedDeclarationSpaces |= declarationSpaces;
                    }
                    else {
                        exportedDeclarationSpaces |= declarationSpaces;
                    }
                }
                else {
                    nonExportedDeclarationSpaces |= declarationSpaces;
                }
            }

            // Spaces for anything not declared a 'default export'.
            const nonDefaultExportedDeclarationSpaces = exportedDeclarationSpaces | nonExportedDeclarationSpaces;

            const commonDeclarationSpacesForExportsAndLocals = exportedDeclarationSpaces & nonExportedDeclarationSpaces;
            const commonDeclarationSpacesForDefaultAndNonDefault = defaultExportedDeclarationSpaces & nonDefaultExportedDeclarationSpaces;

            if (commonDeclarationSpacesForExportsAndLocals || commonDeclarationSpacesForDefaultAndNonDefault) {
                // declaration spaces for exported and non-exported declarations intersect
                for (const d of symbol.declarations) {
                    const declarationSpaces = getDeclarationSpaces(d);

                    const name = getNameOfDeclaration(d);
                    // Only error on the declarations that contributed to the intersecting spaces.
                    if (declarationSpaces & commonDeclarationSpacesForDefaultAndNonDefault) {
                        error(name, Diagnostics.Merged_declaration_0_cannot_include_a_default_export_declaration_Consider_adding_a_separate_export_default_0_declaration_instead, declarationNameToString(name));
                    }
                    else if (declarationSpaces & commonDeclarationSpacesForExportsAndLocals) {
                        error(name, Diagnostics.Individual_declarations_in_merged_declaration_0_must_be_all_exported_or_all_local, declarationNameToString(name));
                    }
                }
            }

            function getDeclarationSpaces(decl: Declaration): DeclarationSpaces {
                let d = decl as Node;
                switch (d.kind) {
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.TypeAliasDeclaration:
                    // A jsdoc typedef and callback are, by definition, type aliases
                    case SyntaxKind.JSDocTypedefTag:
                    case SyntaxKind.JSDocCallbackTag:
                        return DeclarationSpaces.ExportType;
                    case SyntaxKind.ModuleDeclaration:
                        return isAmbientModule(d as ModuleDeclaration) || getModuleInstanceState(d as ModuleDeclaration) !== ModuleInstanceState.NonInstantiated
                            ? DeclarationSpaces.ExportNamespace | DeclarationSpaces.ExportValue
                            : DeclarationSpaces.ExportNamespace;
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.EnumDeclaration:
                        return DeclarationSpaces.ExportType | DeclarationSpaces.ExportValue;
                    case SyntaxKind.SourceFile:
                        return DeclarationSpaces.ExportType | DeclarationSpaces.ExportValue | DeclarationSpaces.ExportNamespace;
                    case SyntaxKind.ExportAssignment:
                        // Export assigned entity name expressions act as aliases and should fall through, otherwise they export values
                        if (!isEntityNameExpression((d as ExportAssignment).expression)) {
                            return DeclarationSpaces.ExportValue;
                        }
                        d = (d as ExportAssignment).expression;
                        /* falls through */
                    // The below options all declare an Alias, which is allowed to merge with other values within the importing module
                    case SyntaxKind.ImportEqualsDeclaration:
                    case SyntaxKind.NamespaceImport:
                    case SyntaxKind.ImportClause:
                        let result = DeclarationSpaces.None;
                        const target = resolveAlias(getSymbolOfNode(d)!);
                        forEach(target.declarations, d => { result |= getDeclarationSpaces(d); });
                        return result;
                    case SyntaxKind.VariableDeclaration:
                    case SyntaxKind.BindingElement:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.ImportSpecifier: // https://github.com/Microsoft/TypeScript/pull/7591
                        return DeclarationSpaces.ExportValue;
                    default:
                        return Debug.fail(Debug.showSyntaxKind(d));
                }
            }
        }

        function getAwaitedTypeOfPromise(type: Type, errorNode?: Node, diagnosticMessage?: DiagnosticMessage): Type | undefined {
            const promisedType = getPromisedTypeOfPromise(type, errorNode);
            return promisedType && getAwaitedType(promisedType, errorNode, diagnosticMessage);
        }

        /**
         * Gets the "promised type" of a promise.
         * @param type The type of the promise.
         * @remarks The "promised type" of a type is the type of the "value" parameter of the "onfulfilled" callback.
         */
        function getPromisedTypeOfPromise(promise: Type, errorNode?: Node): Type | undefined {
            //
            //  { // promise
            //      then( // thenFunction
            //          onfulfilled: ( // onfulfilledParameterType
            //              value: T // valueParameterType
            //          ) => any
            //      ): any;
            //  }
            //

            if (isTypeAny(promise)) {
                return undefined;
            }

            const typeAsPromise = <PromiseOrAwaitableType>promise;
            if (typeAsPromise.promisedTypeOfPromise) {
                return typeAsPromise.promisedTypeOfPromise;
            }

            if (isReferenceToType(promise, getGlobalPromiseType(/*reportErrors*/ false))) {
                return typeAsPromise.promisedTypeOfPromise = (<GenericType>promise).typeArguments![0];
            }

            const thenFunction = getTypeOfPropertyOfType(promise, "then" as __String)!; // TODO: GH#18217
            if (isTypeAny(thenFunction)) {
                return undefined;
            }

            const thenSignatures = thenFunction ? getSignaturesOfType(thenFunction, SignatureKind.Call) : emptyArray;
            if (thenSignatures.length === 0) {
                if (errorNode) {
                    error(errorNode, Diagnostics.A_promise_must_have_a_then_method);
                }
                return undefined;
            }

            const onfulfilledParameterType = getTypeWithFacts(getUnionType(map(thenSignatures, getTypeOfFirstParameterOfSignature)), TypeFacts.NEUndefinedOrNull);
            if (isTypeAny(onfulfilledParameterType)) {
                return undefined;
            }

            const onfulfilledParameterSignatures = getSignaturesOfType(onfulfilledParameterType, SignatureKind.Call);
            if (onfulfilledParameterSignatures.length === 0) {
                if (errorNode) {
                    error(errorNode, Diagnostics.The_first_parameter_of_the_then_method_of_a_promise_must_be_a_callback);
                }
                return undefined;
            }

            return typeAsPromise.promisedTypeOfPromise = getUnionType(map(onfulfilledParameterSignatures, getTypeOfFirstParameterOfSignature), UnionReduction.Subtype);
        }

        /**
         * Gets the "awaited type" of a type.
         * @param type The type to await.
         * @remarks The "awaited type" of an expression is its "promised type" if the expression is a
         * Promise-like type; otherwise, it is the type of the expression. This is used to reflect
         * The runtime behavior of the `await` keyword.
         */
        function checkAwaitedType(type: Type, errorNode: Node, diagnosticMessage: DiagnosticMessage): Type {
            return getAwaitedType(type, errorNode, diagnosticMessage) || errorType;
        }

        function getAwaitedType(type: Type, errorNode?: Node, diagnosticMessage?: DiagnosticMessage): Type | undefined {
            const typeAsAwaitable = <PromiseOrAwaitableType>type;
            if (typeAsAwaitable.awaitedTypeOfType) {
                return typeAsAwaitable.awaitedTypeOfType;
            }

            if (isTypeAny(type)) {
                return typeAsAwaitable.awaitedTypeOfType = type;
            }

            if (type.flags & TypeFlags.Union) {
                let types: Type[] | undefined;
                for (const constituentType of (<UnionType>type).types) {
                    types = append<Type>(types, getAwaitedType(constituentType, errorNode, diagnosticMessage));
                }

                if (!types) {
                    return undefined;
                }

                return typeAsAwaitable.awaitedTypeOfType = getUnionType(types);
            }

            const promisedType = getPromisedTypeOfPromise(type);
            if (promisedType) {
                if (type.id === promisedType.id || awaitedTypeStack.indexOf(promisedType.id) >= 0) {
                    // Verify that we don't have a bad actor in the form of a promise whose
                    // promised type is the same as the promise type, or a mutually recursive
                    // promise. If so, we return undefined as we cannot guess the shape. If this
                    // were the actual case in the JavaScript, this Promise would never resolve.
                    //
                    // An example of a bad actor with a singly-recursive promise type might
                    // be:
                    //
                    //  interface BadPromise {
                    //      then(
                    //          onfulfilled: (value: BadPromise) => any,
                    //          onrejected: (error: any) => any): BadPromise;
                    //  }
                    // The above interface will pass the PromiseLike check, and return a
                    // promised type of `BadPromise`. Since this is a self reference, we
                    // don't want to keep recursing ad infinitum.
                    //
                    // An example of a bad actor in the form of a mutually-recursive
                    // promise type might be:
                    //
                    //  interface BadPromiseA {
                    //      then(
                    //          onfulfilled: (value: BadPromiseB) => any,
                    //          onrejected: (error: any) => any): BadPromiseB;
                    //  }
                    //
                    //  interface BadPromiseB {
                    //      then(
                    //          onfulfilled: (value: BadPromiseA) => any,
                    //          onrejected: (error: any) => any): BadPromiseA;
                    //  }
                    //
                    if (errorNode) {
                        error(errorNode, Diagnostics.Type_is_referenced_directly_or_indirectly_in_the_fulfillment_callback_of_its_own_then_method);
                    }
                    return undefined;
                }

                // Keep track of the type we're about to unwrap to avoid bad recursive promise types.
                // See the comments above for more information.
                awaitedTypeStack.push(type.id);
                const awaitedType = getAwaitedType(promisedType, errorNode, diagnosticMessage);
                awaitedTypeStack.pop();

                if (!awaitedType) {
                    return undefined;
                }

                return typeAsAwaitable.awaitedTypeOfType = awaitedType;
            }

            // The type was not a promise, so it could not be unwrapped any further.
            // As long as the type does not have a callable "then" property, it is
            // safe to return the type; otherwise, an error will be reported in
            // the call to getNonThenableType and we will return undefined.
            //
            // An example of a non-promise "thenable" might be:
            //
            //  await { then(): void {} }
            //
            // The "thenable" does not match the minimal definition for a promise. When
            // a Promise/A+-compatible or ES6 promise tries to adopt this value, the promise
            // will never settle. We treat this as an error to help flag an early indicator
            // of a runtime problem. If the user wants to return this value from an async
            // function, they would need to wrap it in some other value. If they want it to
            // be treated as a promise, they can cast to <any>.
            const thenFunction = getTypeOfPropertyOfType(type, "then" as __String);
            if (thenFunction && getSignaturesOfType(thenFunction, SignatureKind.Call).length > 0) {
                if (errorNode) {
                    if (!diagnosticMessage) return Debug.fail();
                    error(errorNode, diagnosticMessage);
                }
                return undefined;
            }

            return typeAsAwaitable.awaitedTypeOfType = type;
        }

        /**
         * Checks the return type of an async function to ensure it is a compatible
         * Promise implementation.
         *
         * This checks that an async function has a valid Promise-compatible return type.
         * An async function has a valid Promise-compatible return type if the resolved value
         * of the return type has a construct signature that takes in an `initializer` function
         * that in turn supplies a `resolve` function as one of its arguments and results in an
         * object with a callable `then` signature.
         *
         * @param node The signature to check
         */
        function checkAsyncFunctionReturnType(node: FunctionLikeDeclaration | MethodSignature, returnTypeNode: TypeNode) {
            // As part of our emit for an async function, we will need to emit the entity name of
            // the return type annotation as an expression. To meet the necessary runtime semantics
            // for __awaiter, we must also check that the type of the declaration (e.g. the static
            // side or "constructor" of the promise type) is compatible `PromiseConstructorLike`.
            //
            // An example might be (from lib.es6.d.ts):
            //
            //  interface Promise<T> { ... }
            //  interface PromiseConstructor {
            //      new <T>(...): Promise<T>;
            //  }
            //  declare var Promise: PromiseConstructor;
            //
            // When an async function declares a return type annotation of `Promise<T>`, we
            // need to get the type of the `Promise` variable declaration above, which would
            // be `PromiseConstructor`.
            //
            // The same case applies to a class:
            //
            //  declare class Promise<T> {
            //      constructor(...);
            //      then<U>(...): Promise<U>;
            //  }
            //
            const returnType = getTypeFromTypeNode(returnTypeNode);

            if (languageVersion >= ScriptTarget.ES2015) {
                if (returnType === errorType) {
                    return;
                }
                const globalPromiseType = getGlobalPromiseType(/*reportErrors*/ true);
                if (globalPromiseType !== emptyGenericType && !isReferenceToType(returnType, globalPromiseType)) {
                    // The promise type was not a valid type reference to the global promise type, so we
                    // report an error and return the unknown type.
                    error(returnTypeNode, Diagnostics.The_return_type_of_an_async_function_or_method_must_be_the_global_Promise_T_type);
                    return;
                }
            }
            else {
                // Always mark the type node as referenced if it points to a value
                markTypeNodeAsReferenced(returnTypeNode);

                if (returnType === errorType) {
                    return;
                }

                const promiseConstructorName = getEntityNameFromTypeNode(returnTypeNode);
                if (promiseConstructorName === undefined) {
                    error(returnTypeNode, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, typeToString(returnType));
                    return;
                }

                const promiseConstructorSymbol = resolveEntityName(promiseConstructorName, SymbolFlags.Value, /*ignoreErrors*/ true);
                const promiseConstructorType = promiseConstructorSymbol ? getTypeOfSymbol(promiseConstructorSymbol) : errorType;
                if (promiseConstructorType === errorType) {
                    if (promiseConstructorName.kind === SyntaxKind.Identifier && promiseConstructorName.escapedText === "Promise" && getTargetType(returnType) === getGlobalPromiseType(/*reportErrors*/ false)) {
                        error(returnTypeNode, Diagnostics.An_async_function_or_method_in_ES5_SlashES3_requires_the_Promise_constructor_Make_sure_you_have_a_declaration_for_the_Promise_constructor_or_include_ES2015_in_your_lib_option);
                    }
                    else {
                        error(returnTypeNode, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, entityNameToString(promiseConstructorName));
                    }
                    return;
                }

                const globalPromiseConstructorLikeType = getGlobalPromiseConstructorLikeType(/*reportErrors*/ true);
                if (globalPromiseConstructorLikeType === emptyObjectType) {
                    // If we couldn't resolve the global PromiseConstructorLike type we cannot verify
                    // compatibility with __awaiter.
                    error(returnTypeNode, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, entityNameToString(promiseConstructorName));
                    return;
                }

                if (!checkTypeAssignableTo(promiseConstructorType, globalPromiseConstructorLikeType, returnTypeNode,
                    Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value)) {
                    return;
                }

                // Verify there is no local declaration that could collide with the promise constructor.
                const rootName = promiseConstructorName && getFirstIdentifier(promiseConstructorName);
                const collidingSymbol = getSymbol(node.locals!, rootName.escapedText, SymbolFlags.Value);
                if (collidingSymbol) {
                    error(collidingSymbol.valueDeclaration, Diagnostics.Duplicate_identifier_0_Compiler_uses_declaration_1_to_support_async_functions,
                        idText(rootName),
                        entityNameToString(promiseConstructorName));
                    return;
                }
            }
            checkAwaitedType(returnType, node, Diagnostics.The_return_type_of_an_async_function_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
        }

        /** Check a decorator */
        function checkDecorator(node: Decorator): void {
            const signature = getResolvedSignature(node);
            const returnType = getReturnTypeOfSignature(signature);
            if (returnType.flags & TypeFlags.Any) {
                return;
            }

            let expectedReturnType: Type;
            const headMessage = getDiagnosticHeadMessageForDecoratorResolution(node);
            let errorInfo: DiagnosticMessageChain | undefined;
            switch (node.parent.kind) {
                case SyntaxKind.ClassDeclaration:
                    const classSymbol = getSymbolOfNode(node.parent);
                    const classConstructorType = getTypeOfSymbol(classSymbol);
                    expectedReturnType = getUnionType([classConstructorType, voidType]);
                    break;

                case SyntaxKind.Parameter:
                    expectedReturnType = voidType;
                    errorInfo = chainDiagnosticMessages(
                        /*details*/ undefined,
                        Diagnostics.The_return_type_of_a_parameter_decorator_function_must_be_either_void_or_any);

                    break;

                case SyntaxKind.PropertyDeclaration:
                    expectedReturnType = voidType;
                    errorInfo = chainDiagnosticMessages(
                        /*details*/ undefined,
                        Diagnostics.The_return_type_of_a_property_decorator_function_must_be_either_void_or_any);
                    break;

                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    const methodType = getTypeOfNode(node.parent);
                    const descriptorType = createTypedPropertyDescriptorType(methodType);
                    expectedReturnType = getUnionType([descriptorType, voidType]);
                    break;

                default:
                    return Debug.fail();
            }

            checkTypeAssignableTo(
                returnType,
                expectedReturnType,
                node,
                headMessage,
                () => errorInfo);
        }

        /**
         * If a TypeNode can be resolved to a value symbol imported from an external module, it is
         * marked as referenced to prevent import elision.
         */
        function markTypeNodeAsReferenced(node: TypeNode) {
            markEntityNameOrEntityExpressionAsReference(node && getEntityNameFromTypeNode(node));
        }

        function markEntityNameOrEntityExpressionAsReference(typeName: EntityNameOrEntityNameExpression | undefined) {
            if (!typeName) return;

            const rootName = getFirstIdentifier(typeName);
            const meaning = (typeName.kind === SyntaxKind.Identifier ? SymbolFlags.Type : SymbolFlags.Namespace) | SymbolFlags.Alias;
            const rootSymbol = resolveName(rootName, rootName.escapedText, meaning, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isRefernce*/ true);
            if (rootSymbol
                && rootSymbol.flags & SymbolFlags.Alias
                && symbolIsValue(rootSymbol)
                && !isConstEnumOrConstEnumOnlyModule(resolveAlias(rootSymbol))) {
                markAliasSymbolAsReferenced(rootSymbol);
            }
        }

        /**
         * This function marks the type used for metadata decorator as referenced if it is import
         * from external module.
         * This is different from markTypeNodeAsReferenced because it tries to simplify type nodes in
         * union and intersection type
         * @param node
         */
        function markDecoratorMedataDataTypeNodeAsReferenced(node: TypeNode | undefined): void {
            const entityName = getEntityNameForDecoratorMetadata(node);
            if (entityName && isEntityName(entityName)) {
                markEntityNameOrEntityExpressionAsReference(entityName);
            }
        }

        function getEntityNameForDecoratorMetadata(node: TypeNode | undefined): EntityName | undefined {
            if (node) {
                switch (node.kind) {
                    case SyntaxKind.IntersectionType:
                    case SyntaxKind.UnionType:
                        let commonEntityName: EntityName | undefined;
                        for (let typeNode of (<UnionOrIntersectionTypeNode>node).types) {
                            while (typeNode.kind === SyntaxKind.ParenthesizedType) {
                                typeNode = (typeNode as ParenthesizedTypeNode).type; // Skip parens if need be
                            }
                            if (typeNode.kind === SyntaxKind.NeverKeyword) {
                                continue; // Always elide `never` from the union/intersection if possible
                            }
                            if (!strictNullChecks && (typeNode.kind === SyntaxKind.NullKeyword || typeNode.kind === SyntaxKind.UndefinedKeyword)) {
                                continue; // Elide null and undefined from unions for metadata, just like what we did prior to the implementation of strict null checks
                            }
                            const individualEntityName = getEntityNameForDecoratorMetadata(typeNode);
                            if (!individualEntityName) {
                                // Individual is something like string number
                                // So it would be serialized to either that type or object
                                // Safe to return here
                                return undefined;
                            }

                            if (commonEntityName) {
                                // Note this is in sync with the transformation that happens for type node.
                                // Keep this in sync with serializeUnionOrIntersectionType
                                // Verify if they refer to same entity and is identifier
                                // return undefined if they dont match because we would emit object
                                if (!isIdentifier(commonEntityName) ||
                                    !isIdentifier(individualEntityName) ||
                                    commonEntityName.escapedText !== individualEntityName.escapedText) {
                                    return undefined;
                                }
                            }
                            else {
                                commonEntityName = individualEntityName;
                            }
                        }
                        return commonEntityName;

                    case SyntaxKind.ParenthesizedType:
                        return getEntityNameForDecoratorMetadata((<ParenthesizedTypeNode>node).type);

                    case SyntaxKind.TypeReference:
                        return (<TypeReferenceNode>node).typeName;
                }
            }
        }

        function getParameterTypeNodeForDecoratorCheck(node: ParameterDeclaration): TypeNode | undefined {
            const typeNode = getEffectiveTypeAnnotationNode(node);
            return isRestParameter(node) ? getRestParameterElementType(typeNode) : typeNode;
        }

        /** Check the decorators of a node */
        function checkDecorators(node: Node): void {
            if (!node.decorators) {
                return;
            }

            // skip this check for nodes that cannot have decorators. These should have already had an error reported by
            // checkGrammarDecorators.
            if (!nodeCanBeDecorated(node, node.parent, node.parent.parent)) {
                return;
            }

            if (!compilerOptions.experimentalDecorators) {
                error(node, Diagnostics.Experimental_support_for_decorators_is_a_feature_that_is_subject_to_change_in_a_future_release_Set_the_experimentalDecorators_option_to_remove_this_warning);
            }

            const firstDecorator = node.decorators[0];
            checkExternalEmitHelpers(firstDecorator, ExternalEmitHelpers.Decorate);
            if (node.kind === SyntaxKind.Parameter) {
                checkExternalEmitHelpers(firstDecorator, ExternalEmitHelpers.Param);
            }

            if (compilerOptions.emitDecoratorMetadata) {
                checkExternalEmitHelpers(firstDecorator, ExternalEmitHelpers.Metadata);

                // we only need to perform these checks if we are emitting serialized type metadata for the target of a decorator.
                switch (node.kind) {
                    case SyntaxKind.ClassDeclaration:
                        const constructor = getFirstConstructorWithBody(<ClassDeclaration>node);
                        if (constructor) {
                            for (const parameter of constructor.parameters) {
                                markDecoratorMedataDataTypeNodeAsReferenced(getParameterTypeNodeForDecoratorCheck(parameter));
                            }
                        }
                        break;

                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        const otherKind = node.kind === SyntaxKind.GetAccessor ? SyntaxKind.SetAccessor : SyntaxKind.GetAccessor;
                        const otherAccessor = getDeclarationOfKind<AccessorDeclaration>(getSymbolOfNode(node as AccessorDeclaration), otherKind);
                        markDecoratorMedataDataTypeNodeAsReferenced(getAnnotatedAccessorTypeNode(node as AccessorDeclaration) || otherAccessor && getAnnotatedAccessorTypeNode(otherAccessor));
                        break;
                    case SyntaxKind.MethodDeclaration:
                        for (const parameter of (<FunctionLikeDeclaration>node).parameters) {
                            markDecoratorMedataDataTypeNodeAsReferenced(getParameterTypeNodeForDecoratorCheck(parameter));
                        }

                        markDecoratorMedataDataTypeNodeAsReferenced(getEffectiveReturnTypeNode(<FunctionLikeDeclaration>node));
                        break;

                    case SyntaxKind.PropertyDeclaration:
                        markDecoratorMedataDataTypeNodeAsReferenced(getEffectiveTypeAnnotationNode(<ParameterDeclaration>node));
                        break;

                    case SyntaxKind.Parameter:
                        markDecoratorMedataDataTypeNodeAsReferenced(getParameterTypeNodeForDecoratorCheck(<ParameterDeclaration>node));
                        const containingSignature = (node as ParameterDeclaration).parent;
                        for (const parameter of containingSignature.parameters) {
                            markDecoratorMedataDataTypeNodeAsReferenced(getParameterTypeNodeForDecoratorCheck(parameter));
                        }
                        break;
                }
            }

            forEach(node.decorators, checkDecorator);
        }

        function checkFunctionDeclaration(node: FunctionDeclaration): void {
            if (produceDiagnostics) {
                checkFunctionOrMethodDeclaration(node);
                checkGrammarForGenerator(node);
                checkCollisionWithRequireExportsInGeneratedCode(node, node.name!);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name!);
            }
        }

        function checkJSDocTypeAliasTag(node: JSDocTypedefTag | JSDocCallbackTag) {
            if (!node.typeExpression) {
                // If the node had `@property` tags, `typeExpression` would have been set to the first property tag.
                error(node.name, Diagnostics.JSDoc_typedef_tag_should_either_have_a_type_annotation_or_be_followed_by_property_or_member_tags);
            }

            if (node.name) {
                checkTypeNameIsReserved(node.name, Diagnostics.Type_alias_name_cannot_be_0);
            }
            checkSourceElement(node.typeExpression);
        }

        function checkJSDocTemplateTag(node: JSDocTemplateTag): void {
            checkSourceElement(node.constraint);
            for (const tp of node.typeParameters) {
                checkSourceElement(tp);
            }
        }

        function checkJSDocTypeTag(node: JSDocTypeTag) {
            checkSourceElement(node.typeExpression);
        }

        function checkJSDocParameterTag(node: JSDocParameterTag) {
            checkSourceElement(node.typeExpression);
            if (!getParameterSymbolFromJSDoc(node)) {
                const decl = getHostSignatureFromJSDoc(node);
                // don't issue an error for invalid hosts -- just functions --
                // and give a better error message when the host function mentions `arguments`
                // but the tag doesn't have an array type
                if (decl) {
                    const i = getJSDocTags(decl).filter(isJSDocParameterTag).indexOf(node);
                    if (i > -1 && i < decl.parameters.length && isBindingPattern(decl.parameters[i].name)) {
                        return;
                    }
                    if (!containsArgumentsReference(decl)) {
                        error(node.name,
                            Diagnostics.JSDoc_param_tag_has_name_0_but_there_is_no_parameter_with_that_name,
                            idText(node.name.kind === SyntaxKind.QualifiedName ? node.name.right : node.name));
                    }
                    else if (findLast(getJSDocTags(decl), isJSDocParameterTag) === node &&
                        node.typeExpression && node.typeExpression.type &&
                        !isArrayType(getTypeFromTypeNode(node.typeExpression.type))) {
                        error(node.name,
                              Diagnostics.JSDoc_param_tag_has_name_0_but_there_is_no_parameter_with_that_name_It_would_match_arguments_if_it_had_an_array_type,
                              idText(node.name.kind === SyntaxKind.QualifiedName ? node.name.right : node.name));
                    }
                }
            }
        }

        function checkJSDocAugmentsTag(node: JSDocAugmentsTag): void {
            const classLike = getJSDocHost(node);
            if (!isClassDeclaration(classLike) && !isClassExpression(classLike)) {
                error(classLike, Diagnostics.JSDoc_0_is_not_attached_to_a_class, idText(node.tagName));
                return;
            }

            const augmentsTags = getJSDocTags(classLike).filter(isJSDocAugmentsTag);
            Debug.assert(augmentsTags.length > 0);
            if (augmentsTags.length > 1) {
                error(augmentsTags[1], Diagnostics.Class_declarations_cannot_have_more_than_one_augments_or_extends_tag);
            }

            const name = getIdentifierFromEntityNameExpression(node.class.expression);
            const extend = getClassExtendsHeritageElement(classLike);
            if (extend) {
                const className = getIdentifierFromEntityNameExpression(extend.expression);
                if (className && name.escapedText !== className.escapedText) {
                    error(name, Diagnostics.JSDoc_0_1_does_not_match_the_extends_2_clause, idText(node.tagName), idText(name), idText(className));
                }
            }
        }

        function getIdentifierFromEntityNameExpression(node: Identifier | PropertyAccessExpression): Identifier;
        function getIdentifierFromEntityNameExpression(node: Expression): Identifier | undefined;
        function getIdentifierFromEntityNameExpression(node: Expression): Identifier | undefined {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                    return node as Identifier;
                case SyntaxKind.PropertyAccessExpression:
                    return (node as PropertyAccessExpression).name;
                default:
                    return undefined;
            }
        }

        function checkFunctionOrMethodDeclaration(node: FunctionDeclaration | MethodDeclaration | MethodSignature): void {
            checkDecorators(node);
            checkSignatureDeclaration(node);
            const functionFlags = getFunctionFlags(node);

            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name && node.name.kind === SyntaxKind.ComputedPropertyName) {
                // This check will account for methods in class/interface declarations,
                // as well as accessors in classes/object literals
                checkComputedPropertyName(node.name);
            }

            if (!hasNonBindableDynamicName(node)) {
                // first we want to check the local symbol that contain this declaration
                // - if node.localSymbol !== undefined - this is current declaration is exported and localSymbol points to the local symbol
                // - if node.localSymbol === undefined - this node is non-exported so we can just pick the result of getSymbolOfNode
                const symbol = getSymbolOfNode(node);
                const localSymbol = node.localSymbol || symbol;

                // Since the javascript won't do semantic analysis like typescript,
                // if the javascript file comes before the typescript file and both contain same name functions,
                // checkFunctionOrConstructorSymbol wouldn't be called if we didnt ignore javascript function.
                const firstDeclaration = find(localSymbol.declarations,
                    // Get first non javascript function declaration
                    declaration => declaration.kind === node.kind && !(declaration.flags & NodeFlags.JavaScriptFile));

                // Only type check the symbol once
                if (node === firstDeclaration) {
                    checkFunctionOrConstructorSymbol(localSymbol);
                }

                if (symbol.parent) {
                    // run check once for the first declaration
                    if (getDeclarationOfKind(symbol, node.kind) === node) {
                        // run check on export symbol to check that modifiers agree across all exported declarations
                        checkFunctionOrConstructorSymbol(symbol);
                    }
                }
            }

            const body = node.kind === SyntaxKind.MethodSignature ? undefined : node.body;
            checkSourceElement(body);

            if ((functionFlags & FunctionFlags.Generator) === 0) { // Async function or normal function
                const returnOrPromisedType = getReturnOrPromisedType(node, functionFlags);
                checkAllCodePathsInNonVoidFunctionReturnOrThrow(node, returnOrPromisedType);
            }

            if (produceDiagnostics && !getEffectiveReturnTypeNode(node)) {
                // Report an implicit any error if there is no body, no explicit return type, and node is not a private method
                // in an ambient context
                if (noImplicitAny && nodeIsMissing(body) && !isPrivateWithinAmbient(node)) {
                    reportImplicitAnyError(node, anyType);
                }

                if (functionFlags & FunctionFlags.Generator && nodeIsPresent(body)) {
                    // A generator with a body and no type annotation can still cause errors. It can error if the
                    // yielded values have no common supertype, or it can give an implicit any error if it has no
                    // yielded values. The only way to trigger these errors is to try checking its return type.
                    getReturnTypeOfSignature(getSignatureFromDeclaration(node));
                }
            }

            // A js function declaration can have a @type tag instead of a return type node, but that type must have a call signature
            if (isInJSFile(node)) {
                const typeTag = getJSDocTypeTag(node);
                if (typeTag && typeTag.typeExpression && !getContextualCallSignature(getTypeFromTypeNode(typeTag.typeExpression), node)) {
                    error(typeTag, Diagnostics.The_type_of_a_function_declaration_must_match_the_function_s_signature);
                }
            }
        }

        function registerForUnusedIdentifiersCheck(node: PotentiallyUnusedIdentifier): void {
            // May be in a call such as getTypeOfNode that happened to call this. But potentiallyUnusedIdentifiers is only defined in the scope of `checkSourceFile`.
            if (produceDiagnostics) {
                const sourceFile = getSourceFileOfNode(node);
                let potentiallyUnusedIdentifiers = allPotentiallyUnusedIdentifiers.get(sourceFile.path);
                if (!potentiallyUnusedIdentifiers) {
                    potentiallyUnusedIdentifiers = [];
                    allPotentiallyUnusedIdentifiers.set(sourceFile.path, potentiallyUnusedIdentifiers);
                }
                // TODO: GH#22580
                // Debug.assert(addToSeen(seenPotentiallyUnusedIdentifiers, getNodeId(node)), "Adding potentially-unused identifier twice");
                potentiallyUnusedIdentifiers.push(node);
            }
        }

        type PotentiallyUnusedIdentifier =
            | SourceFile | ModuleDeclaration | ClassLikeDeclaration | InterfaceDeclaration
            | Block | CaseBlock | ForStatement | ForInStatement | ForOfStatement
            | Exclude<SignatureDeclaration, IndexSignatureDeclaration | JSDocFunctionType> | TypeAliasDeclaration
            | InferTypeNode;

        function checkUnusedIdentifiers(potentiallyUnusedIdentifiers: ReadonlyArray<PotentiallyUnusedIdentifier>, addDiagnostic: AddUnusedDiagnostic) {
            for (const node of potentiallyUnusedIdentifiers) {
                switch (node.kind) {
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.ClassExpression:
                        checkUnusedClassMembers(node, addDiagnostic);
                        checkUnusedTypeParameters(node, addDiagnostic);
                        break;
                    case SyntaxKind.InterfaceDeclaration:
                        checkUnusedTypeParameters(node, addDiagnostic);
                        break;
                    case SyntaxKind.SourceFile:
                    case SyntaxKind.ModuleDeclaration:
                    case SyntaxKind.Block:
                    case SyntaxKind.CaseBlock:
                    case SyntaxKind.ForStatement:
                    case SyntaxKind.ForInStatement:
                    case SyntaxKind.ForOfStatement:
                        checkUnusedLocalsAndParameters(node, addDiagnostic);
                        break;
                    case SyntaxKind.Constructor:
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.ArrowFunction:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        if (node.body) {
                            checkUnusedLocalsAndParameters(node, addDiagnostic);
                        }
                        checkUnusedTypeParameters(node, addDiagnostic);
                        break;
                    case SyntaxKind.MethodSignature:
                    case SyntaxKind.CallSignature:
                    case SyntaxKind.ConstructSignature:
                    case SyntaxKind.FunctionType:
                    case SyntaxKind.ConstructorType:
                    case SyntaxKind.TypeAliasDeclaration:
                    case SyntaxKind.InferType:
                        checkUnusedTypeParameters(node, addDiagnostic);
                        break;
                    default:
                        Debug.assertNever(node, "Node should not have been registered for unused identifiers check");
                }
            }
        }

        function errorUnusedLocal(declaration: Declaration, name: string, addDiagnostic: AddUnusedDiagnostic) {
            const node = getNameOfDeclaration(declaration) || declaration;
            const message = isTypeDeclaration(declaration) ? Diagnostics._0_is_declared_but_never_used : Diagnostics._0_is_declared_but_its_value_is_never_read;
            addDiagnostic(declaration, UnusedKind.Local, createDiagnosticForNode(node, message, name));
        }

        function isIdentifierThatStartsWithUnderscore(node: Node) {
            return isIdentifier(node) && idText(node).charCodeAt(0) === CharacterCodes._;
        }

        function checkUnusedClassMembers(node: ClassDeclaration | ClassExpression, addDiagnostic: AddUnusedDiagnostic): void {
            if (!(node.flags & NodeFlags.Ambient)) {
                for (const member of node.members) {
                    switch (member.kind) {
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.PropertyDeclaration:
                        case SyntaxKind.GetAccessor:
                        case SyntaxKind.SetAccessor:
                            if (member.kind === SyntaxKind.SetAccessor && member.symbol.flags & SymbolFlags.GetAccessor) {
                                // Already would have reported an error on the getter.
                                break;
                            }
                            const symbol = getSymbolOfNode(member);
                            if (!symbol.isReferenced && hasModifier(member, ModifierFlags.Private)) {
                                addDiagnostic(member, UnusedKind.Local, createDiagnosticForNode(member.name!, Diagnostics._0_is_declared_but_its_value_is_never_read, symbolToString(symbol)));
                            }
                            break;
                        case SyntaxKind.Constructor:
                            for (const parameter of (<ConstructorDeclaration>member).parameters) {
                                if (!parameter.symbol.isReferenced && hasModifier(parameter, ModifierFlags.Private)) {
                                    addDiagnostic(parameter, UnusedKind.Local, createDiagnosticForNode(parameter.name, Diagnostics.Property_0_is_declared_but_its_value_is_never_read, symbolName(parameter.symbol)));
                                }
                            }
                            break;
                        case SyntaxKind.IndexSignature:
                        case SyntaxKind.SemicolonClassElement:
                            // Can't be private
                            break;
                        default:
                            Debug.fail();
                    }
                }
            }
        }

        function checkUnusedTypeParameters(node: ClassLikeDeclaration | SignatureDeclaration | InterfaceDeclaration | TypeAliasDeclaration | InferTypeNode, addDiagnostic: AddUnusedDiagnostic): void {
            // Only report errors on the last declaration for the type parameter container;
            // this ensures that all uses have been accounted for.
            if (node.flags & NodeFlags.Ambient || node.kind !== SyntaxKind.InferType && last(getSymbolOfNode(node).declarations) !== node) return;

            if (node.kind === SyntaxKind.InferType) {
                const { typeParameter } = node;
                if (isTypeParameterUnused(typeParameter)) {
                    addDiagnostic(node, UnusedKind.Parameter, createDiagnosticForNode(node, Diagnostics._0_is_declared_but_its_value_is_never_read, idText(typeParameter.name)));
                }
            }
            else {
                const typeParameters = getEffectiveTypeParameterDeclarations(node);
                const seenParentsWithEveryUnused = new NodeSet<DeclarationWithTypeParameterChildren>();

                for (const typeParameter of typeParameters) {
                    if (!isTypeParameterUnused(typeParameter)) continue;

                    const name = idText(typeParameter.name);
                    const { parent } = typeParameter;
                    if (parent.kind !== SyntaxKind.InferType && parent.typeParameters!.every(isTypeParameterUnused)) {
                        if (seenParentsWithEveryUnused.tryAdd(parent)) {
                            const range = isJSDocTemplateTag(parent)
                                // Whole @template tag
                                ? rangeOfNode(parent)
                                // Include the `<>` in the error message
                                : rangeOfTypeParameters(parent.typeParameters!);
                            const only = typeParameters.length === 1;
                            const message = only ? Diagnostics._0_is_declared_but_its_value_is_never_read : Diagnostics.All_type_parameters_are_unused;
                            const arg0 = only ? name : undefined;
                            addDiagnostic(typeParameter, UnusedKind.Parameter, createFileDiagnostic(getSourceFileOfNode(parent), range.pos, range.end - range.pos, message, arg0));
                        }
                    }
                    else {
                        addDiagnostic(typeParameter, UnusedKind.Parameter, createDiagnosticForNode(typeParameter, Diagnostics._0_is_declared_but_its_value_is_never_read, name));
                    }
                }
            }
        }
        function isTypeParameterUnused(typeParameter: TypeParameterDeclaration): boolean {
            return !(getMergedSymbol(typeParameter.symbol).isReferenced! & SymbolFlags.TypeParameter) && !isIdentifierThatStartsWithUnderscore(typeParameter.name);
        }

        function addToGroup<K, V>(map: Map<[K, V[]]>, key: K, value: V, getKey: (key: K) => number | string): void {
            const keyString = String(getKey(key));
            const group = map.get(keyString);
            if (group) {
                group[1].push(value);
            }
            else {
                map.set(keyString, [key, [value]]);
            }
        }

        function tryGetRootParameterDeclaration(node: Node): ParameterDeclaration | undefined {
            return tryCast(getRootDeclaration(node), isParameter);
        }

        function checkUnusedLocalsAndParameters(nodeWithLocals: Node, addDiagnostic: AddUnusedDiagnostic): void {
            if (nodeWithLocals.flags & NodeFlags.Ambient) return;

            // Ideally we could use the ImportClause directly as a key, but must wait until we have full ES6 maps. So must store key along with value.
            const unusedImports = createMap<[ImportClause, ImportedDeclaration[]]>();
            const unusedDestructures = createMap<[ObjectBindingPattern, BindingElement[]]>();
            const unusedVariables = createMap<[VariableDeclarationList, VariableDeclaration[]]>();
            nodeWithLocals.locals!.forEach(local => {
                // If it's purely a type parameter, ignore, will be checked in `checkUnusedTypeParameters`.
                // If it's a type parameter merged with a parameter, check if the parameter-side is used.
                if (local.flags & SymbolFlags.TypeParameter ? !(local.flags & SymbolFlags.Variable && !(local.isReferenced! & SymbolFlags.Variable)) : local.isReferenced || local.exportSymbol) {
                    return;
                }

                for (const declaration of local.declarations) {
                    if (isAmbientModule(declaration) ||
                        (isVariableDeclaration(declaration) && isForInOrOfStatement(declaration.parent.parent) || isImportedDeclaration(declaration)) && isIdentifierThatStartsWithUnderscore(declaration.name!)) {
                        continue;
                    }

                    if (isImportedDeclaration(declaration)) {
                        addToGroup(unusedImports, importClauseFromImported(declaration), declaration, getNodeId);
                    }
                    else if (isBindingElement(declaration) && isObjectBindingPattern(declaration.parent)) {
                        // In `{ a, ...b }, `a` is considered used since it removes a property from `b`. `b` may still be unused though.
                        const lastElement = last(declaration.parent.elements);
                        if (declaration === lastElement || !last(declaration.parent.elements).dotDotDotToken) {
                            addToGroup(unusedDestructures, declaration.parent, declaration, getNodeId);
                        }
                    }
                    else if (isVariableDeclaration(declaration)) {
                        addToGroup(unusedVariables, declaration.parent, declaration, getNodeId);
                    }
                    else {
                        const parameter = local.valueDeclaration && tryGetRootParameterDeclaration(local.valueDeclaration);
                        const name = local.valueDeclaration && getNameOfDeclaration(local.valueDeclaration);
                        if (parameter && name) {
                            if (!isParameterPropertyDeclaration(parameter) && !parameterIsThisKeyword(parameter) && !isIdentifierThatStartsWithUnderscore(name)) {
                                addDiagnostic(parameter, UnusedKind.Parameter, createDiagnosticForNode(name, Diagnostics._0_is_declared_but_its_value_is_never_read, symbolName(local)));
                            }
                        }
                        else {
                            errorUnusedLocal(declaration, symbolName(local), addDiagnostic);
                        }
                    }
                }
            });
            unusedImports.forEach(([importClause, unuseds]) => {
                const importDecl = importClause.parent;
                const nDeclarations = (importClause.name ? 1 : 0) +
                    (importClause.namedBindings ?
                        (importClause.namedBindings.kind === SyntaxKind.NamespaceImport ? 1 : importClause.namedBindings.elements.length)
                        : 0);
                if (nDeclarations === unuseds.length) {
                    addDiagnostic(importDecl, UnusedKind.Local, unuseds.length === 1
                        ? createDiagnosticForNode(importDecl, Diagnostics._0_is_declared_but_its_value_is_never_read, idText(first(unuseds).name!))
                        : createDiagnosticForNode(importDecl, Diagnostics.All_imports_in_import_declaration_are_unused));
                }
                else {
                    for (const unused of unuseds) errorUnusedLocal(unused, idText(unused.name!), addDiagnostic);
                }
            });
            unusedDestructures.forEach(([bindingPattern, bindingElements]) => {
                const kind = tryGetRootParameterDeclaration(bindingPattern.parent) ? UnusedKind.Parameter : UnusedKind.Local;
                if (bindingPattern.elements.length === bindingElements.length) {
                    if (bindingElements.length === 1 && bindingPattern.parent.kind === SyntaxKind.VariableDeclaration && bindingPattern.parent.parent.kind === SyntaxKind.VariableDeclarationList) {
                        addToGroup(unusedVariables, bindingPattern.parent.parent, bindingPattern.parent, getNodeId);
                    }
                    else {
                        addDiagnostic(bindingPattern, kind, bindingElements.length === 1
                            ? createDiagnosticForNode(bindingPattern, Diagnostics._0_is_declared_but_its_value_is_never_read, bindingNameText(first(bindingElements).name))
                            : createDiagnosticForNode(bindingPattern, Diagnostics.All_destructured_elements_are_unused));
                    }
                }
                else {
                    for (const e of bindingElements) {
                        addDiagnostic(e, kind, createDiagnosticForNode(e, Diagnostics._0_is_declared_but_its_value_is_never_read, bindingNameText(e.name)));
                    }
                }
            });
            unusedVariables.forEach(([declarationList, declarations]) => {
                if (declarationList.declarations.length === declarations.length) {
                    addDiagnostic(declarationList, UnusedKind.Local, declarations.length === 1
                        ? createDiagnosticForNode(first(declarations).name, Diagnostics._0_is_declared_but_its_value_is_never_read, bindingNameText(first(declarations).name))
                        : createDiagnosticForNode(declarationList.parent.kind === SyntaxKind.VariableStatement ? declarationList.parent : declarationList, Diagnostics.All_variables_are_unused));
                }
                else {
                    for (const decl of declarations) {
                        addDiagnostic(decl, UnusedKind.Local, createDiagnosticForNode(decl, Diagnostics._0_is_declared_but_its_value_is_never_read, bindingNameText(decl.name)));
                    }
                }
            });
        }

        function bindingNameText(name: BindingName): string {
            switch (name.kind) {
                case SyntaxKind.Identifier:
                    return idText(name);
                case SyntaxKind.ArrayBindingPattern:
                case SyntaxKind.ObjectBindingPattern:
                    return bindingNameText(cast(first(name.elements), isBindingElement).name);
                default:
                    return Debug.assertNever(name);
            }
        }

        type ImportedDeclaration = ImportClause | ImportSpecifier | NamespaceImport;
        function isImportedDeclaration(node: Node): node is ImportedDeclaration {
            return node.kind === SyntaxKind.ImportClause || node.kind === SyntaxKind.ImportSpecifier || node.kind === SyntaxKind.NamespaceImport;
        }
        function importClauseFromImported(decl: ImportedDeclaration): ImportClause {
            return decl.kind === SyntaxKind.ImportClause ? decl : decl.kind === SyntaxKind.NamespaceImport ? decl.parent : decl.parent.parent;
        }

        function checkBlock(node: Block) {
            // Grammar checking for SyntaxKind.Block
            if (node.kind === SyntaxKind.Block) {
                checkGrammarStatementInAmbientContext(node);
            }
            if (isFunctionOrModuleBlock(node)) {
                const saveFlowAnalysisDisabled = flowAnalysisDisabled;
                forEach(node.statements, checkSourceElement);
                flowAnalysisDisabled = saveFlowAnalysisDisabled;
            }
            else {
                forEach(node.statements, checkSourceElement);
            }
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkCollisionWithArgumentsInGeneratedCode(node: SignatureDeclaration) {
            // no rest parameters \ declaration context \ overload - no codegen impact
            if (languageVersion >= ScriptTarget.ES2015 || compilerOptions.noEmit || !hasRestParameter(node) || node.flags & NodeFlags.Ambient || nodeIsMissing((<FunctionLikeDeclaration>node).body)) {
                return;
            }

            forEach(node.parameters, p => {
                if (p.name && !isBindingPattern(p.name) && p.name.escapedText === argumentsSymbol.escapedName) {
                    error(p, Diagnostics.Duplicate_identifier_arguments_Compiler_uses_arguments_to_initialize_rest_parameters);
                }
            });
        }

        function needCollisionCheckForIdentifier(node: Node, identifier: Identifier | undefined, name: string): boolean {
            if (!(identifier && identifier.escapedText === name)) {
                return false;
            }

            if (node.kind === SyntaxKind.PropertyDeclaration ||
                node.kind === SyntaxKind.PropertySignature ||
                node.kind === SyntaxKind.MethodDeclaration ||
                node.kind === SyntaxKind.MethodSignature ||
                node.kind === SyntaxKind.GetAccessor ||
                node.kind === SyntaxKind.SetAccessor) {
                // it is ok to have member named '_super' or '_this' - member access is always qualified
                return false;
            }

            if (node.flags & NodeFlags.Ambient) {
                // ambient context - no codegen impact
                return false;
            }

            const root = getRootDeclaration(node);
            if (root.kind === SyntaxKind.Parameter && nodeIsMissing((<FunctionLikeDeclaration>root.parent).body)) {
                // just an overload - no codegen impact
                return false;
            }

            return true;
        }

        // this function will run after checking the source file so 'CaptureThis' is correct for all nodes
        function checkIfThisIsCapturedInEnclosingScope(node: Node): void {
            findAncestor(node, current => {
                if (getNodeCheckFlags(current) & NodeCheckFlags.CaptureThis) {
                    const isDeclaration = node.kind !== SyntaxKind.Identifier;
                    if (isDeclaration) {
                        error(getNameOfDeclaration(<Declaration>node), Diagnostics.Duplicate_identifier_this_Compiler_uses_variable_declaration_this_to_capture_this_reference);
                    }
                    else {
                        error(node, Diagnostics.Expression_resolves_to_variable_declaration_this_that_compiler_uses_to_capture_this_reference);
                    }
                    return true;
                }
                return false;
            });
        }

        function checkIfNewTargetIsCapturedInEnclosingScope(node: Node): void {
            findAncestor(node, current => {
                if (getNodeCheckFlags(current) & NodeCheckFlags.CaptureNewTarget) {
                    const isDeclaration = node.kind !== SyntaxKind.Identifier;
                    if (isDeclaration) {
                        error(getNameOfDeclaration(<Declaration>node), Diagnostics.Duplicate_identifier_newTarget_Compiler_uses_variable_declaration_newTarget_to_capture_new_target_meta_property_reference);
                    }
                    else {
                        error(node, Diagnostics.Expression_resolves_to_variable_declaration_newTarget_that_compiler_uses_to_capture_new_target_meta_property_reference);
                    }
                    return true;
                }
                return false;
            });
        }

        function checkCollisionWithRequireExportsInGeneratedCode(node: Node, name: Identifier) {
            // No need to check for require or exports for ES6 modules and later
            if (moduleKind >= ModuleKind.ES2015 || compilerOptions.noEmit) {
                return;
            }

            if (!needCollisionCheckForIdentifier(node, name, "require") && !needCollisionCheckForIdentifier(node, name, "exports")) {
                return;
            }

            // Uninstantiated modules shouldnt do this check
            if (isModuleDeclaration(node) && getModuleInstanceState(node) !== ModuleInstanceState.Instantiated) {
                return;
            }

            // In case of variable declaration, node.parent is variable statement so look at the variable statement's parent
            const parent = getDeclarationContainer(node);
            if (parent.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>parent)) {
                // If the declaration happens to be in external module, report error that require and exports are reserved keywords
                error(name, Diagnostics.Duplicate_identifier_0_Compiler_reserves_name_1_in_top_level_scope_of_a_module,
                    declarationNameToString(name), declarationNameToString(name));
            }
        }

        function checkCollisionWithGlobalPromiseInGeneratedCode(node: Node, name: Identifier): void {
            if (languageVersion >= ScriptTarget.ES2017 || compilerOptions.noEmit || !needCollisionCheckForIdentifier(node, name, "Promise")) {
                return;
            }

            // Uninstantiated modules shouldnt do this check
            if (isModuleDeclaration(node) && getModuleInstanceState(node) !== ModuleInstanceState.Instantiated) {
                return;
            }

            // In case of variable declaration, node.parent is variable statement so look at the variable statement's parent
            const parent = getDeclarationContainer(node);
            if (parent.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>parent) && parent.flags & NodeFlags.HasAsyncFunctions) {
                // If the declaration happens to be in external module, report error that Promise is a reserved identifier.
                error(name, Diagnostics.Duplicate_identifier_0_Compiler_reserves_name_1_in_top_level_scope_of_a_module_containing_async_functions,
                    declarationNameToString(name), declarationNameToString(name));
            }
        }

        function checkVarDeclaredNamesNotShadowed(node: VariableDeclaration | BindingElement) {
            // - ScriptBody : StatementList
            // It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList
            // also occurs in the VarDeclaredNames of StatementList.

            // - Block : { StatementList }
            // It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList
            // also occurs in the VarDeclaredNames of StatementList.

            // Variable declarations are hoisted to the top of their function scope. They can shadow
            // block scoped declarations, which bind tighter. this will not be flagged as duplicate definition
            // by the binder as the declaration scope is different.
            // A non-initialized declaration is a no-op as the block declaration will resolve before the var
            // declaration. the problem is if the declaration has an initializer. this will act as a write to the
            // block declared value. this is fine for let, but not const.
            // Only consider declarations with initializers, uninitialized const declarations will not
            // step on a let/const variable.
            // Do not consider const and const declarations, as duplicate block-scoped declarations
            // are handled by the binder.
            // We are only looking for const declarations that step on let\const declarations from a
            // different scope. e.g.:
            //      {
            //          const x = 0; // localDeclarationSymbol obtained after name resolution will correspond to this declaration
            //          const x = 0; // symbol for this declaration will be 'symbol'
            //      }

            // skip block-scoped variables and parameters
            if ((getCombinedNodeFlags(node) & NodeFlags.BlockScoped) !== 0 || isParameterDeclaration(node)) {
                return;
            }

            // skip variable declarations that don't have initializers
            // NOTE: in ES6 spec initializer is required in variable declarations where name is binding pattern
            // so we'll always treat binding elements as initialized
            if (node.kind === SyntaxKind.VariableDeclaration && !node.initializer) {
                return;
            }

            const symbol = getSymbolOfNode(node);
            if (symbol.flags & SymbolFlags.FunctionScopedVariable) {
                if (!isIdentifier(node.name)) return Debug.fail();
                const localDeclarationSymbol = resolveName(node, node.name.escapedText, SymbolFlags.Variable, /*nodeNotFoundErrorMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ false);
                if (localDeclarationSymbol &&
                    localDeclarationSymbol !== symbol &&
                    localDeclarationSymbol.flags & SymbolFlags.BlockScopedVariable) {
                    if (getDeclarationNodeFlagsFromSymbol(localDeclarationSymbol) & NodeFlags.BlockScoped) {
                        const varDeclList = getAncestor(localDeclarationSymbol.valueDeclaration, SyntaxKind.VariableDeclarationList)!;
                        const container =
                            varDeclList.parent.kind === SyntaxKind.VariableStatement && varDeclList.parent.parent
                                ? varDeclList.parent.parent
                                : undefined;

                        // names of block-scoped and function scoped variables can collide only
                        // if block scoped variable is defined in the function\module\source file scope (because of variable hoisting)
                        const namesShareScope =
                            container &&
                            (container.kind === SyntaxKind.Block && isFunctionLike(container.parent) ||
                                container.kind === SyntaxKind.ModuleBlock ||
                                container.kind === SyntaxKind.ModuleDeclaration ||
                                container.kind === SyntaxKind.SourceFile);

                        // here we know that function scoped variable is shadowed by block scoped one
                        // if they are defined in the same scope - binder has already reported redeclaration error
                        // otherwise if variable has an initializer - show error that initialization will fail
                        // since LHS will be block scoped name instead of function scoped
                        if (!namesShareScope) {
                            const name = symbolToString(localDeclarationSymbol);
                            error(node, Diagnostics.Cannot_initialize_outer_scoped_variable_0_in_the_same_scope_as_block_scoped_declaration_1, name, name);
                        }
                    }
                }
            }
        }

        // Check that a parameter initializer contains no references to parameters declared to the right of itself
        function checkParameterInitializer(node: HasExpressionInitializer): void {
            if (getRootDeclaration(node).kind !== SyntaxKind.Parameter) {
                return;
            }

            const func = getContainingFunction(node);
            visit(node.initializer!);

            function visit(n: Node): void {
                if (isTypeNode(n) || isDeclarationName(n)) {
                    // do not dive in types
                    // skip declaration names (i.e. in object literal expressions)
                    return;
                }
                if (n.kind === SyntaxKind.PropertyAccessExpression) {
                    // skip property names in property access expression
                    return visit((<PropertyAccessExpression>n).expression);
                }
                else if (n.kind === SyntaxKind.Identifier) {
                    // check FunctionLikeDeclaration.locals (stores parameters\function local variable)
                    // if it contains entry with a specified name
                    const symbol = resolveName(n, (<Identifier>n).escapedText, SymbolFlags.Value | SymbolFlags.Alias, /*nameNotFoundMessage*/undefined, /*nameArg*/undefined, /*isUse*/ false);
                    if (!symbol || symbol === unknownSymbol || !symbol.valueDeclaration) {
                        return;
                    }
                    if (symbol.valueDeclaration === node) {
                        error(n, Diagnostics.Parameter_0_cannot_be_referenced_in_its_initializer, declarationNameToString(node.name));
                        return;
                    }
                    // locals map for function contain both parameters and function locals
                    // so we need to do a bit of extra work to check if reference is legal
                    const enclosingContainer = getEnclosingBlockScopeContainer(symbol.valueDeclaration);
                    if (enclosingContainer === func) {
                        if (symbol.valueDeclaration.kind === SyntaxKind.Parameter ||
                           symbol.valueDeclaration.kind === SyntaxKind.BindingElement) {
                            // it is ok to reference parameter in initializer if either
                            // - parameter is located strictly on the left of current parameter declaration
                            if (symbol.valueDeclaration.pos < node.pos) {
                                return;
                            }
                            // - parameter is wrapped in function-like entity
                            if (findAncestor(
                                n,
                                current => {
                                    if (current === node.initializer) {
                                        return "quit";
                                    }
                                    return isFunctionLike(current.parent) ||
                                        // computed property names/initializers in instance property declaration of class like entities
                                        // are executed in constructor and thus deferred
                                        (current.parent.kind === SyntaxKind.PropertyDeclaration &&
                                         !(hasModifier(current.parent, ModifierFlags.Static)) &&
                                         isClassLike(current.parent.parent));
                                    })) {
                                return;
                            }
                            // fall through to report error
                        }
                        error(n, Diagnostics.Initializer_of_parameter_0_cannot_reference_identifier_1_declared_after_it, declarationNameToString(node.name), declarationNameToString(<Identifier>n));
                    }
                }
                else {
                    return forEachChild(n, visit);
                }
            }
        }

        function convertAutoToAny(type: Type) {
            return type === autoType ? anyType : type === autoArrayType ? anyArrayType : type;
        }

        // Check variable, parameter, or property declaration
        function checkVariableLikeDeclaration(node: ParameterDeclaration | PropertyDeclaration | PropertySignature | VariableDeclaration | BindingElement) {
            checkDecorators(node);
            if (!isBindingElement(node)) {
                checkSourceElement(node.type);
            }

            // JSDoc `function(string, string): string` syntax results in parameters with no name
            if (!node.name) {
                return;
            }
            // For a computed property, just check the initializer and exit
            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                checkComputedPropertyName(node.name);
                if (node.initializer) {
                    checkExpressionCached(node.initializer);
                }
            }

            if (node.kind === SyntaxKind.BindingElement) {
                if (node.parent.kind === SyntaxKind.ObjectBindingPattern && languageVersion < ScriptTarget.ESNext) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.Rest);
                }
                // check computed properties inside property names of binding elements
                if (node.propertyName && node.propertyName.kind === SyntaxKind.ComputedPropertyName) {
                    checkComputedPropertyName(node.propertyName);
                }

                // check private/protected variable access
                const parent = node.parent.parent;
                const parentType = getTypeForBindingElementParent(parent);
                const name = node.propertyName || node.name;
                if (!isBindingPattern(name)) {
                    const nameText = getTextOfPropertyName(name);
                    if (nameText) {
                        const property = getPropertyOfType(parentType!, nameText)!; // TODO: GH#18217
                        markPropertyAsReferenced(property, /*nodeForCheckWriteOnly*/ undefined, /*isThisAccess*/ false); // A destructuring is never a write-only reference.
                        if (parent.initializer && property) {
                            checkPropertyAccessibility(parent, parent.initializer.kind === SyntaxKind.SuperKeyword, parentType!, property);
                        }
                    }
                }
            }

            // For a binding pattern, check contained binding elements
            if (isBindingPattern(node.name)) {
                if (node.name.kind === SyntaxKind.ArrayBindingPattern && languageVersion < ScriptTarget.ES2015 && compilerOptions.downlevelIteration) {
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.Read);
                }

                forEach(node.name.elements, checkSourceElement);
            }
            // For a parameter declaration with an initializer, error and exit if the containing function doesn't have a body
            if (node.initializer && getRootDeclaration(node).kind === SyntaxKind.Parameter && nodeIsMissing((getContainingFunction(node) as FunctionLikeDeclaration).body)) {
                error(node, Diagnostics.A_parameter_initializer_is_only_allowed_in_a_function_or_constructor_implementation);
                return;
            }
            // For a binding pattern, validate the initializer and exit
            if (isBindingPattern(node.name)) {
                // Don't validate for-in initializer as it is already an error
                if (node.initializer && node.parent.parent.kind !== SyntaxKind.ForInStatement) {
                    const initializerType = checkExpressionCached(node.initializer);
                    if (strictNullChecks && node.name.elements.length === 0) {
                        checkNonNullType(initializerType, node);
                    }
                    else {
                        checkTypeAssignableToAndOptionallyElaborate(initializerType, getWidenedTypeForVariableLikeDeclaration(node), node, node.initializer);
                    }
                    checkParameterInitializer(node);
                }
                return;
            }
            const symbol = getSymbolOfNode(node);
            const type = convertAutoToAny(getTypeOfSymbol(symbol));
            if (node === symbol.valueDeclaration) {
                // Node is the primary declaration of the symbol, just validate the initializer
                // Don't validate for-in initializer as it is already an error
                const initializer = getEffectiveInitializer(node);
                if (initializer) {
                    const isJSObjectLiteralInitializer = isInJSFile(node) &&
                        isObjectLiteralExpression(initializer) &&
                        (initializer.properties.length === 0 || isPrototypeAccess(node.name)) &&
                        hasEntries(symbol.exports);
                    if (!isJSObjectLiteralInitializer && node.parent.parent.kind !== SyntaxKind.ForInStatement) {
                        checkTypeAssignableToAndOptionallyElaborate(checkExpressionCached(initializer), type, node, initializer, /*headMessage*/ undefined);
                        checkParameterInitializer(node);
                    }
                }
            }
            else {
                // Node is a secondary declaration, check that type is identical to primary declaration and check that
                // initializer is consistent with type associated with the node
                const declarationType = convertAutoToAny(getWidenedTypeForVariableLikeDeclaration(node));

                if (type !== errorType && declarationType !== errorType &&
                    !isTypeIdenticalTo(type, declarationType) &&
                    !(symbol.flags & SymbolFlags.Assignment)) {
                    errorNextVariableOrPropertyDeclarationMustHaveSameType(type, node, declarationType);
                }
                if (node.initializer) {
                    checkTypeAssignableToAndOptionallyElaborate(checkExpressionCached(node.initializer), declarationType, node, node.initializer, /*headMessage*/ undefined);
                }
                if (!areDeclarationFlagsIdentical(node, symbol.valueDeclaration)) {
                    error(getNameOfDeclaration(symbol.valueDeclaration), Diagnostics.All_declarations_of_0_must_have_identical_modifiers, declarationNameToString(node.name));
                    error(node.name, Diagnostics.All_declarations_of_0_must_have_identical_modifiers, declarationNameToString(node.name));
                }
            }
            if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature) {
                // We know we don't have a binding pattern or computed name here
                checkExportsOnMergedDeclarations(node);
                if (node.kind === SyntaxKind.VariableDeclaration || node.kind === SyntaxKind.BindingElement) {
                    checkVarDeclaredNamesNotShadowed(node);
                }
                checkCollisionWithRequireExportsInGeneratedCode(node, <Identifier>node.name);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, <Identifier>node.name);
            }
        }

        function errorNextVariableOrPropertyDeclarationMustHaveSameType(firstType: Type, nextDeclaration: Declaration, nextType: Type): void {
            const nextDeclarationName = getNameOfDeclaration(nextDeclaration);
            const message = nextDeclaration.kind === SyntaxKind.PropertyDeclaration || nextDeclaration.kind === SyntaxKind.PropertySignature
                ? Diagnostics.Subsequent_property_declarations_must_have_the_same_type_Property_0_must_be_of_type_1_but_here_has_type_2
                : Diagnostics.Subsequent_variable_declarations_must_have_the_same_type_Variable_0_must_be_of_type_1_but_here_has_type_2;
            error(
                nextDeclarationName,
                message,
                declarationNameToString(nextDeclarationName),
                typeToString(firstType),
                typeToString(nextType));
        }

        function areDeclarationFlagsIdentical(left: Declaration, right: Declaration) {
            if ((left.kind === SyntaxKind.Parameter && right.kind === SyntaxKind.VariableDeclaration) ||
                (left.kind === SyntaxKind.VariableDeclaration && right.kind === SyntaxKind.Parameter)) {
                // Differences in optionality between parameters and variables are allowed.
                return true;
            }

            if (hasQuestionToken(left) !== hasQuestionToken(right)) {
                return false;
            }

            const interestingFlags = ModifierFlags.Private |
                ModifierFlags.Protected |
                ModifierFlags.Async |
                ModifierFlags.Abstract |
                ModifierFlags.Readonly |
                ModifierFlags.Static;

            return getSelectedModifierFlags(left, interestingFlags) === getSelectedModifierFlags(right, interestingFlags);
        }

        function checkVariableDeclaration(node: VariableDeclaration) {
            checkGrammarVariableDeclaration(node);
            return checkVariableLikeDeclaration(node);
        }

        function checkBindingElement(node: BindingElement) {
            checkGrammarBindingElement(node);
            return checkVariableLikeDeclaration(node);
        }

        function checkVariableStatement(node: VariableStatement) {
            // Grammar checking
            if (!checkGrammarDecoratorsAndModifiers(node) && !checkGrammarVariableDeclarationList(node.declarationList)) checkGrammarForDisallowedLetOrConstStatement(node);
            forEach(node.declarationList.declarations, checkSourceElement);
        }

        function checkExpressionStatement(node: ExpressionStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkExpression(node.expression);
        }

        function checkIfStatement(node: IfStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkTruthinessExpression(node.expression);
            checkSourceElement(node.thenStatement);

            if (node.thenStatement.kind === SyntaxKind.EmptyStatement) {
                error(node.thenStatement, Diagnostics.The_body_of_an_if_statement_cannot_be_the_empty_statement);
            }

            checkSourceElement(node.elseStatement);
        }

        function checkDoStatement(node: DoStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkSourceElement(node.statement);
            checkTruthinessExpression(node.expression);
        }

        function checkWhileStatement(node: WhileStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkTruthinessExpression(node.expression);
            checkSourceElement(node.statement);
        }

        function checkTruthinessExpression(node: Expression, checkMode?: CheckMode) {
            const type = checkExpression(node, checkMode);
            if (type.flags & TypeFlags.Void) {
                error(node, Diagnostics.An_expression_of_type_void_cannot_be_tested_for_truthiness);
            }
            return type;
        }

        function checkForStatement(node: ForStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                if (node.initializer && node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                    checkGrammarVariableDeclarationList(<VariableDeclarationList>node.initializer);
                }
            }

            if (node.initializer) {
                if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                    forEach((<VariableDeclarationList>node.initializer).declarations, checkVariableDeclaration);
                }
                else {
                    checkExpression(node.initializer);
                }
            }

            if (node.condition) checkTruthinessExpression(node.condition);
            if (node.incrementor) checkExpression(node.incrementor);
            checkSourceElement(node.statement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkForOfStatement(node: ForOfStatement): void {
            checkGrammarForInOrForOfStatement(node);

            if (node.awaitModifier) {
                const functionFlags = getFunctionFlags(getContainingFunction(node));
                if ((functionFlags & (FunctionFlags.Invalid | FunctionFlags.Async)) === FunctionFlags.Async && languageVersion < ScriptTarget.ESNext) {
                    // for..await..of in an async function or async generator function prior to ESNext requires the __asyncValues helper
                    checkExternalEmitHelpers(node, ExternalEmitHelpers.ForAwaitOfIncludes);
                }
            }
            else if (compilerOptions.downlevelIteration && languageVersion < ScriptTarget.ES2015) {
                // for..of prior to ES2015 requires the __values helper when downlevelIteration is enabled
                checkExternalEmitHelpers(node, ExternalEmitHelpers.ForOfIncludes);
            }

            // Check the LHS and RHS
            // If the LHS is a declaration, just check it as a variable declaration, which will in turn check the RHS
            // via checkRightHandSideOfForOf.
            // If the LHS is an expression, check the LHS, as a destructuring assignment or as a reference.
            // Then check that the RHS is assignable to it.
            if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                checkForInOrForOfVariableDeclaration(node);
            }
            else {
                const varExpr = node.initializer;
                const iteratedType = checkRightHandSideOfForOf(node.expression, node.awaitModifier);

                // There may be a destructuring assignment on the left side
                if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
                    // iteratedType may be undefined. In this case, we still want to check the structure of
                    // varExpr, in particular making sure it's a valid LeftHandSideExpression. But we'd like
                    // to short circuit the type relation checking as much as possible, so we pass the unknownType.
                    checkDestructuringAssignment(varExpr, iteratedType || errorType);
                }
                else {
                    const leftType = checkExpression(varExpr);
                    checkReferenceExpression(varExpr, Diagnostics.The_left_hand_side_of_a_for_of_statement_must_be_a_variable_or_a_property_access);

                    // iteratedType will be undefined if the rightType was missing properties/signatures
                    // required to get its iteratedType (like [Symbol.iterator] or next). This may be
                    // because we accessed properties from anyType, or it may have led to an error inside
                    // getElementTypeOfIterable.
                    if (iteratedType) {
                        checkTypeAssignableToAndOptionallyElaborate(iteratedType, leftType, varExpr, node.expression);
                    }
                }
            }

            checkSourceElement(node.statement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkForInStatement(node: ForInStatement) {
            // Grammar checking
            checkGrammarForInOrForOfStatement(node);

            const rightType = checkNonNullExpression(node.expression);
            // TypeScript 1.0 spec (April 2014): 5.4
            // In a 'for-in' statement of the form
            // for (let VarDecl in Expr) Statement
            //   VarDecl must be a variable declaration without a type annotation that declares a variable of type Any,
            //   and Expr must be an expression of type Any, an object type, or a type parameter type.
            if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                const variable = (<VariableDeclarationList>node.initializer).declarations[0];
                if (variable && isBindingPattern(variable.name)) {
                    error(variable.name, Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern);
                }
                checkForInOrForOfVariableDeclaration(node);
            }
            else {
                // In a 'for-in' statement of the form
                // for (Var in Expr) Statement
                //   Var must be an expression classified as a reference of type Any or the String primitive type,
                //   and Expr must be an expression of type Any, an object type, or a type parameter type.
                const varExpr = node.initializer;
                const leftType = checkExpression(varExpr);
                if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
                    error(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern);
                }
                else if (!isTypeAssignableTo(getIndexTypeOrString(rightType), leftType)) {
                    error(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_must_be_of_type_string_or_any);
                }
                else {
                    // run check only former check succeeded to avoid cascading errors
                    checkReferenceExpression(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_must_be_a_variable_or_a_property_access);
                }
            }

            // unknownType is returned i.e. if node.expression is identifier whose name cannot be resolved
            // in this case error about missing name is already reported - do not report extra one
            if (rightType === neverType || !isTypeAssignableToKind(rightType, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive)) {
                error(node.expression, Diagnostics.The_right_hand_side_of_a_for_in_statement_must_be_of_type_any_an_object_type_or_a_type_parameter_but_here_has_type_0, typeToString(rightType));
            }

            checkSourceElement(node.statement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkForInOrForOfVariableDeclaration(iterationStatement: ForInOrOfStatement): void {
            const variableDeclarationList = <VariableDeclarationList>iterationStatement.initializer;
            // checkGrammarForInOrForOfStatement will check that there is exactly one declaration.
            if (variableDeclarationList.declarations.length >= 1) {
                const decl = variableDeclarationList.declarations[0];
                checkVariableDeclaration(decl);
            }
        }

        function checkRightHandSideOfForOf(rhsExpression: Expression, awaitModifier: AwaitKeywordToken | undefined): Type {
            const expressionType = checkNonNullExpression(rhsExpression);
            return checkIteratedTypeOrElementType(expressionType, rhsExpression, /*allowStringInput*/ true, awaitModifier !== undefined);
        }

        function checkIteratedTypeOrElementType(inputType: Type, errorNode: Node | undefined, allowStringInput: boolean, allowAsyncIterables: boolean): Type {
            if (isTypeAny(inputType)) {
                return inputType;
            }

            return getIteratedTypeOrElementType(inputType, errorNode, allowStringInput, allowAsyncIterables, /*checkAssignability*/ true) || anyType;
        }

        /**
         * When consuming an iterable type in a for..of, spread, or iterator destructuring assignment
         * we want to get the iterated type of an iterable for ES2015 or later, or the iterated type
         * of a iterable (if defined globally) or element type of an array like for ES2015 or earlier.
         */
        function getIteratedTypeOrElementType(inputType: Type, errorNode: Node | undefined, allowStringInput: boolean, allowAsyncIterables: boolean, checkAssignability: boolean): Type | undefined {
            if (inputType === neverType) {
                reportTypeNotIterableError(errorNode!, inputType, allowAsyncIterables); // TODO: GH#18217
                return undefined;
            }

            const uplevelIteration = languageVersion >= ScriptTarget.ES2015;
            const downlevelIteration = !uplevelIteration && compilerOptions.downlevelIteration;

            // Get the iterated type of an `Iterable<T>` or `IterableIterator<T>` only in ES2015
            // or higher, when inside of an async generator or for-await-if, or when
            // downlevelIteration is requested.
            if (uplevelIteration || downlevelIteration || allowAsyncIterables) {
                // We only report errors for an invalid iterable type in ES2015 or higher.
                const iteratedType = getIteratedTypeOfIterable(inputType, uplevelIteration ? errorNode : undefined, allowAsyncIterables, /*allowSyncIterables*/ true, checkAssignability);
                if (iteratedType || uplevelIteration) {
                    return iteratedType;
                }
            }

            let arrayType = inputType;
            let reportedError = false;
            let hasStringConstituent = false;

            // If strings are permitted, remove any string-like constituents from the array type.
            // This allows us to find other non-string element types from an array unioned with
            // a string.
            if (allowStringInput) {
                if (arrayType.flags & TypeFlags.Union) {
                    // After we remove all types that are StringLike, we will know if there was a string constituent
                    // based on whether the result of filter is a new array.
                    const arrayTypes = (<UnionType>inputType).types;
                    const filteredTypes = filter(arrayTypes, t => !(t.flags & TypeFlags.StringLike));
                    if (filteredTypes !== arrayTypes) {
                        arrayType = getUnionType(filteredTypes, UnionReduction.Subtype);
                    }
                }
                else if (arrayType.flags & TypeFlags.StringLike) {
                    arrayType = neverType;
                }

                hasStringConstituent = arrayType !== inputType;
                if (hasStringConstituent) {
                    if (languageVersion < ScriptTarget.ES5) {
                        if (errorNode) {
                            error(errorNode, Diagnostics.Using_a_string_in_a_for_of_statement_is_only_supported_in_ECMAScript_5_and_higher);
                            reportedError = true;
                        }
                    }

                    // Now that we've removed all the StringLike types, if no constituents remain, then the entire
                    // arrayOrStringType was a string.
                    if (arrayType.flags & TypeFlags.Never) {
                        return stringType;
                    }
                }
            }

            if (!isArrayLikeType(arrayType)) {
                if (errorNode && !reportedError) {
                    // Which error we report depends on whether we allow strings or if there was a
                    // string constituent. For example, if the input type is number | string, we
                    // want to say that number is not an array type. But if the input was just
                    // number and string input is allowed, we want to say that number is not an
                    // array type or a string type.
                    const isIterable = !!getIteratedTypeOfIterable(inputType, /* errorNode */ undefined, allowAsyncIterables, /*allowSyncIterables*/ true, checkAssignability);
                    const diagnostic = !allowStringInput || hasStringConstituent
                        ? downlevelIteration
                            ? Diagnostics.Type_0_is_not_an_array_type_or_does_not_have_a_Symbol_iterator_method_that_returns_an_iterator
                            : isIterable
                                ? Diagnostics.Type_0_is_not_an_array_type_Use_compiler_option_downlevelIteration_to_allow_iterating_of_iterators
                                : Diagnostics.Type_0_is_not_an_array_type
                        : downlevelIteration
                            ? Diagnostics.Type_0_is_not_an_array_type_or_a_string_type_or_does_not_have_a_Symbol_iterator_method_that_returns_an_iterator
                            : isIterable
                                ? Diagnostics.Type_0_is_not_an_array_type_or_a_string_type_Use_compiler_option_downlevelIteration_to_allow_iterating_of_iterators
                                : Diagnostics.Type_0_is_not_an_array_type_or_a_string_type;
                    error(errorNode, diagnostic, typeToString(arrayType));
                }
                return hasStringConstituent ? stringType : undefined;
            }

            const arrayElementType = getIndexTypeOfType(arrayType, IndexKind.Number);
            if (hasStringConstituent && arrayElementType) {
                // This is just an optimization for the case where arrayOrStringType is string | string[]
                if (arrayElementType.flags & TypeFlags.StringLike) {
                    return stringType;
                }

                return getUnionType([arrayElementType, stringType], UnionReduction.Subtype);
            }

            return arrayElementType;
        }

        /**
         * We want to treat type as an iterable, and get the type it is an iterable of. The iterable
         * must have the following structure (annotated with the names of the variables below):
         *
         *     { // iterable
         *         [Symbol.iterator]: { // iteratorMethod
         *             (): Iterator<T>
         *         }
         *     }
         *
         * For an async iterable, we expect the following structure:
         *
         *     { // iterable
         *         [Symbol.asyncIterator]: { // iteratorMethod
         *             (): AsyncIterator<T>
         *         }
         *     }
         *
         * T is the type we are after. At every level that involves analyzing return types
         * of signatures, we union the return types of all the signatures.
         *
         * Another thing to note is that at any step of this process, we could run into a dead end,
         * meaning either the property is missing, or we run into the anyType. If either of these things
         * happens, we return undefined to signal that we could not find the iterated type. If a property
         * is missing, and the previous step did not result in 'any', then we also give an error if the
         * caller requested it. Then the caller can decide what to do in the case where there is no iterated
         * type. This is different from returning anyType, because that would signify that we have matched the
         * whole pattern and that T (above) is 'any'.
         *
         * For a **for-of** statement, `yield*` (in a normal generator), spread, array
         * destructuring, or normal generator we will only ever look for a `[Symbol.iterator]()`
         * method.
         *
         * For an async generator we will only ever look at the `[Symbol.asyncIterator]()` method.
         *
         * For a **for-await-of** statement or a `yield*` in an async generator we will look for
         * the `[Symbol.asyncIterator]()` method first, and then the `[Symbol.iterator]()` method.
         */
        function getIteratedTypeOfIterable(type: Type, errorNode: Node | undefined, allowAsyncIterables: boolean, allowSyncIterables: boolean, checkAssignability: boolean): Type | undefined {
            if (isTypeAny(type)) {
                return undefined;
            }

            return mapType(type, getIteratedType);

            function getIteratedType(type: Type) {
                const typeAsIterable = <IterableOrIteratorType>type;
                if (allowAsyncIterables) {
                    if (typeAsIterable.iteratedTypeOfAsyncIterable) {
                        return typeAsIterable.iteratedTypeOfAsyncIterable;
                    }

                    // As an optimization, if the type is an instantiation of the global `AsyncIterable<T>`
                    // or the global `AsyncIterableIterator<T>` then just grab its type argument.
                    if (isReferenceToType(type, getGlobalAsyncIterableType(/*reportErrors*/ false)) ||
                        isReferenceToType(type, getGlobalAsyncIterableIteratorType(/*reportErrors*/ false))) {
                        return typeAsIterable.iteratedTypeOfAsyncIterable = (<GenericType>type).typeArguments![0];
                    }
                }

                if (allowSyncIterables) {
                    if (typeAsIterable.iteratedTypeOfIterable) {
                        return typeAsIterable.iteratedTypeOfIterable;
                    }

                    // As an optimization, if the type is an instantiation of the global `Iterable<T>` or
                    // `IterableIterator<T>` then just grab its type argument.
                    if (isReferenceToType(type, getGlobalIterableType(/*reportErrors*/ false)) ||
                        isReferenceToType(type, getGlobalIterableIteratorType(/*reportErrors*/ false))) {
                        return typeAsIterable.iteratedTypeOfIterable = (<GenericType>type).typeArguments![0];
                    }
                }

                const asyncMethodType = allowAsyncIterables && getTypeOfPropertyOfType(type, getPropertyNameForKnownSymbolName("asyncIterator"));
                const methodType = asyncMethodType || (allowSyncIterables ? getTypeOfPropertyOfType(type, getPropertyNameForKnownSymbolName("iterator")) : undefined);
                if (isTypeAny(methodType)) {
                    return undefined;
                }

                const signatures = methodType ? getSignaturesOfType(methodType, SignatureKind.Call) : undefined;
                if (!some(signatures)) {
                    if (errorNode) {
                        // only report on the first error
                        reportTypeNotIterableError(errorNode, type, allowAsyncIterables);
                        errorNode = undefined;
                    }
                    return undefined;
                }

                const returnType = getUnionType(map(signatures, getReturnTypeOfSignature), UnionReduction.Subtype);
                const iteratedType = getIteratedTypeOfIterator(returnType, errorNode, /*isAsyncIterator*/ !!asyncMethodType);
                if (checkAssignability && errorNode && iteratedType) {
                    // If `checkAssignability` was specified, we were called from
                    // `checkIteratedTypeOrElementType`. As such, we need to validate that
                    // the type passed in is actually an Iterable.
                    checkTypeAssignableTo(type, asyncMethodType
                        ? createAsyncIterableType(iteratedType)
                        : createIterableType(iteratedType), errorNode);
                }

                return asyncMethodType
                    ? typeAsIterable.iteratedTypeOfAsyncIterable = iteratedType
                    : typeAsIterable.iteratedTypeOfIterable = iteratedType;
            }
        }

        function reportTypeNotIterableError(errorNode: Node, type: Type, allowAsyncIterables: boolean): void {
            error(errorNode, allowAsyncIterables
                ? Diagnostics.Type_0_must_have_a_Symbol_asyncIterator_method_that_returns_an_async_iterator
                : Diagnostics.Type_0_must_have_a_Symbol_iterator_method_that_returns_an_iterator, typeToString(type));
        }

        /**
         * This function has very similar logic as getIteratedTypeOfIterable, except that it operates on
         * Iterators instead of Iterables. Here is the structure:
         *
         *  { // iterator
         *      next: { // nextMethod
         *          (): { // nextResult
         *              value: T // nextValue
         *          }
         *      }
         *  }
         *
         * For an async iterator, we expect the following structure:
         *
         *  { // iterator
         *      next: { // nextMethod
         *          (): PromiseLike<{ // nextResult
         *              value: T // nextValue
         *          }>
         *      }
         *  }
         */
        function getIteratedTypeOfIterator(type: Type, errorNode: Node | undefined, isAsyncIterator: boolean): Type | undefined {
            if (isTypeAny(type)) {
                return undefined;
            }

            const typeAsIterator = <IterableOrIteratorType>type;
            if (isAsyncIterator ? typeAsIterator.iteratedTypeOfAsyncIterator : typeAsIterator.iteratedTypeOfIterator) {
                return isAsyncIterator ? typeAsIterator.iteratedTypeOfAsyncIterator : typeAsIterator.iteratedTypeOfIterator;
            }

            // As an optimization, if the type is an instantiation of the global `Iterator<T>` (for
            // a non-async iterator) or the global `AsyncIterator<T>` (for an async-iterator) then
            // just grab its type argument.
            const getIteratorType = isAsyncIterator ? getGlobalAsyncIteratorType : getGlobalIteratorType;
            if (isReferenceToType(type, getIteratorType(/*reportErrors*/ false))) {
                return isAsyncIterator
                    ? typeAsIterator.iteratedTypeOfAsyncIterator = (<GenericType>type).typeArguments![0]
                    : typeAsIterator.iteratedTypeOfIterator = (<GenericType>type).typeArguments![0];
            }

            // Both async and non-async iterators must have a `next` method.
            const nextMethod = getTypeOfPropertyOfType(type, "next" as __String);
            if (isTypeAny(nextMethod)) {
                return undefined;
            }

            const nextMethodSignatures = nextMethod ? getSignaturesOfType(nextMethod, SignatureKind.Call) : emptyArray;
            if (nextMethodSignatures.length === 0) {
                if (errorNode) {
                    error(errorNode, isAsyncIterator
                        ? Diagnostics.An_async_iterator_must_have_a_next_method
                        : Diagnostics.An_iterator_must_have_a_next_method);
                }
                return undefined;
            }

            let nextResult: Type | undefined = getUnionType(map(nextMethodSignatures, getReturnTypeOfSignature), UnionReduction.Subtype);
            if (isTypeAny(nextResult)) {
                return undefined;
            }

            // For an async iterator, we must get the awaited type of the return type.
            if (isAsyncIterator) {
                nextResult = getAwaitedTypeOfPromise(nextResult, errorNode, Diagnostics.The_type_returned_by_the_next_method_of_an_async_iterator_must_be_a_promise_for_a_type_with_a_value_property);
                if (isTypeAny(nextResult)) {
                    return undefined;
                }
            }

            const nextValue = nextResult && getTypeOfPropertyOfType(nextResult, "value" as __String);
            if (!nextValue) {
                if (errorNode) {
                    error(errorNode, isAsyncIterator
                        ? Diagnostics.The_type_returned_by_the_next_method_of_an_async_iterator_must_be_a_promise_for_a_type_with_a_value_property
                        : Diagnostics.The_type_returned_by_the_next_method_of_an_iterator_must_have_a_value_property);
                }
                return undefined;
            }

            return isAsyncIterator
                ? typeAsIterator.iteratedTypeOfAsyncIterator = nextValue
                : typeAsIterator.iteratedTypeOfIterator = nextValue;
        }

        /**
         * A generator may have a return type of `Iterator<T>`, `Iterable<T>`, or
         * `IterableIterator<T>`. An async generator may have a return type of `AsyncIterator<T>`,
         * `AsyncIterable<T>`, or `AsyncIterableIterator<T>`. This function can be used to extract
         * the iterated type from this return type for contextual typing and verifying signatures.
         */
        function getIteratedTypeOfGenerator(returnType: Type, isAsyncGenerator: boolean): Type | undefined {
            if (isTypeAny(returnType)) {
                return undefined;
            }

            return getIteratedTypeOfIterable(returnType, /*errorNode*/ undefined, /*allowAsyncIterables*/ isAsyncGenerator, /*allowSyncIterables*/ !isAsyncGenerator, /*checkAssignability*/ false)
                || getIteratedTypeOfIterator(returnType, /*errorNode*/ undefined, isAsyncGenerator);
        }

        function checkBreakOrContinueStatement(node: BreakOrContinueStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) checkGrammarBreakOrContinueStatement(node);

            // TODO: Check that target label is valid
        }

        function isUnwrappedReturnTypeVoidOrAny(func: SignatureDeclaration, returnType: Type): boolean {
            const unwrappedReturnType = (getFunctionFlags(func) & FunctionFlags.AsyncGenerator) === FunctionFlags.Async
                ? getPromisedTypeOfPromise(returnType) // Async function
                : returnType; // AsyncGenerator function, Generator function, or normal function
            return !!unwrappedReturnType && maybeTypeOfKind(unwrappedReturnType, TypeFlags.Void | TypeFlags.AnyOrUnknown);
        }

        function checkReturnStatement(node: ReturnStatement) {
            // Grammar checking
            if (checkGrammarStatementInAmbientContext(node)) {
                return;
            }

            const func = getContainingFunction(node);
            if (!func) {
                grammarErrorOnFirstToken(node, Diagnostics.A_return_statement_can_only_be_used_within_a_function_body);
                return;
            }

            const signature = getSignatureFromDeclaration(func);
            const returnType = getReturnTypeOfSignature(signature);
            const functionFlags = getFunctionFlags(func);
            const isGenerator = functionFlags & FunctionFlags.Generator;
            if (strictNullChecks || node.expression || returnType.flags & TypeFlags.Never) {
                const exprType = node.expression ? checkExpressionCached(node.expression) : undefinedType;
                if (isGenerator) { // AsyncGenerator function or Generator function
                    // A generator does not need its return expressions checked against its return type.
                    // Instead, the yield expressions are checked against the element type.
                    // TODO: Check return types of generators when return type tracking is added
                    // for generators.
                    return;
                }
                else if (func.kind === SyntaxKind.SetAccessor) {
                    if (node.expression) {
                        error(node, Diagnostics.Setters_cannot_return_a_value);
                    }
                }
                else if (func.kind === SyntaxKind.Constructor) {
                    if (node.expression && !checkTypeAssignableToAndOptionallyElaborate(exprType, returnType, node, node.expression)) {
                        error(node, Diagnostics.Return_type_of_constructor_signature_must_be_assignable_to_the_instance_type_of_the_class);
                    }
                }
                else if (getReturnTypeFromAnnotation(func)) {
                    if (functionFlags & FunctionFlags.Async) { // Async function
                        const promisedType = getPromisedTypeOfPromise(returnType);
                        const awaitedType = checkAwaitedType(exprType, node, Diagnostics.The_return_type_of_an_async_function_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
                        if (promisedType) {
                            // If the function has a return type, but promisedType is
                            // undefined, an error will be reported in checkAsyncFunctionReturnType
                            // so we don't need to report one here.
                            checkTypeAssignableTo(awaitedType, promisedType, node);
                        }
                    }
                    else {
                        checkTypeAssignableToAndOptionallyElaborate(exprType, returnType, node, node.expression);
                    }
                }
            }
            else if (func.kind !== SyntaxKind.Constructor && compilerOptions.noImplicitReturns && !isUnwrappedReturnTypeVoidOrAny(func, returnType) && !isGenerator) {
                // The function has a return type, but the return statement doesn't have an expression.
                error(node, Diagnostics.Not_all_code_paths_return_a_value);
            }
        }

        function checkWithStatement(node: WithStatement) {
            // Grammar checking for withStatement
            if (!checkGrammarStatementInAmbientContext(node)) {
                if (node.flags & NodeFlags.AwaitContext) {
                    grammarErrorOnFirstToken(node, Diagnostics.with_statements_are_not_allowed_in_an_async_function_block);
                }
            }

            checkExpression(node.expression);

            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                const start = getSpanOfTokenAtPosition(sourceFile, node.pos).start;
                const end = node.statement.pos;
                grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.The_with_statement_is_not_supported_All_symbols_in_a_with_block_will_have_type_any);
            }
        }

        function checkSwitchStatement(node: SwitchStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            let firstDefaultClause: CaseOrDefaultClause;
            let hasDuplicateDefaultClause = false;

            const expressionType = checkExpression(node.expression);
            const expressionIsLiteral = isLiteralType(expressionType);
            forEach(node.caseBlock.clauses, clause => {
                // Grammar check for duplicate default clauses, skip if we already report duplicate default clause
                if (clause.kind === SyntaxKind.DefaultClause && !hasDuplicateDefaultClause) {
                    if (firstDefaultClause === undefined) {
                        firstDefaultClause = clause;
                    }
                    else {
                        const sourceFile = getSourceFileOfNode(node);
                        const start = skipTrivia(sourceFile.text, clause.pos);
                        const end = clause.statements.length > 0 ? clause.statements[0].pos : clause.end;
                        grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.A_default_clause_cannot_appear_more_than_once_in_a_switch_statement);
                        hasDuplicateDefaultClause = true;
                    }
                }

                if (produceDiagnostics && clause.kind === SyntaxKind.CaseClause) {
                    // TypeScript 1.0 spec (April 2014): 5.9
                    // In a 'switch' statement, each 'case' expression must be of a type that is comparable
                    // to or from the type of the 'switch' expression.
                    let caseType = checkExpression(clause.expression);
                    const caseIsLiteral = isLiteralType(caseType);
                    let comparedExpressionType = expressionType;
                    if (!caseIsLiteral || !expressionIsLiteral) {
                        caseType = caseIsLiteral ? getBaseTypeOfLiteralType(caseType) : caseType;
                        comparedExpressionType = getBaseTypeOfLiteralType(expressionType);
                    }
                    if (!isTypeEqualityComparableTo(comparedExpressionType, caseType)) {
                        // expressionType is not comparable to caseType, try the reversed check and report errors if it fails
                        checkTypeComparableTo(caseType, comparedExpressionType, clause.expression, /*headMessage*/ undefined);
                    }
                }
                forEach(clause.statements, checkSourceElement);
            });
            if (node.caseBlock.locals) {
                registerForUnusedIdentifiersCheck(node.caseBlock);
            }
        }

        function checkLabeledStatement(node: LabeledStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                findAncestor(node.parent,
                             current => {
                                 if (isFunctionLike(current)) {
                                     return "quit";
                                 }
                                 if (current.kind === SyntaxKind.LabeledStatement && (<LabeledStatement>current).label.escapedText === node.label.escapedText) {
                                     grammarErrorOnNode(node.label, Diagnostics.Duplicate_label_0, getTextOfNode(node.label));
                                     return true;
                                 }
                                 return false;
                             });
            }

            // ensure that label is unique
            checkSourceElement(node.statement);
        }

        function checkThrowStatement(node: ThrowStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                if (node.expression === undefined) {
                    grammarErrorAfterFirstToken(node, Diagnostics.Line_break_not_permitted_here);
                }
            }

            if (node.expression) {
                checkExpression(node.expression);
            }
        }

        function checkTryStatement(node: TryStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkBlock(node.tryBlock);
            const catchClause = node.catchClause;
            if (catchClause) {
                // Grammar checking
                if (catchClause.variableDeclaration) {
                    if (catchClause.variableDeclaration.type) {
                        grammarErrorOnFirstToken(catchClause.variableDeclaration.type, Diagnostics.Catch_clause_variable_cannot_have_a_type_annotation);
                    }
                    else if (catchClause.variableDeclaration.initializer) {
                        grammarErrorOnFirstToken(catchClause.variableDeclaration.initializer, Diagnostics.Catch_clause_variable_cannot_have_an_initializer);
                    }
                    else {
                        const blockLocals = catchClause.block.locals;
                        if (blockLocals) {
                            forEachKey(catchClause.locals!, caughtName => {
                                const blockLocal = blockLocals.get(caughtName);
                                if (blockLocal && (blockLocal.flags & SymbolFlags.BlockScopedVariable) !== 0) {
                                    grammarErrorOnNode(blockLocal.valueDeclaration, Diagnostics.Cannot_redeclare_identifier_0_in_catch_clause, caughtName);
                                }
                            });
                        }
                    }
                }

                checkBlock(catchClause.block);
            }

            if (node.finallyBlock) {
                checkBlock(node.finallyBlock);
            }
        }

        function checkIndexConstraints(type: Type) {
            const declaredNumberIndexer = getIndexDeclarationOfSymbol(type.symbol, IndexKind.Number);
            const declaredStringIndexer = getIndexDeclarationOfSymbol(type.symbol, IndexKind.String);

            const stringIndexType = getIndexTypeOfType(type, IndexKind.String);
            const numberIndexType = getIndexTypeOfType(type, IndexKind.Number);

            if (stringIndexType || numberIndexType) {
                forEach(getPropertiesOfObjectType(type), prop => {
                    const propType = getTypeOfSymbol(prop);
                    checkIndexConstraintForProperty(prop, propType, type, declaredStringIndexer, stringIndexType, IndexKind.String);
                    checkIndexConstraintForProperty(prop, propType, type, declaredNumberIndexer, numberIndexType, IndexKind.Number);
                });

                const classDeclaration = type.symbol.valueDeclaration;
                if (getObjectFlags(type) & ObjectFlags.Class && isClassLike(classDeclaration)) {
                    for (const member of classDeclaration.members) {
                        // Only process instance properties with computed names here.
                        // Static properties cannot be in conflict with indexers,
                        // and properties with literal names were already checked.
                        if (!hasModifier(member, ModifierFlags.Static) && hasNonBindableDynamicName(member)) {
                            const symbol = getSymbolOfNode(member);
                            const propType = getTypeOfSymbol(symbol);
                            checkIndexConstraintForProperty(symbol, propType, type, declaredStringIndexer, stringIndexType, IndexKind.String);
                            checkIndexConstraintForProperty(symbol, propType, type, declaredNumberIndexer, numberIndexType, IndexKind.Number);
                        }
                    }
                }
            }

            let errorNode: Node | undefined;
            if (stringIndexType && numberIndexType) {
                errorNode = declaredNumberIndexer || declaredStringIndexer;
                // condition 'errorNode === undefined' may appear if types does not declare nor string neither number indexer
                if (!errorNode && (getObjectFlags(type) & ObjectFlags.Interface)) {
                    const someBaseTypeHasBothIndexers = forEach(getBaseTypes(<InterfaceType>type), base => getIndexTypeOfType(base, IndexKind.String) && getIndexTypeOfType(base, IndexKind.Number));
                    errorNode = someBaseTypeHasBothIndexers ? undefined : type.symbol.declarations[0];
                }
            }

            if (errorNode && !isTypeAssignableTo(numberIndexType!, stringIndexType!)) { // TODO: GH#18217
                error(errorNode, Diagnostics.Numeric_index_type_0_is_not_assignable_to_string_index_type_1,
                    typeToString(numberIndexType!), typeToString(stringIndexType!));
            }

            function checkIndexConstraintForProperty(
                prop: Symbol,
                propertyType: Type,
                containingType: Type,
                indexDeclaration: Declaration | undefined,
                indexType: Type | undefined,
                indexKind: IndexKind): void {

                // ESSymbol properties apply to neither string nor numeric indexers.
                if (!indexType || isKnownSymbol(prop)) {
                    return;
                }

                const propDeclaration = prop.valueDeclaration;
                const name = propDeclaration && getNameOfDeclaration(propDeclaration);

                // index is numeric and property name is not valid numeric literal
                if (indexKind === IndexKind.Number && !(name ? isNumericName(name) : isNumericLiteralName(prop.escapedName))) {
                    return;
                }

                // perform property check if property or indexer is declared in 'type'
                // this allows us to rule out cases when both property and indexer are inherited from the base class
                let errorNode: Node | undefined;
                if (propDeclaration && name &&
                    (propDeclaration.kind === SyntaxKind.BinaryExpression ||
                     name.kind === SyntaxKind.ComputedPropertyName ||
                     prop.parent === containingType.symbol)) {
                    errorNode = propDeclaration;
                }
                else if (indexDeclaration) {
                    errorNode = indexDeclaration;
                }
                else if (getObjectFlags(containingType) & ObjectFlags.Interface) {
                    // for interfaces property and indexer might be inherited from different bases
                    // check if any base class already has both property and indexer.
                    // check should be performed only if 'type' is the first type that brings property\indexer together
                    const someBaseClassHasBothPropertyAndIndexer = forEach(getBaseTypes(<InterfaceType>containingType), base => getPropertyOfObjectType(base, prop.escapedName) && getIndexTypeOfType(base, indexKind));
                    errorNode = someBaseClassHasBothPropertyAndIndexer ? undefined : containingType.symbol.declarations[0];
                }

                if (errorNode && !isTypeAssignableTo(propertyType, indexType)) {
                    const errorMessage =
                        indexKind === IndexKind.String
                            ? Diagnostics.Property_0_of_type_1_is_not_assignable_to_string_index_type_2
                            : Diagnostics.Property_0_of_type_1_is_not_assignable_to_numeric_index_type_2;
                    error(errorNode, errorMessage, symbolToString(prop), typeToString(propertyType), typeToString(indexType));
                }
            }
        }

        function checkTypeNameIsReserved(name: Identifier, message: DiagnosticMessage): void {
            // TS 1.0 spec (April 2014): 3.6.1
            // The predefined type keywords are reserved and cannot be used as names of user defined types.
            switch (name.escapedText) {
                case "any":
                case "unknown":
                case "number":
                case "boolean":
                case "string":
                case "symbol":
                case "void":
                case "object":
                    error(name, message, name.escapedText as string);
            }
        }

        /**
         * The name cannot be used as 'Object' of user defined types with special target.
         */
        function checkClassNameCollisionWithObject(name: Identifier): void {
            if (languageVersion === ScriptTarget.ES5 && name.escapedText === "Object"
                && moduleKind !== ModuleKind.ES2015 && moduleKind !== ModuleKind.ESNext) {
                error(name, Diagnostics.Class_name_cannot_be_Object_when_targeting_ES5_with_module_0, ModuleKind[moduleKind]); // https://github.com/Microsoft/TypeScript/issues/17494
            }
        }

        /**
         * Check each type parameter and check that type parameters have no duplicate type parameter declarations
         */
        function checkTypeParameters(typeParameterDeclarations: ReadonlyArray<TypeParameterDeclaration> | undefined) {
            if (typeParameterDeclarations) {
                let seenDefault = false;
                for (let i = 0; i < typeParameterDeclarations.length; i++) {
                    const node = typeParameterDeclarations[i];
                    checkTypeParameter(node);

                    if (produceDiagnostics) {
                        if (node.default) {
                            seenDefault = true;
                        }
                        else if (seenDefault) {
                            error(node, Diagnostics.Required_type_parameters_may_not_follow_optional_type_parameters);
                        }
                        for (let j = 0; j < i; j++) {
                            if (typeParameterDeclarations[j].symbol === node.symbol) {
                                error(node.name, Diagnostics.Duplicate_identifier_0, declarationNameToString(node.name));
                            }
                        }
                    }
                }
            }
        }

        /** Check that type parameter lists are identical across multiple declarations */
        function checkTypeParameterListsIdentical(symbol: Symbol) {
            if (symbol.declarations.length === 1) {
                return;
            }

            const links = getSymbolLinks(symbol);
            if (!links.typeParametersChecked) {
                links.typeParametersChecked = true;
                const declarations = getClassOrInterfaceDeclarationsOfSymbol(symbol);
                if (declarations.length <= 1) {
                    return;
                }

                const type = <InterfaceType>getDeclaredTypeOfSymbol(symbol);
                if (!areTypeParametersIdentical(declarations, type.localTypeParameters!)) {
                    // Report an error on every conflicting declaration.
                    const name = symbolToString(symbol);
                    for (const declaration of declarations) {
                        error(declaration.name, Diagnostics.All_declarations_of_0_must_have_identical_type_parameters, name);
                    }
                }
            }
        }

        function areTypeParametersIdentical(declarations: ReadonlyArray<ClassDeclaration | InterfaceDeclaration>, targetParameters: TypeParameter[]) {
            const maxTypeArgumentCount = length(targetParameters);
            const minTypeArgumentCount = getMinTypeArgumentCount(targetParameters);

            for (const declaration of declarations) {
                // If this declaration has too few or too many type parameters, we report an error
                const sourceParameters = getEffectiveTypeParameterDeclarations(declaration);
                const numTypeParameters = sourceParameters.length;
                if (numTypeParameters < minTypeArgumentCount || numTypeParameters > maxTypeArgumentCount) {
                    return false;
                }

                for (let i = 0; i < numTypeParameters; i++) {
                    const source = sourceParameters[i];
                    const target = targetParameters[i];

                    // If the type parameter node does not have the same as the resolved type
                    // parameter at this position, we report an error.
                    if (source.name.escapedText !== target.symbol.escapedName) {
                        return false;
                    }

                    // If the type parameter node does not have an identical constraint as the resolved
                    // type parameter at this position, we report an error.
                    const constraint = getEffectiveConstraintOfTypeParameter(source);
                    const sourceConstraint = constraint && getTypeFromTypeNode(constraint);
                    const targetConstraint = getConstraintOfTypeParameter(target);
                    if (sourceConstraint) {
                        // relax check if later interface augmentation has no constraint
                        if (!targetConstraint || !isTypeIdenticalTo(sourceConstraint, targetConstraint)) {
                            return false;
                        }
                    }

                    // If the type parameter node has a default and it is not identical to the default
                    // for the type parameter at this position, we report an error.
                    const sourceDefault = source.default && getTypeFromTypeNode(source.default);
                    const targetDefault = getDefaultFromTypeParameter(target);
                    if (sourceDefault && targetDefault && !isTypeIdenticalTo(sourceDefault, targetDefault)) {
                        return false;
                    }
                }
            }

            return true;
        }

        function checkClassExpression(node: ClassExpression): Type {
            checkClassLikeDeclaration(node);
            checkNodeDeferred(node);
            return getTypeOfSymbol(getSymbolOfNode(node));
        }

        function checkClassExpressionDeferred(node: ClassExpression) {
            forEach(node.members, checkSourceElement);
            registerForUnusedIdentifiersCheck(node);
        }

        function checkClassDeclaration(node: ClassDeclaration) {
            if (!node.name && !hasModifier(node, ModifierFlags.Default)) {
                grammarErrorOnFirstToken(node, Diagnostics.A_class_declaration_without_the_default_modifier_must_have_a_name);
            }
            checkClassLikeDeclaration(node);
            forEach(node.members, checkSourceElement);

            registerForUnusedIdentifiersCheck(node);
        }

        function checkClassLikeDeclaration(node: ClassLikeDeclaration) {
            checkGrammarClassLikeDeclaration(node);
            checkDecorators(node);
            if (node.name) {
                checkTypeNameIsReserved(node.name, Diagnostics.Class_name_cannot_be_0);
                checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
                if (!(node.flags & NodeFlags.Ambient)) {
                    checkClassNameCollisionWithObject(node.name);
                }
            }
            checkTypeParameters(getEffectiveTypeParameterDeclarations(node));
            checkExportsOnMergedDeclarations(node);
            const symbol = getSymbolOfNode(node);
            const type = <InterfaceType>getDeclaredTypeOfSymbol(symbol);
            const typeWithThis = getTypeWithThisArgument(type);
            const staticType = <ObjectType>getTypeOfSymbol(symbol);
            checkTypeParameterListsIdentical(symbol);
            checkClassForDuplicateDeclarations(node);

            // Only check for reserved static identifiers on non-ambient context.
            if (!(node.flags & NodeFlags.Ambient)) {
                checkClassForStaticPropertyNameConflicts(node);
            }

            const baseTypeNode = getEffectiveBaseTypeNode(node);
            if (baseTypeNode) {
                if (languageVersion < ScriptTarget.ES2015) {
                    checkExternalEmitHelpers(baseTypeNode.parent, ExternalEmitHelpers.Extends);
                }

                const baseTypes = getBaseTypes(type);
                if (baseTypes.length && produceDiagnostics) {
                    const baseType = baseTypes[0];
                    const baseConstructorType = getBaseConstructorTypeOfClass(type);
                    const staticBaseType = getApparentType(baseConstructorType);
                    checkBaseTypeAccessibility(staticBaseType, baseTypeNode);
                    checkSourceElement(baseTypeNode.expression);
                    const extendsNode = getClassExtendsHeritageElement(node);
                    if (extendsNode && extendsNode !== baseTypeNode) {
                        checkExpression(extendsNode.expression);
                    }
                    if (some(baseTypeNode.typeArguments)) {
                        forEach(baseTypeNode.typeArguments, checkSourceElement);
                        for (const constructor of getConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments, baseTypeNode)) {
                            if (!checkTypeArgumentConstraints(baseTypeNode, constructor.typeParameters!)) {
                                break;
                            }
                        }
                    }
                    const baseWithThis = getTypeWithThisArgument(baseType, type.thisType);
                    if (!checkTypeAssignableTo(typeWithThis, baseWithThis, /*errorNode*/ undefined)) {
                        issueMemberSpecificError(node, typeWithThis, baseWithThis, Diagnostics.Class_0_incorrectly_extends_base_class_1);
                    }
                    checkTypeAssignableTo(staticType, getTypeWithoutSignatures(staticBaseType), node.name || node,
                        Diagnostics.Class_static_side_0_incorrectly_extends_base_class_static_side_1);
                    if (baseConstructorType.flags & TypeFlags.TypeVariable && !isMixinConstructorType(staticType)) {
                        error(node.name || node, Diagnostics.A_mixin_class_must_have_a_constructor_with_a_single_rest_parameter_of_type_any);
                    }

                    if (!(staticBaseType.symbol && staticBaseType.symbol.flags & SymbolFlags.Class) && !(baseConstructorType.flags & TypeFlags.TypeVariable)) {
                        // When the static base type is a "class-like" constructor function (but not actually a class), we verify
                        // that all instantiated base constructor signatures return the same type. We can simply compare the type
                        // references (as opposed to checking the structure of the types) because elsewhere we have already checked
                        // that the base type is a class or interface type (and not, for example, an anonymous object type).
                        // (Javascript constructor functions have this property trivially true since their return type is ignored.)
                        const constructors = getInstantiatedConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments, baseTypeNode);
                        if (forEach(constructors, sig => !isJavascriptConstructor(sig.declaration) && getReturnTypeOfSignature(sig) !== baseType)) {
                            error(baseTypeNode.expression, Diagnostics.Base_constructors_must_all_have_the_same_return_type);
                        }
                    }
                    checkKindsOfPropertyMemberOverrides(type, baseType);
                }
            }

            const implementedTypeNodes = getClassImplementsHeritageClauseElements(node);
            if (implementedTypeNodes) {
                for (const typeRefNode of implementedTypeNodes) {
                    if (!isEntityNameExpression(typeRefNode.expression)) {
                        error(typeRefNode.expression, Diagnostics.A_class_can_only_implement_an_identifier_Slashqualified_name_with_optional_type_arguments);
                    }
                    checkTypeReferenceNode(typeRefNode);
                    if (produceDiagnostics) {
                        const t = getTypeFromTypeNode(typeRefNode);
                        if (t !== errorType) {
                            if (isValidBaseType(t)) {
                                const genericDiag = t.symbol && t.symbol.flags & SymbolFlags.Class ?
                                    Diagnostics.Class_0_incorrectly_implements_class_1_Did_you_mean_to_extend_1_and_inherit_its_members_as_a_subclass :
                                    Diagnostics.Class_0_incorrectly_implements_interface_1;
                                const baseWithThis = getTypeWithThisArgument(t, type.thisType);
                                if (!checkTypeAssignableTo(typeWithThis, baseWithThis, /*errorNode*/ undefined)) {
                                    issueMemberSpecificError(node, typeWithThis, baseWithThis, genericDiag);
                                }
                            }
                            else {
                                error(typeRefNode, Diagnostics.A_class_may_only_implement_another_class_or_interface);
                            }
                        }
                    }
                }
            }

            if (produceDiagnostics) {
                checkIndexConstraints(type);
                checkTypeForDuplicateIndexSignatures(node);
                checkPropertyInitialization(node);
            }
        }

        function issueMemberSpecificError(node: ClassLikeDeclaration, typeWithThis: Type, baseWithThis: Type, broadDiag: DiagnosticMessage) {
            // iterate over all implemented properties and issue errors on each one which isn't compatible, rather than the class as a whole, if possible
            let issuedMemberError = false;
            for (const member of node.members) {
                if (hasStaticModifier(member)) {
                    continue;
                }
                const declaredProp = member.name && getSymbolAtLocation(member.name) || getSymbolAtLocation(member);
                if (declaredProp) {
                    const prop = getPropertyOfType(typeWithThis, declaredProp.escapedName);
                    const baseProp = getPropertyOfType(baseWithThis, declaredProp.escapedName);
                    if (prop && baseProp) {
                        const rootChain = () => chainDiagnosticMessages(
                            /*details*/ undefined,
                            Diagnostics.Property_0_in_type_1_is_not_assignable_to_the_same_property_in_base_type_2,
                            symbolToString(declaredProp),
                            typeToString(typeWithThis),
                            typeToString(baseWithThis)
                        );
                        if (!checkTypeAssignableTo(getTypeOfSymbol(prop), getTypeOfSymbol(baseProp), member.name || member, /*message*/ undefined, rootChain)) {
                            issuedMemberError = true;
                        }
                    }
                }
            }
            if (!issuedMemberError) {
                // check again with diagnostics to generate a less-specific error
                checkTypeAssignableTo(typeWithThis, baseWithThis, node.name || node, broadDiag);
            }
        }

        function checkBaseTypeAccessibility(type: Type, node: ExpressionWithTypeArguments) {
            const signatures = getSignaturesOfType(type, SignatureKind.Construct);
            if (signatures.length) {
                const declaration = signatures[0].declaration;
                if (declaration && hasModifier(declaration, ModifierFlags.Private)) {
                    const typeClassDeclaration = getClassLikeDeclarationOfSymbol(type.symbol)!;
                    if (!isNodeWithinClass(node, typeClassDeclaration)) {
                        error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, getFullyQualifiedName(type.symbol));
                    }
                }
            }
        }

        function getTargetSymbol(s: Symbol) {
            // if symbol is instantiated its flags are not copied from the 'target'
            // so we'll need to get back original 'target' symbol to work with correct set of flags
            return getCheckFlags(s) & CheckFlags.Instantiated ? (<TransientSymbol>s).target! : s;
        }

        function getClassOrInterfaceDeclarationsOfSymbol(symbol: Symbol) {
            return filter(symbol.declarations, (d: Declaration): d is ClassDeclaration | InterfaceDeclaration =>
                d.kind === SyntaxKind.ClassDeclaration || d.kind === SyntaxKind.InterfaceDeclaration);
        }

        function checkKindsOfPropertyMemberOverrides(type: InterfaceType, baseType: BaseType): void {

            // TypeScript 1.0 spec (April 2014): 8.2.3
            // A derived class inherits all members from its base class it doesn't override.
            // Inheritance means that a derived class implicitly contains all non - overridden members of the base class.
            // Both public and private property members are inherited, but only public property members can be overridden.
            // A property member in a derived class is said to override a property member in a base class
            // when the derived class property member has the same name and kind(instance or static)
            // as the base class property member.
            // The type of an overriding property member must be assignable(section 3.8.4)
            // to the type of the overridden property member, or otherwise a compile - time error occurs.
            // Base class instance member functions can be overridden by derived class instance member functions,
            // but not by other kinds of members.
            // Base class instance member variables and accessors can be overridden by
            // derived class instance member variables and accessors, but not by other kinds of members.

            // NOTE: assignability is checked in checkClassDeclaration
            const baseProperties = getPropertiesOfType(baseType);
            for (const baseProperty of baseProperties) {
                const base = getTargetSymbol(baseProperty);

                if (base.flags & SymbolFlags.Prototype) {
                    continue;
                }

                const derived = getTargetSymbol(getPropertyOfObjectType(type, base.escapedName)!); // TODO: GH#18217
                const baseDeclarationFlags = getDeclarationModifierFlagsFromSymbol(base);

                Debug.assert(!!derived, "derived should point to something, even if it is the base class' declaration.");

                if (derived) {
                    // In order to resolve whether the inherited method was overridden in the base class or not,
                    // we compare the Symbols obtained. Since getTargetSymbol returns the symbol on the *uninstantiated*
                    // type declaration, derived and base resolve to the same symbol even in the case of generic classes.
                    if (derived === base) {
                        // derived class inherits base without override/redeclaration

                        const derivedClassDecl = getClassLikeDeclarationOfSymbol(type.symbol)!;

                        // It is an error to inherit an abstract member without implementing it or being declared abstract.
                        // If there is no declaration for the derived class (as in the case of class expressions),
                        // then the class cannot be declared abstract.
                        if (baseDeclarationFlags & ModifierFlags.Abstract && (!derivedClassDecl || !hasModifier(derivedClassDecl, ModifierFlags.Abstract))) {
                            if (derivedClassDecl.kind === SyntaxKind.ClassExpression) {
                                error(derivedClassDecl, Diagnostics.Non_abstract_class_expression_does_not_implement_inherited_abstract_member_0_from_class_1,
                                    symbolToString(baseProperty), typeToString(baseType));
                            }
                            else {
                                error(derivedClassDecl, Diagnostics.Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2,
                                    typeToString(type), symbolToString(baseProperty), typeToString(baseType));
                            }
                        }
                    }
                    else {
                        // derived overrides base.
                        const derivedDeclarationFlags = getDeclarationModifierFlagsFromSymbol(derived);
                        if (baseDeclarationFlags & ModifierFlags.Private || derivedDeclarationFlags & ModifierFlags.Private) {
                            // either base or derived property is private - not override, skip it
                            continue;
                        }

                        if (isPrototypeProperty(base) || base.flags & SymbolFlags.PropertyOrAccessor && derived.flags & SymbolFlags.PropertyOrAccessor) {
                            // method is overridden with method or property/accessor is overridden with property/accessor - correct case
                            continue;
                        }

                        let errorMessage: DiagnosticMessage;
                        if (isPrototypeProperty(base)) {
                            if (derived.flags & SymbolFlags.Accessor) {
                                errorMessage = Diagnostics.Class_0_defines_instance_member_function_1_but_extended_class_2_defines_it_as_instance_member_accessor;
                            }
                            else {
                                errorMessage = Diagnostics.Class_0_defines_instance_member_function_1_but_extended_class_2_defines_it_as_instance_member_property;
                            }
                        }
                        else if (base.flags & SymbolFlags.Accessor) {
                            errorMessage = Diagnostics.Class_0_defines_instance_member_accessor_1_but_extended_class_2_defines_it_as_instance_member_function;
                        }
                        else {
                            errorMessage = Diagnostics.Class_0_defines_instance_member_property_1_but_extended_class_2_defines_it_as_instance_member_function;
                        }

                        error(getNameOfDeclaration(derived.valueDeclaration) || derived.valueDeclaration, errorMessage, typeToString(baseType), symbolToString(base), typeToString(type));
                    }
                }
            }
        }

        function checkInheritedPropertiesAreIdentical(type: InterfaceType, typeNode: Node): boolean {
            const baseTypes = getBaseTypes(type);
            if (baseTypes.length < 2) {
                return true;
            }

            interface InheritanceInfoMap { prop: Symbol; containingType: Type; }
            const seen = createUnderscoreEscapedMap<InheritanceInfoMap>();
            forEach(resolveDeclaredMembers(type).declaredProperties, p => { seen.set(p.escapedName, { prop: p, containingType: type }); });
            let ok = true;

            for (const base of baseTypes) {
                const properties = getPropertiesOfType(getTypeWithThisArgument(base, type.thisType));
                for (const prop of properties) {
                    const existing = seen.get(prop.escapedName);
                    if (!existing) {
                        seen.set(prop.escapedName, { prop, containingType: base });
                    }
                    else {
                        const isInheritedProperty = existing.containingType !== type;
                        if (isInheritedProperty && !isPropertyIdenticalTo(existing.prop, prop)) {
                            ok = false;

                            const typeName1 = typeToString(existing.containingType);
                            const typeName2 = typeToString(base);

                            let errorInfo = chainDiagnosticMessages(/*details*/ undefined, Diagnostics.Named_property_0_of_types_1_and_2_are_not_identical, symbolToString(prop), typeName1, typeName2);
                            errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Interface_0_cannot_simultaneously_extend_types_1_and_2, typeToString(type), typeName1, typeName2);
                            diagnostics.add(createDiagnosticForNodeFromMessageChain(typeNode, errorInfo));
                        }
                    }
                }
            }

            return ok;
        }

        function checkPropertyInitialization(node: ClassLikeDeclaration) {
            if (!strictNullChecks || !strictPropertyInitialization || node.flags & NodeFlags.Ambient) {
                return;
            }
            const constructor = findConstructorDeclaration(node);
            for (const member of node.members) {
                if (isInstancePropertyWithoutInitializer(member)) {
                    const propName = (<PropertyDeclaration>member).name;
                    if (isIdentifier(propName)) {
                        const type = getTypeOfSymbol(getSymbolOfNode(member));
                        if (!(type.flags & TypeFlags.AnyOrUnknown || getFalsyFlags(type) & TypeFlags.Undefined)) {
                            if (!constructor || !isPropertyInitializedInConstructor(propName, type, constructor)) {
                                error(member.name, Diagnostics.Property_0_has_no_initializer_and_is_not_definitely_assigned_in_the_constructor, declarationNameToString(propName));
                            }
                        }
                    }
                }
            }
        }

        function isInstancePropertyWithoutInitializer(node: Node) {
            return node.kind === SyntaxKind.PropertyDeclaration &&
                !hasModifier(node, ModifierFlags.Static | ModifierFlags.Abstract) &&
                !(<PropertyDeclaration>node).exclamationToken &&
                !(<PropertyDeclaration>node).initializer;
        }

        function isPropertyInitializedInConstructor(propName: Identifier, propType: Type, constructor: ConstructorDeclaration) {
            const reference = createPropertyAccess(createThis(), propName);
            reference.flowNode = constructor.returnFlowNode;
            const flowType = getFlowTypeOfReference(reference, propType, getOptionalType(propType));
            return !(getFalsyFlags(flowType) & TypeFlags.Undefined);
        }

        function checkInterfaceDeclaration(node: InterfaceDeclaration) {
            // Grammar checking
            if (!checkGrammarDecoratorsAndModifiers(node)) checkGrammarInterfaceDeclaration(node);

            checkTypeParameters(node.typeParameters);
            if (produceDiagnostics) {
                checkTypeNameIsReserved(node.name, Diagnostics.Interface_name_cannot_be_0);

                checkExportsOnMergedDeclarations(node);
                const symbol = getSymbolOfNode(node);
                checkTypeParameterListsIdentical(symbol);

                // Only check this symbol once
                const firstInterfaceDecl = getDeclarationOfKind<InterfaceDeclaration>(symbol, SyntaxKind.InterfaceDeclaration);
                if (node === firstInterfaceDecl) {
                    const type = <InterfaceType>getDeclaredTypeOfSymbol(symbol);
                    const typeWithThis = getTypeWithThisArgument(type);
                    // run subsequent checks only if first set succeeded
                    if (checkInheritedPropertiesAreIdentical(type, node.name)) {
                        for (const baseType of getBaseTypes(type)) {
                            checkTypeAssignableTo(typeWithThis, getTypeWithThisArgument(baseType, type.thisType), node.name, Diagnostics.Interface_0_incorrectly_extends_interface_1);
                        }
                        checkIndexConstraints(type);
                    }
                }
                checkObjectTypeForDuplicateDeclarations(node);
            }
            forEach(getInterfaceBaseTypeNodes(node), heritageElement => {
                if (!isEntityNameExpression(heritageElement.expression)) {
                    error(heritageElement.expression, Diagnostics.An_interface_can_only_extend_an_identifier_Slashqualified_name_with_optional_type_arguments);
                }
                checkTypeReferenceNode(heritageElement);
            });

            forEach(node.members, checkSourceElement);

            if (produceDiagnostics) {
                checkTypeForDuplicateIndexSignatures(node);
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkTypeAliasDeclaration(node: TypeAliasDeclaration) {
            // Grammar checking
            checkGrammarDecoratorsAndModifiers(node);

            checkTypeNameIsReserved(node.name, Diagnostics.Type_alias_name_cannot_be_0);
            checkTypeParameters(node.typeParameters);
            checkSourceElement(node.type);
            registerForUnusedIdentifiersCheck(node);
        }

        function computeEnumMemberValues(node: EnumDeclaration) {
            const nodeLinks = getNodeLinks(node);
            if (!(nodeLinks.flags & NodeCheckFlags.EnumValuesComputed)) {
                nodeLinks.flags |= NodeCheckFlags.EnumValuesComputed;
                let autoValue: number | undefined = 0;
                for (const member of node.members) {
                    const value = computeMemberValue(member, autoValue);
                    getNodeLinks(member).enumMemberValue = value;
                    autoValue = typeof value === "number" ? value + 1 : undefined;
                }
            }
        }

        function computeMemberValue(member: EnumMember, autoValue: number | undefined) {
            if (isComputedNonLiteralName(member.name)) {
                error(member.name, Diagnostics.Computed_property_names_are_not_allowed_in_enums);
            }
            else {
                const text = getTextOfPropertyName(member.name);
                if (isNumericLiteralName(text) && !isInfinityOrNaNString(text)) {
                    error(member.name, Diagnostics.An_enum_member_cannot_have_a_numeric_name);
                }
            }
            if (member.initializer) {
                return computeConstantValue(member);
            }
            // In ambient enum declarations that specify no const modifier, enum member declarations that omit
            // a value are considered computed members (as opposed to having auto-incremented values).
            if (member.parent.flags & NodeFlags.Ambient && !isEnumConst(member.parent)) {
                return undefined;
            }
            // If the member declaration specifies no value, the member is considered a constant enum member.
            // If the member is the first member in the enum declaration, it is assigned the value zero.
            // Otherwise, it is assigned the value of the immediately preceding member plus one, and an error
            // occurs if the immediately preceding member is not a constant enum member.
            if (autoValue !== undefined) {
                return autoValue;
            }
            error(member.name, Diagnostics.Enum_member_must_have_initializer);
            return undefined;
        }

        function computeConstantValue(member: EnumMember): string | number | undefined {
            const enumKind = getEnumKind(getSymbolOfNode(member.parent));
            const isConstEnum = isEnumConst(member.parent);
            const initializer = member.initializer!;
            const value = enumKind === EnumKind.Literal && !isLiteralEnumMember(member) ? undefined : evaluate(initializer);
            if (value !== undefined) {
                if (isConstEnum && typeof value === "number" && !isFinite(value)) {
                    error(initializer, isNaN(value) ?
                        Diagnostics.const_enum_member_initializer_was_evaluated_to_disallowed_value_NaN :
                        Diagnostics.const_enum_member_initializer_was_evaluated_to_a_non_finite_value);
                }
            }
            else if (enumKind === EnumKind.Literal) {
                error(initializer, Diagnostics.Computed_values_are_not_permitted_in_an_enum_with_string_valued_members);
                return 0;
            }
            else if (isConstEnum) {
                error(initializer, Diagnostics.In_const_enum_declarations_member_initializer_must_be_constant_expression);
            }
            else if (member.parent.flags & NodeFlags.Ambient) {
                error(initializer, Diagnostics.In_ambient_enum_declarations_member_initializer_must_be_constant_expression);
            }
            else {
                // Only here do we need to check that the initializer is assignable to the enum type.
                checkTypeAssignableTo(checkExpression(initializer), getDeclaredTypeOfSymbol(getSymbolOfNode(member.parent)), initializer, /*headMessage*/ undefined);
            }
            return value;

            function evaluate(expr: Expression): string | number | undefined {
                switch (expr.kind) {
                    case SyntaxKind.PrefixUnaryExpression:
                        const value = evaluate((<PrefixUnaryExpression>expr).operand);
                        if (typeof value === "number") {
                            switch ((<PrefixUnaryExpression>expr).operator) {
                                case SyntaxKind.PlusToken: return value;
                                case SyntaxKind.MinusToken: return -value;
                                case SyntaxKind.TildeToken: return ~value;
                            }
                        }
                        break;
                    case SyntaxKind.BinaryExpression:
                        const left = evaluate((<BinaryExpression>expr).left);
                        const right = evaluate((<BinaryExpression>expr).right);
                        if (typeof left === "number" && typeof right === "number") {
                            switch ((<BinaryExpression>expr).operatorToken.kind) {
                                case SyntaxKind.BarToken: return left | right;
                                case SyntaxKind.AmpersandToken: return left & right;
                                case SyntaxKind.GreaterThanGreaterThanToken: return left >> right;
                                case SyntaxKind.GreaterThanGreaterThanGreaterThanToken: return left >>> right;
                                case SyntaxKind.LessThanLessThanToken: return left << right;
                                case SyntaxKind.CaretToken: return left ^ right;
                                case SyntaxKind.AsteriskToken: return left * right;
                                case SyntaxKind.SlashToken: return left / right;
                                case SyntaxKind.PlusToken: return left + right;
                                case SyntaxKind.MinusToken: return left - right;
                                case SyntaxKind.PercentToken: return left % right;
                                case SyntaxKind.AsteriskAsteriskToken: return left ** right;
                            }
                        }
                        else if (typeof left === "string" && typeof right === "string" && (<BinaryExpression>expr).operatorToken.kind === SyntaxKind.PlusToken) {
                            return left + right;
                        }
                        break;
                    case SyntaxKind.StringLiteral:
                        return (<StringLiteral>expr).text;
                    case SyntaxKind.NumericLiteral:
                        checkGrammarNumericLiteral(<NumericLiteral>expr);
                        return +(<NumericLiteral>expr).text;
                    case SyntaxKind.ParenthesizedExpression:
                        return evaluate((<ParenthesizedExpression>expr).expression);
                    case SyntaxKind.Identifier:
                        const identifier = <Identifier>expr;
                        if (isInfinityOrNaNString(identifier.escapedText)) {
                            return +(identifier.escapedText);
                        }
                        return nodeIsMissing(expr) ? 0 : evaluateEnumMember(expr, getSymbolOfNode(member.parent), identifier.escapedText);
                    case SyntaxKind.ElementAccessExpression:
                    case SyntaxKind.PropertyAccessExpression:
                        const ex = <PropertyAccessExpression | ElementAccessExpression>expr;
                        if (isConstantMemberAccess(ex)) {
                            const type = getTypeOfExpression(ex.expression);
                            if (type.symbol && type.symbol.flags & SymbolFlags.Enum) {
                                let name: __String;
                                if (ex.kind === SyntaxKind.PropertyAccessExpression) {
                                    name = ex.name.escapedText;
                                }
                                else {
                                    const argument = ex.argumentExpression;
                                    Debug.assert(isLiteralExpression(argument));
                                    name = escapeLeadingUnderscores((argument as LiteralExpression).text);
                                }
                                return evaluateEnumMember(expr, type.symbol, name);
                            }
                        }
                        break;
                }
                return undefined;
            }

            function evaluateEnumMember(expr: Expression, enumSymbol: Symbol, name: __String) {
                const memberSymbol = enumSymbol.exports!.get(name);
                if (memberSymbol) {
                    const declaration = memberSymbol.valueDeclaration;
                    if (declaration !== member) {
                        if (isBlockScopedNameDeclaredBeforeUse(declaration, member)) {
                            return getEnumMemberValue(declaration as EnumMember);
                        }
                        error(expr, Diagnostics.A_member_initializer_in_a_enum_declaration_cannot_reference_members_declared_after_it_including_members_defined_in_other_enums);
                        return 0;
                    }
                }
                return undefined;
            }
        }

        function isConstantMemberAccess(node: Expression): boolean {
            return node.kind === SyntaxKind.Identifier ||
                node.kind === SyntaxKind.PropertyAccessExpression && isConstantMemberAccess((<PropertyAccessExpression>node).expression) ||
                node.kind === SyntaxKind.ElementAccessExpression && isConstantMemberAccess((<ElementAccessExpression>node).expression) &&
                    (<ElementAccessExpression>node).argumentExpression.kind === SyntaxKind.StringLiteral;
        }

        function checkEnumDeclaration(node: EnumDeclaration) {
            if (!produceDiagnostics) {
                return;
            }

            // Grammar checking
            checkGrammarDecoratorsAndModifiers(node);

            checkTypeNameIsReserved(node.name, Diagnostics.Enum_name_cannot_be_0);
            checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
            checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
            checkExportsOnMergedDeclarations(node);

            computeEnumMemberValues(node);

            const enumIsConst = isEnumConst(node);
            if (compilerOptions.isolatedModules && enumIsConst && node.flags & NodeFlags.Ambient) {
                error(node.name, Diagnostics.Ambient_const_enums_are_not_allowed_when_the_isolatedModules_flag_is_provided);
            }

            // Spec 2014 - Section 9.3:
            // It isn't possible for one enum declaration to continue the automatic numbering sequence of another,
            // and when an enum type has multiple declarations, only one declaration is permitted to omit a value
            // for the first member.
            //
            // Only perform this check once per symbol
            const enumSymbol = getSymbolOfNode(node);
            const firstDeclaration = getDeclarationOfKind(enumSymbol, node.kind);
            if (node === firstDeclaration) {
                if (enumSymbol.declarations.length > 1) {
                    // check that const is placed\omitted on all enum declarations
                    forEach(enumSymbol.declarations, decl => {
                        if (isEnumDeclaration(decl) && isEnumConst(decl) !== enumIsConst) {
                            error(getNameOfDeclaration(decl), Diagnostics.Enum_declarations_must_all_be_const_or_non_const);
                        }
                    });
                }

                let seenEnumMissingInitialInitializer = false;
                forEach(enumSymbol.declarations, declaration => {
                    // return true if we hit a violation of the rule, false otherwise
                    if (declaration.kind !== SyntaxKind.EnumDeclaration) {
                        return false;
                    }

                    const enumDeclaration = <EnumDeclaration>declaration;
                    if (!enumDeclaration.members.length) {
                        return false;
                    }

                    const firstEnumMember = enumDeclaration.members[0];
                    if (!firstEnumMember.initializer) {
                        if (seenEnumMissingInitialInitializer) {
                            error(firstEnumMember.name, Diagnostics.In_an_enum_with_multiple_declarations_only_one_declaration_can_omit_an_initializer_for_its_first_enum_element);
                        }
                        else {
                            seenEnumMissingInitialInitializer = true;
                        }
                    }
                });
            }
        }

        function getFirstNonAmbientClassOrFunctionDeclaration(symbol: Symbol): Declaration | undefined {
            const declarations = symbol.declarations;
            for (const declaration of declarations) {
                if ((declaration.kind === SyntaxKind.ClassDeclaration ||
                    (declaration.kind === SyntaxKind.FunctionDeclaration && nodeIsPresent((<FunctionLikeDeclaration>declaration).body))) &&
                    !(declaration.flags & NodeFlags.Ambient)) {
                    return declaration;
                }
            }
            return undefined;
        }

        function inSameLexicalScope(node1: Node, node2: Node) {
            const container1 = getEnclosingBlockScopeContainer(node1);
            const container2 = getEnclosingBlockScopeContainer(node2);
            if (isGlobalSourceFile(container1)) {
                return isGlobalSourceFile(container2);
            }
            else if (isGlobalSourceFile(container2)) {
                return false;
            }
            else {
                return container1 === container2;
            }
        }

        function checkModuleDeclaration(node: ModuleDeclaration) {
            if (produceDiagnostics) {
                // Grammar checking
                const isGlobalAugmentation = isGlobalScopeAugmentation(node);
                const inAmbientContext = node.flags & NodeFlags.Ambient;
                if (isGlobalAugmentation && !inAmbientContext) {
                    error(node.name, Diagnostics.Augmentations_for_the_global_scope_should_have_declare_modifier_unless_they_appear_in_already_ambient_context);
                }

                const isAmbientExternalModule = isAmbientModule(node);
                const contextErrorMessage = isAmbientExternalModule
                    ? Diagnostics.An_ambient_module_declaration_is_only_allowed_at_the_top_level_in_a_file
                    : Diagnostics.A_namespace_declaration_is_only_allowed_in_a_namespace_or_module;
                if (checkGrammarModuleElementContext(node, contextErrorMessage)) {
                    // If we hit a module declaration in an illegal context, just bail out to avoid cascading errors.
                    return;
                }

                if (!checkGrammarDecoratorsAndModifiers(node)) {
                    if (!inAmbientContext && node.name.kind === SyntaxKind.StringLiteral) {
                        grammarErrorOnNode(node.name, Diagnostics.Only_ambient_modules_can_use_quoted_names);
                    }
                }

                if (isIdentifier(node.name)) {
                    checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
                    checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
                }

                checkExportsOnMergedDeclarations(node);
                const symbol = getSymbolOfNode(node);

                // The following checks only apply on a non-ambient instantiated module declaration.
                if (symbol.flags & SymbolFlags.ValueModule
                    && symbol.declarations.length > 1
                    && !inAmbientContext
                    && isInstantiatedModule(node, !!compilerOptions.preserveConstEnums || !!compilerOptions.isolatedModules)) {
                    const firstNonAmbientClassOrFunc = getFirstNonAmbientClassOrFunctionDeclaration(symbol);
                    if (firstNonAmbientClassOrFunc) {
                        if (getSourceFileOfNode(node) !== getSourceFileOfNode(firstNonAmbientClassOrFunc)) {
                            error(node.name, Diagnostics.A_namespace_declaration_cannot_be_in_a_different_file_from_a_class_or_function_with_which_it_is_merged);
                        }
                        else if (node.pos < firstNonAmbientClassOrFunc.pos) {
                            error(node.name, Diagnostics.A_namespace_declaration_cannot_be_located_prior_to_a_class_or_function_with_which_it_is_merged);
                        }
                    }

                    // if the module merges with a class declaration in the same lexical scope,
                    // we need to track this to ensure the correct emit.
                    const mergedClass = getDeclarationOfKind(symbol, SyntaxKind.ClassDeclaration);
                    if (mergedClass &&
                        inSameLexicalScope(node, mergedClass)) {
                        getNodeLinks(node).flags |= NodeCheckFlags.LexicalModuleMergesWithClass;
                    }
                }

                if (isAmbientExternalModule) {
                    if (isExternalModuleAugmentation(node)) {
                        // body of the augmentation should be checked for consistency only if augmentation was applied to its target (either global scope or module)
                        // otherwise we'll be swamped in cascading errors.
                        // We can detect if augmentation was applied using following rules:
                        // - augmentation for a global scope is always applied
                        // - augmentation for some external module is applied if symbol for augmentation is merged (it was combined with target module).
                        const checkBody = isGlobalAugmentation || (getSymbolOfNode(node).flags & SymbolFlags.Transient);
                        if (checkBody && node.body) {
                            for (const statement of node.body.statements) {
                                checkModuleAugmentationElement(statement, isGlobalAugmentation);
                            }
                        }
                    }
                    else if (isGlobalSourceFile(node.parent)) {
                        if (isGlobalAugmentation) {
                            error(node.name, Diagnostics.Augmentations_for_the_global_scope_can_only_be_directly_nested_in_external_modules_or_ambient_module_declarations);
                        }
                        else if (isExternalModuleNameRelative(getTextOfIdentifierOrLiteral(node.name))) {
                            error(node.name, Diagnostics.Ambient_module_declaration_cannot_specify_relative_module_name);
                        }
                    }
                    else {
                        if (isGlobalAugmentation) {
                            error(node.name, Diagnostics.Augmentations_for_the_global_scope_can_only_be_directly_nested_in_external_modules_or_ambient_module_declarations);
                        }
                        else {
                            // Node is not an augmentation and is not located on the script level.
                            // This means that this is declaration of ambient module that is located in other module or namespace which is prohibited.
                            error(node.name, Diagnostics.Ambient_modules_cannot_be_nested_in_other_modules_or_namespaces);
                        }
                    }
                }
            }

            if (node.body) {
                checkSourceElement(node.body);
                if (!isGlobalScopeAugmentation(node)) {
                    registerForUnusedIdentifiersCheck(node);
                }
            }
        }

        function checkModuleAugmentationElement(node: Node, isGlobalAugmentation: boolean): void {
            switch (node.kind) {
                case SyntaxKind.VariableStatement:
                    // error each individual name in variable statement instead of marking the entire variable statement
                    for (const decl of (<VariableStatement>node).declarationList.declarations) {
                        checkModuleAugmentationElement(decl, isGlobalAugmentation);
                    }
                    break;
                case SyntaxKind.ExportAssignment:
                case SyntaxKind.ExportDeclaration:
                    grammarErrorOnFirstToken(node, Diagnostics.Exports_and_export_assignments_are_not_permitted_in_module_augmentations);
                    break;
                case SyntaxKind.ImportEqualsDeclaration:
                case SyntaxKind.ImportDeclaration:
                    grammarErrorOnFirstToken(node, Diagnostics.Imports_are_not_permitted_in_module_augmentations_Consider_moving_them_to_the_enclosing_external_module);
                    break;
                case SyntaxKind.BindingElement:
                case SyntaxKind.VariableDeclaration:
                    const name = (<VariableDeclaration | BindingElement>node).name;
                    if (isBindingPattern(name)) {
                        for (const el of name.elements) {
                            // mark individual names in binding pattern
                            checkModuleAugmentationElement(el, isGlobalAugmentation);
                        }
                        break;
                    }
                    // falls through
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.EnumDeclaration:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.InterfaceDeclaration:
                case SyntaxKind.ModuleDeclaration:
                case SyntaxKind.TypeAliasDeclaration:
                    if (isGlobalAugmentation) {
                        return;
                    }
                    const symbol = getSymbolOfNode(node);
                    if (symbol) {
                        // module augmentations cannot introduce new names on the top level scope of the module
                        // this is done it two steps
                        // 1. quick check - if symbol for node is not merged - this is local symbol to this augmentation - report error
                        // 2. main check - report error if value declaration of the parent symbol is module augmentation)
                        let reportError = !(symbol.flags & SymbolFlags.Transient);
                        if (!reportError) {
                            // symbol should not originate in augmentation
                            reportError = isExternalModuleAugmentation(symbol.parent!.declarations[0]);
                        }
                    }
                    break;
            }
        }

        function getFirstIdentifier(node: EntityNameOrEntityNameExpression): Identifier {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                    return node;
                case SyntaxKind.QualifiedName:
                    do {
                        node = node.left;
                    } while (node.kind !== SyntaxKind.Identifier);
                    return node;
                case SyntaxKind.PropertyAccessExpression:
                    do {
                        node = node.expression;
                    } while (node.kind !== SyntaxKind.Identifier);
                    return node;
            }
        }

        function checkExternalImportOrExportDeclaration(node: ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration): boolean {
            const moduleName = getExternalModuleName(node);
            if (!moduleName || nodeIsMissing(moduleName)) {
                // Should be a parse error.
                return false;
            }
            if (!isStringLiteral(moduleName)) {
                error(moduleName, Diagnostics.String_literal_expected);
                return false;
            }
            const inAmbientExternalModule = node.parent.kind === SyntaxKind.ModuleBlock && isAmbientModule(node.parent.parent);
            if (node.parent.kind !== SyntaxKind.SourceFile && !inAmbientExternalModule) {
                error(moduleName, node.kind === SyntaxKind.ExportDeclaration ?
                    Diagnostics.Export_declarations_are_not_permitted_in_a_namespace :
                    Diagnostics.Import_declarations_in_a_namespace_cannot_reference_a_module);
                return false;
            }
            if (inAmbientExternalModule && isExternalModuleNameRelative(moduleName.text)) {
                // we have already reported errors on top level imports\exports in external module augmentations in checkModuleDeclaration
                // no need to do this again.
                if (!isTopLevelInExternalModuleAugmentation(node)) {
                    // TypeScript 1.0 spec (April 2013): 12.1.6
                    // An ExternalImportDeclaration in an AmbientExternalModuleDeclaration may reference
                    // other external modules only through top - level external module names.
                    // Relative external module names are not permitted.
                    error(node, Diagnostics.Import_or_export_declaration_in_an_ambient_module_declaration_cannot_reference_module_through_relative_module_name);
                    return false;
                }
            }
            return true;
        }

        function checkAliasSymbol(node: ImportEqualsDeclaration | ImportClause | NamespaceImport | ImportSpecifier | ExportSpecifier) {
            const symbol = getSymbolOfNode(node);
            const target = resolveAlias(symbol);
            if (target !== unknownSymbol) {
                // For external modules symbol represent local symbol for an alias.
                // This local symbol will merge any other local declarations (excluding other aliases)
                // and symbol.flags will contains combined representation for all merged declaration.
                // Based on symbol.flags we can compute a set of excluded meanings (meaning that resolved alias should not have,
                // otherwise it will conflict with some local declaration). Note that in addition to normal flags we include matching SymbolFlags.Export*
                // in order to prevent collisions with declarations that were exported from the current module (they still contribute to local names).
                const excludedMeanings =
                    (symbol.flags & (SymbolFlags.Value | SymbolFlags.ExportValue) ? SymbolFlags.Value : 0) |
                    (symbol.flags & SymbolFlags.Type ? SymbolFlags.Type : 0) |
                    (symbol.flags & SymbolFlags.Namespace ? SymbolFlags.Namespace : 0);
                if (target.flags & excludedMeanings) {
                    const message = node.kind === SyntaxKind.ExportSpecifier ?
                        Diagnostics.Export_declaration_conflicts_with_exported_declaration_of_0 :
                        Diagnostics.Import_declaration_conflicts_with_local_declaration_of_0;
                    error(node, message, symbolToString(symbol));
                }

                // Don't allow to re-export something with no value side when `--isolatedModules` is set.
                if (compilerOptions.isolatedModules
                    && node.kind === SyntaxKind.ExportSpecifier
                    && !(target.flags & SymbolFlags.Value)
                    && !(node.flags & NodeFlags.Ambient)) {
                    error(node, Diagnostics.Cannot_re_export_a_type_when_the_isolatedModules_flag_is_provided);
                }
            }
        }

        function checkImportBinding(node: ImportEqualsDeclaration | ImportClause | NamespaceImport | ImportSpecifier) {
            checkCollisionWithRequireExportsInGeneratedCode(node, node.name!);
            checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name!);
            checkAliasSymbol(node);
        }

        function checkImportDeclaration(node: ImportDeclaration) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_import_declaration_can_only_be_used_in_a_namespace_or_module)) {
                // If we hit an import declaration in an illegal context, just bail out to avoid cascading errors.
                return;
            }
            if (!checkGrammarDecoratorsAndModifiers(node) && hasModifiers(node)) {
                grammarErrorOnFirstToken(node, Diagnostics.An_import_declaration_cannot_have_modifiers);
            }
            if (checkExternalImportOrExportDeclaration(node)) {
                const importClause = node.importClause;
                if (importClause) {
                    if (importClause.name) {
                        checkImportBinding(importClause);
                    }
                    if (importClause.namedBindings) {
                        if (importClause.namedBindings.kind === SyntaxKind.NamespaceImport) {
                            checkImportBinding(importClause.namedBindings);
                        }
                        else {
                            const moduleExisted = resolveExternalModuleName(node, node.moduleSpecifier);
                            if (moduleExisted) {
                                forEach(importClause.namedBindings.elements, checkImportBinding);
                            }
                        }
                    }
                }
            }
        }

        function checkImportEqualsDeclaration(node: ImportEqualsDeclaration) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_import_declaration_can_only_be_used_in_a_namespace_or_module)) {
                // If we hit an import declaration in an illegal context, just bail out to avoid cascading errors.
                return;
            }

            checkGrammarDecoratorsAndModifiers(node);
            if (isInternalModuleImportEqualsDeclaration(node) || checkExternalImportOrExportDeclaration(node)) {
                checkImportBinding(node);
                if (hasModifier(node, ModifierFlags.Export)) {
                    markExportAsReferenced(node);
                }
                if (node.moduleReference.kind !== SyntaxKind.ExternalModuleReference) {
                    const target = resolveAlias(getSymbolOfNode(node));
                    if (target !== unknownSymbol) {
                        if (target.flags & SymbolFlags.Value) {
                            // Target is a value symbol, check that it is not hidden by a local declaration with the same name
                            const moduleName = getFirstIdentifier(node.moduleReference);
                            if (!(resolveEntityName(moduleName, SymbolFlags.Value | SymbolFlags.Namespace)!.flags & SymbolFlags.Namespace)) {
                                error(moduleName, Diagnostics.Module_0_is_hidden_by_a_local_declaration_with_the_same_name, declarationNameToString(moduleName));
                            }
                        }
                        if (target.flags & SymbolFlags.Type) {
                            checkTypeNameIsReserved(node.name, Diagnostics.Import_name_cannot_be_0);
                        }
                    }
                }
                else {
                    if (moduleKind >= ModuleKind.ES2015 && !(node.flags & NodeFlags.Ambient)) {
                        // Import equals declaration is deprecated in es6 or above
                        grammarErrorOnNode(node, Diagnostics.Import_assignment_cannot_be_used_when_targeting_ECMAScript_modules_Consider_using_import_Asterisk_as_ns_from_mod_import_a_from_mod_import_d_from_mod_or_another_module_format_instead);
                    }
                }
            }
        }

        function checkExportDeclaration(node: ExportDeclaration) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_export_declaration_can_only_be_used_in_a_module)) {
                // If we hit an export in an illegal context, just bail out to avoid cascading errors.
                return;
            }

            if (!checkGrammarDecoratorsAndModifiers(node) && hasModifiers(node)) {
                grammarErrorOnFirstToken(node, Diagnostics.An_export_declaration_cannot_have_modifiers);
            }

            if (!node.moduleSpecifier || checkExternalImportOrExportDeclaration(node)) {
                if (node.exportClause) {
                    // export { x, y }
                    // export { x, y } from "foo"
                    forEach(node.exportClause.elements, checkExportSpecifier);

                    const inAmbientExternalModule = node.parent.kind === SyntaxKind.ModuleBlock && isAmbientModule(node.parent.parent);
                    const inAmbientNamespaceDeclaration = !inAmbientExternalModule && node.parent.kind === SyntaxKind.ModuleBlock &&
                        !node.moduleSpecifier && node.flags & NodeFlags.Ambient;
                    if (node.parent.kind !== SyntaxKind.SourceFile && !inAmbientExternalModule && !inAmbientNamespaceDeclaration) {
                        error(node, Diagnostics.Export_declarations_are_not_permitted_in_a_namespace);
                    }
                }
                else {
                    // export * from "foo"
                    const moduleSymbol = resolveExternalModuleName(node, node.moduleSpecifier!);
                    if (moduleSymbol && hasExportAssignmentSymbol(moduleSymbol)) {
                        error(node.moduleSpecifier, Diagnostics.Module_0_uses_export_and_cannot_be_used_with_export_Asterisk, symbolToString(moduleSymbol));
                    }

                    if (moduleKind !== ModuleKind.System && moduleKind !== ModuleKind.ES2015 && moduleKind !== ModuleKind.ESNext) {
                        checkExternalEmitHelpers(node, ExternalEmitHelpers.ExportStar);
                    }
                }
            }
        }

        function checkGrammarModuleElementContext(node: Statement, errorMessage: DiagnosticMessage): boolean {
            const isInAppropriateContext = node.parent.kind === SyntaxKind.SourceFile || node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.ModuleDeclaration;
            if (!isInAppropriateContext) {
                grammarErrorOnFirstToken(node, errorMessage);
            }
            return !isInAppropriateContext;
        }

        function checkExportSpecifier(node: ExportSpecifier) {
            checkAliasSymbol(node);
            if (getEmitDeclarations(compilerOptions)) {
                collectLinkedAliases(node.propertyName || node.name, /*setVisibility*/ true);
            }
            if (!node.parent.parent.moduleSpecifier) {
                const exportedName = node.propertyName || node.name;
                // find immediate value referenced by exported name (SymbolFlags.Alias is set so we don't chase down aliases)
                const symbol = resolveName(exportedName, exportedName.escapedText, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias,
                    /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ true);
                if (symbol && (symbol === undefinedSymbol || isGlobalSourceFile(getDeclarationContainer(symbol.declarations[0])))) {
                    error(exportedName, Diagnostics.Cannot_export_0_Only_local_declarations_can_be_exported_from_a_module, idText(exportedName));
                }
                else {
                    markExportAsReferenced(node);
                }
            }
        }

        function checkExportAssignment(node: ExportAssignment) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_export_assignment_can_only_be_used_in_a_module)) {
                // If we hit an export assignment in an illegal context, just bail out to avoid cascading errors.
                return;
            }

            const container = node.parent.kind === SyntaxKind.SourceFile ? node.parent : <ModuleDeclaration>node.parent.parent;
            if (container.kind === SyntaxKind.ModuleDeclaration && !isAmbientModule(container)) {
                if (node.isExportEquals) {
                    error(node, Diagnostics.An_export_assignment_cannot_be_used_in_a_namespace);
                }
                else {
                    error(node, Diagnostics.A_default_export_can_only_be_used_in_an_ECMAScript_style_module);
                }

                return;
            }
            // Grammar checking
            if (!checkGrammarDecoratorsAndModifiers(node) && hasModifiers(node)) {
                grammarErrorOnFirstToken(node, Diagnostics.An_export_assignment_cannot_have_modifiers);
            }
            if (node.expression.kind === SyntaxKind.Identifier) {
                markExportAsReferenced(node);

                if (getEmitDeclarations(compilerOptions)) {
                    collectLinkedAliases(node.expression as Identifier, /*setVisibility*/ true);
                }
            }
            else {
                checkExpressionCached(node.expression);
            }

            checkExternalModuleExports(container);

            if ((node.flags & NodeFlags.Ambient) && !isEntityNameExpression(node.expression)) {
                grammarErrorOnNode(node.expression, Diagnostics.The_expression_of_an_export_assignment_must_be_an_identifier_or_qualified_name_in_an_ambient_context);
            }

            if (node.isExportEquals && !(node.flags & NodeFlags.Ambient)) {
                if (moduleKind >= ModuleKind.ES2015) {
                    // export assignment is not supported in es6 modules
                    grammarErrorOnNode(node, Diagnostics.Export_assignment_cannot_be_used_when_targeting_ECMAScript_modules_Consider_using_export_default_or_another_module_format_instead);
                }
                else if (moduleKind === ModuleKind.System) {
                    // system modules does not support export assignment
                    grammarErrorOnNode(node, Diagnostics.Export_assignment_is_not_supported_when_module_flag_is_system);
                }
            }
        }

        function hasExportedMembers(moduleSymbol: Symbol) {
            return forEachEntry(moduleSymbol.exports!, (_, id) => id !== "export=");
        }

        function checkExternalModuleExports(node: SourceFile | ModuleDeclaration) {
            const moduleSymbol = getSymbolOfNode(node);
            const links = getSymbolLinks(moduleSymbol);
            if (!links.exportsChecked) {
                const exportEqualsSymbol = moduleSymbol.exports!.get("export=" as __String);
                if (exportEqualsSymbol && hasExportedMembers(moduleSymbol)) {
                    const declaration = getDeclarationOfAliasSymbol(exportEqualsSymbol) || exportEqualsSymbol.valueDeclaration;
                    if (!isTopLevelInExternalModuleAugmentation(declaration) && !isInJSFile(declaration)) {
                        error(declaration, Diagnostics.An_export_assignment_cannot_be_used_in_a_module_with_other_exported_elements);
                    }
                }
                // Checks for export * conflicts
                const exports = getExportsOfModule(moduleSymbol);
                if (exports) {
                    exports.forEach(({ declarations, flags }, id) => {
                        if (id === "__export") {
                            return;
                        }
                        // ECMA262: 15.2.1.1 It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries.
                        // (TS Exceptions: namespaces, function overloads, enums, and interfaces)
                        if (flags & (SymbolFlags.Namespace | SymbolFlags.Interface | SymbolFlags.Enum)) {
                            return;
                        }
                        const exportedDeclarationsCount = countWhere(declarations, isNotOverloadAndNotAccessor);
                        if (flags & SymbolFlags.TypeAlias && exportedDeclarationsCount <= 2) {
                            // it is legal to merge type alias with other values
                            // so count should be either 1 (just type alias) or 2 (type alias + merged value)
                            return;
                        }
                        if (exportedDeclarationsCount > 1) {
                            for (const declaration of declarations) {
                                if (isNotOverload(declaration)) {
                                    diagnostics.add(createDiagnosticForNode(declaration, Diagnostics.Cannot_redeclare_exported_variable_0, unescapeLeadingUnderscores(id)));
                                }
                            }
                        }
                    });
                }
                links.exportsChecked = true;
            }
        }

        function isNotAccessor(declaration: Declaration): boolean {
            // Accessors check for their own matching duplicates, and in contexts where they are valid, there are already duplicate identifier checks
            return !isAccessor(declaration);
        }

        function isNotOverload(declaration: Declaration): boolean {
            return (declaration.kind !== SyntaxKind.FunctionDeclaration && declaration.kind !== SyntaxKind.MethodDeclaration) ||
                    !!(declaration as FunctionDeclaration).body;
        }

        function checkSourceElement(node: Node | undefined): void {
            if (!node) {
                return;
            }

            if (isInJSFile(node)) {
                forEach((node as JSDocContainer).jsDoc, ({ tags }) => forEach(tags, checkSourceElement));
            }

            const kind = node.kind;
            if (cancellationToken) {
                // Only bother checking on a few construct kinds.  We don't want to be excessively
                // hitting the cancellation token on every node we check.
                switch (kind) {
                    case SyntaxKind.ModuleDeclaration:
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.FunctionDeclaration:
                        cancellationToken.throwIfCancellationRequested();
                }
            }

            switch (kind) {
                case SyntaxKind.TypeParameter:
                    return checkTypeParameter(<TypeParameterDeclaration>node);
                case SyntaxKind.Parameter:
                    return checkParameter(<ParameterDeclaration>node);
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                    return checkPropertyDeclaration(<PropertyDeclaration | PropertySignature>node);
                case SyntaxKind.FunctionType:
                case SyntaxKind.ConstructorType:
                case SyntaxKind.CallSignature:
                case SyntaxKind.ConstructSignature:
                case SyntaxKind.IndexSignature:
                    return checkSignatureDeclaration(<SignatureDeclaration>node);
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                    return checkMethodDeclaration(<MethodDeclaration | MethodSignature>node);
                case SyntaxKind.Constructor:
                    return checkConstructorDeclaration(<ConstructorDeclaration>node);
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    return checkAccessorDeclaration(<AccessorDeclaration>node);
                case SyntaxKind.TypeReference:
                    return checkTypeReferenceNode(<TypeReferenceNode>node);
                case SyntaxKind.TypePredicate:
                    return checkTypePredicate(<TypePredicateNode>node);
                case SyntaxKind.TypeQuery:
                    return checkTypeQuery(<TypeQueryNode>node);
                case SyntaxKind.TypeLiteral:
                    return checkTypeLiteral(<TypeLiteralNode>node);
                case SyntaxKind.ArrayType:
                    return checkArrayType(<ArrayTypeNode>node);
                case SyntaxKind.TupleType:
                    return checkTupleType(<TupleTypeNode>node);
                case SyntaxKind.UnionType:
                case SyntaxKind.IntersectionType:
                    return checkUnionOrIntersectionType(<UnionOrIntersectionTypeNode>node);
                case SyntaxKind.ParenthesizedType:
                case SyntaxKind.OptionalType:
                case SyntaxKind.RestType:
                    return checkSourceElement((<ParenthesizedTypeNode | OptionalTypeNode | RestTypeNode>node).type);
                case SyntaxKind.ThisType:
                    return checkThisType(<ThisTypeNode>node);
                case SyntaxKind.TypeOperator:
                    return checkTypeOperator(<TypeOperatorNode>node);
                case SyntaxKind.ConditionalType:
                    return checkConditionalType(<ConditionalTypeNode>node);
                case SyntaxKind.InferType:
                    return checkInferType(<InferTypeNode>node);
                case SyntaxKind.ImportType:
                    return checkImportType(<ImportTypeNode>node);
                case SyntaxKind.JSDocAugmentsTag:
                    return checkJSDocAugmentsTag(node as JSDocAugmentsTag);
                case SyntaxKind.JSDocTypedefTag:
                case SyntaxKind.JSDocCallbackTag:
                    return checkJSDocTypeAliasTag(node as JSDocTypedefTag);
                case SyntaxKind.JSDocTemplateTag:
                    return checkJSDocTemplateTag(node as JSDocTemplateTag);
                case SyntaxKind.JSDocTypeTag:
                    return checkJSDocTypeTag(node as JSDocTypeTag);
                case SyntaxKind.JSDocParameterTag:
                    return checkJSDocParameterTag(node as JSDocParameterTag);
                case SyntaxKind.JSDocFunctionType:
                    checkSignatureDeclaration(node as JSDocFunctionType);
                    // falls through
                case SyntaxKind.JSDocNonNullableType:
                case SyntaxKind.JSDocNullableType:
                case SyntaxKind.JSDocAllType:
                case SyntaxKind.JSDocUnknownType:
                case SyntaxKind.JSDocTypeLiteral:
                    checkJSDocTypeIsInJsFile(node);
                    forEachChild(node, checkSourceElement);
                    return;
                case SyntaxKind.JSDocVariadicType:
                    checkJSDocVariadicType(node as JSDocVariadicType);
                    return;
                case SyntaxKind.JSDocTypeExpression:
                    return checkSourceElement((node as JSDocTypeExpression).type);
                case SyntaxKind.IndexedAccessType:
                    return checkIndexedAccessType(<IndexedAccessTypeNode>node);
                case SyntaxKind.MappedType:
                    return checkMappedType(<MappedTypeNode>node);
                case SyntaxKind.FunctionDeclaration:
                    return checkFunctionDeclaration(<FunctionDeclaration>node);
                case SyntaxKind.Block:
                case SyntaxKind.ModuleBlock:
                    return checkBlock(<Block>node);
                case SyntaxKind.VariableStatement:
                    return checkVariableStatement(<VariableStatement>node);
                case SyntaxKind.ExpressionStatement:
                    return checkExpressionStatement(<ExpressionStatement>node);
                case SyntaxKind.IfStatement:
                    return checkIfStatement(<IfStatement>node);
                case SyntaxKind.DoStatement:
                    return checkDoStatement(<DoStatement>node);
                case SyntaxKind.WhileStatement:
                    return checkWhileStatement(<WhileStatement>node);
                case SyntaxKind.ForStatement:
                    return checkForStatement(<ForStatement>node);
                case SyntaxKind.ForInStatement:
                    return checkForInStatement(<ForInStatement>node);
                case SyntaxKind.ForOfStatement:
                    return checkForOfStatement(<ForOfStatement>node);
                case SyntaxKind.ContinueStatement:
                case SyntaxKind.BreakStatement:
                    return checkBreakOrContinueStatement(<BreakOrContinueStatement>node);
                case SyntaxKind.ReturnStatement:
                    return checkReturnStatement(<ReturnStatement>node);
                case SyntaxKind.WithStatement:
                    return checkWithStatement(<WithStatement>node);
                case SyntaxKind.SwitchStatement:
                    return checkSwitchStatement(<SwitchStatement>node);
                case SyntaxKind.LabeledStatement:
                    return checkLabeledStatement(<LabeledStatement>node);
                case SyntaxKind.ThrowStatement:
                    return checkThrowStatement(<ThrowStatement>node);
                case SyntaxKind.TryStatement:
                    return checkTryStatement(<TryStatement>node);
                case SyntaxKind.VariableDeclaration:
                    return checkVariableDeclaration(<VariableDeclaration>node);
                case SyntaxKind.BindingElement:
                    return checkBindingElement(<BindingElement>node);
                case SyntaxKind.ClassDeclaration:
                    return checkClassDeclaration(<ClassDeclaration>node);
                case SyntaxKind.InterfaceDeclaration:
                    return checkInterfaceDeclaration(<InterfaceDeclaration>node);
                case SyntaxKind.TypeAliasDeclaration:
                    return checkTypeAliasDeclaration(<TypeAliasDeclaration>node);
                case SyntaxKind.EnumDeclaration:
                    return checkEnumDeclaration(<EnumDeclaration>node);
                case SyntaxKind.ModuleDeclaration:
                    return checkModuleDeclaration(<ModuleDeclaration>node);
                case SyntaxKind.ImportDeclaration:
                    return checkImportDeclaration(<ImportDeclaration>node);
                case SyntaxKind.ImportEqualsDeclaration:
                    return checkImportEqualsDeclaration(<ImportEqualsDeclaration>node);
                case SyntaxKind.ExportDeclaration:
                    return checkExportDeclaration(<ExportDeclaration>node);
                case SyntaxKind.ExportAssignment:
                    return checkExportAssignment(<ExportAssignment>node);
                case SyntaxKind.EmptyStatement:
                case SyntaxKind.DebuggerStatement:
                    checkGrammarStatementInAmbientContext(node);
                    return;
                case SyntaxKind.MissingDeclaration:
                    return checkMissingDeclaration(node);
            }
        }

        function checkJSDocTypeIsInJsFile(node: Node): void {
            if (!isInJSFile(node)) {
                grammarErrorOnNode(node, Diagnostics.JSDoc_types_can_only_be_used_inside_documentation_comments);
            }
        }

        function checkJSDocVariadicType(node: JSDocVariadicType): void {
            checkJSDocTypeIsInJsFile(node);
            checkSourceElement(node.type);

            // Only legal location is in the *last* parameter tag or last parameter of a JSDoc function.
            const { parent } = node;
            if (isParameter(parent) && isJSDocFunctionType(parent.parent)) {
                if (last(parent.parent.parameters) !== parent) {
                    error(node, Diagnostics.A_rest_parameter_must_be_last_in_a_parameter_list);
                }
                return;
            }

            if (!isJSDocTypeExpression(parent)) {
                error(node, Diagnostics.JSDoc_may_only_appear_in_the_last_parameter_of_a_signature);
            }

            const paramTag = node.parent.parent;
            if (!isJSDocParameterTag(paramTag)) {
                error(node, Diagnostics.JSDoc_may_only_appear_in_the_last_parameter_of_a_signature);
                return;
            }

            const param = getParameterSymbolFromJSDoc(paramTag);
            if (!param) {
                // We will error in `checkJSDocParameterTag`.
                return;
            }

            const host = getHostSignatureFromJSDoc(paramTag);
            if (!host || last(host.parameters).symbol !== param) {
                error(node, Diagnostics.A_rest_parameter_must_be_last_in_a_parameter_list);
            }
        }

        function getTypeFromJSDocVariadicType(node: JSDocVariadicType): Type {
            const type = getTypeFromTypeNode(node.type);
            const { parent } = node;
            const paramTag = node.parent.parent;
            if (isJSDocTypeExpression(node.parent) && isJSDocParameterTag(paramTag)) {
                // Else we will add a diagnostic, see `checkJSDocVariadicType`.
                const host = getHostSignatureFromJSDoc(paramTag);
                if (host) {
                    /*
                    Only return an array type if the corresponding parameter is marked as a rest parameter, or if there are no parameters.
                    So in the following situation we will not create an array type:
                        /** @param {...number} a * /
                        function f(a) {}
                    Because `a` will just be of type `number | undefined`. A synthetic `...args` will also be added, which *will* get an array type.
                    */
                    const lastParamDeclaration = lastOrUndefined(host.parameters);
                    const symbol = getParameterSymbolFromJSDoc(paramTag);
                    if (!lastParamDeclaration ||
                        symbol && lastParamDeclaration.symbol === symbol && isRestParameter(lastParamDeclaration)) {
                        return createArrayType(type);
                    }
                }
            }
            if (isParameter(parent) && isJSDocFunctionType(parent.parent)) {
                return createArrayType(type);
            }
            return addOptionality(type);
        }

        // Function and class expression bodies are checked after all statements in the enclosing body. This is
        // to ensure constructs like the following are permitted:
        //     const foo = function () {
        //        const s = foo();
        //        return "hello";
        //     }
        // Here, performing a full type check of the body of the function expression whilst in the process of
        // determining the type of foo would cause foo to be given type any because of the recursive reference.
        // Delaying the type check of the body ensures foo has been assigned a type.
        function checkNodeDeferred(node: Node) {
            if (deferredNodes) {
                const id = "" + getNodeId(node);
                deferredNodes.set(id, node);
            }
        }

        function checkDeferredNodes() {
            deferredNodes!.forEach(node => {
                switch (node.kind) {
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.ArrowFunction:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                        checkFunctionExpressionOrObjectLiteralMethodDeferred(<FunctionExpression>node);
                        break;
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        checkAccessorDeclaration(<AccessorDeclaration>node);
                        break;
                    case SyntaxKind.ClassExpression:
                        checkClassExpressionDeferred(<ClassExpression>node);
                        break;
                    case SyntaxKind.JsxSelfClosingElement:
                        checkJsxSelfClosingElementDeferred(<JsxSelfClosingElement>node);
                        break;
                    case SyntaxKind.JsxElement:
                        checkJsxElementDeferred(<JsxElement>node);
                        break;
                }
            });
        }

        function checkSourceFile(node: SourceFile) {
            performance.mark("beforeCheck");
            checkSourceFileWorker(node);
            performance.mark("afterCheck");
            performance.measure("Check", "beforeCheck", "afterCheck");
        }

        function unusedIsError(kind: UnusedKind): boolean {
            switch (kind) {
                case UnusedKind.Local:
                    return !!compilerOptions.noUnusedLocals;
                case UnusedKind.Parameter:
                    return !!compilerOptions.noUnusedParameters;
                default:
                    return Debug.assertNever(kind);
            }
        }

        function getPotentiallyUnusedIdentifiers(sourceFile: SourceFile): ReadonlyArray<PotentiallyUnusedIdentifier> {
            return allPotentiallyUnusedIdentifiers.get(sourceFile.path) || emptyArray;
        }

        // Fully type check a source file and collect the relevant diagnostics.
        function checkSourceFileWorker(node: SourceFile) {
            const links = getNodeLinks(node);
            if (!(links.flags & NodeCheckFlags.TypeChecked)) {
                if (skipTypeChecking(node, compilerOptions)) {
                    return;
                }

                // Grammar checking
                checkGrammarSourceFile(node);

                clear(potentialThisCollisions);
                clear(potentialNewTargetCollisions);

                deferredNodes = createMap<Node>();
                forEach(node.statements, checkSourceElement);

                checkDeferredNodes();

                if (isExternalOrCommonJsModule(node)) {
                    registerForUnusedIdentifiersCheck(node);
                }

                if (!node.isDeclarationFile && (compilerOptions.noUnusedLocals || compilerOptions.noUnusedParameters)) {
                    checkUnusedIdentifiers(getPotentiallyUnusedIdentifiers(node), (containingNode, kind, diag) => {
                        if (!containsParseError(containingNode) && unusedIsError(kind)) {
                            diagnostics.add(diag);
                        }
                    });
                }

                deferredNodes = undefined;

                if (isExternalOrCommonJsModule(node)) {
                    checkExternalModuleExports(node);
                }

                if (potentialThisCollisions.length) {
                    forEach(potentialThisCollisions, checkIfThisIsCapturedInEnclosingScope);
                    clear(potentialThisCollisions);
                }

                if (potentialNewTargetCollisions.length) {
                    forEach(potentialNewTargetCollisions, checkIfNewTargetIsCapturedInEnclosingScope);
                    clear(potentialNewTargetCollisions);
                }

                links.flags |= NodeCheckFlags.TypeChecked;
            }
        }

        function getDiagnostics(sourceFile: SourceFile, ct: CancellationToken): Diagnostic[] {
            try {
                // Record the cancellation token so it can be checked later on during checkSourceElement.
                // Do this in a finally block so we can ensure that it gets reset back to nothing after
                // this call is done.
                cancellationToken = ct;
                return getDiagnosticsWorker(sourceFile);
            }
            finally {
                cancellationToken = undefined;
            }
        }

        function getDiagnosticsWorker(sourceFile: SourceFile): Diagnostic[] {
            throwIfNonDiagnosticsProducing();
            if (sourceFile) {
                // Some global diagnostics are deferred until they are needed and
                // may not be reported in the firt call to getGlobalDiagnostics.
                // We should catch these changes and report them.
                const previousGlobalDiagnostics = diagnostics.getGlobalDiagnostics();
                const previousGlobalDiagnosticsSize = previousGlobalDiagnostics.length;

                checkSourceFile(sourceFile);

                const semanticDiagnostics = diagnostics.getDiagnostics(sourceFile.fileName);
                const currentGlobalDiagnostics = diagnostics.getGlobalDiagnostics();
                if (currentGlobalDiagnostics !== previousGlobalDiagnostics) {
                    // If the arrays are not the same reference, new diagnostics were added.
                    const deferredGlobalDiagnostics = relativeComplement(previousGlobalDiagnostics, currentGlobalDiagnostics, compareDiagnostics);
                    return concatenate(deferredGlobalDiagnostics, semanticDiagnostics);
                }
                else if (previousGlobalDiagnosticsSize === 0 && currentGlobalDiagnostics.length > 0) {
                    // If the arrays are the same reference, but the length has changed, a single
                    // new diagnostic was added as DiagnosticCollection attempts to reuse the
                    // same array.
                    return concatenate(currentGlobalDiagnostics, semanticDiagnostics);
                }

                return semanticDiagnostics;
            }

            // Global diagnostics are always added when a file is not provided to
            // getDiagnostics
            forEach(host.getSourceFiles(), checkSourceFile);
            return diagnostics.getDiagnostics();
        }

        function getGlobalDiagnostics(): Diagnostic[] {
            throwIfNonDiagnosticsProducing();
            return diagnostics.getGlobalDiagnostics();
        }

        function throwIfNonDiagnosticsProducing() {
            if (!produceDiagnostics) {
                throw new Error("Trying to get diagnostics from a type checker that does not produce them.");
            }
        }

        // Language service support

        function getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[] {
            if (location.flags & NodeFlags.InWithStatement) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return [];
            }

            const symbols = createSymbolTable();
            let isStatic = false;

            populateSymbols();

            symbols.delete(InternalSymbolName.This); // Not a symbol, a keyword
            return symbolsToArray(symbols);

            function populateSymbols() {
                while (location) {
                    if (location.locals && !isGlobalSourceFile(location)) {
                        copySymbols(location.locals, meaning);
                    }

                    switch (location.kind) {
                        case SyntaxKind.ModuleDeclaration:
                            copySymbols(getSymbolOfNode(location as ModuleDeclaration).exports!, meaning & SymbolFlags.ModuleMember);
                            break;
                        case SyntaxKind.EnumDeclaration:
                            copySymbols(getSymbolOfNode(location as EnumDeclaration).exports!, meaning & SymbolFlags.EnumMember);
                            break;
                        case SyntaxKind.ClassExpression:
                            const className = (location as ClassExpression).name;
                            if (className) {
                                copySymbol(location.symbol, meaning);
                            }
                        // falls through
                        // this fall-through is necessary because we would like to handle
                        // type parameter inside class expression similar to how we handle it in classDeclaration and interface Declaration
                        case SyntaxKind.ClassDeclaration:
                        case SyntaxKind.InterfaceDeclaration:
                            // If we didn't come from static member of class or interface,
                            // add the type parameters into the symbol table
                            // (type parameters of classDeclaration/classExpression and interface are in member property of the symbol.
                            // Note: that the memberFlags come from previous iteration.
                            if (!isStatic) {
                                copySymbols(getMembersOfSymbol(getSymbolOfNode(location as ClassDeclaration | InterfaceDeclaration)), meaning & SymbolFlags.Type);
                            }
                            break;
                        case SyntaxKind.FunctionExpression:
                            const funcName = (location as FunctionExpression).name;
                            if (funcName) {
                                copySymbol(location.symbol, meaning);
                            }
                            break;
                    }

                    if (introducesArgumentsExoticObject(location)) {
                        copySymbol(argumentsSymbol, meaning);
                    }

                    isStatic = hasModifier(location, ModifierFlags.Static);
                    location = location.parent;
                }

                copySymbols(globals, meaning);
            }

            /**
             * Copy the given symbol into symbol tables if the symbol has the given meaning
             * and it doesn't already existed in the symbol table
             * @param key a key for storing in symbol table; if undefined, use symbol.name
             * @param symbol the symbol to be added into symbol table
             * @param meaning meaning of symbol to filter by before adding to symbol table
             */
            function copySymbol(symbol: Symbol, meaning: SymbolFlags): void {
                if (getCombinedLocalAndExportSymbolFlags(symbol) & meaning) {
                    const id = symbol.escapedName;
                    // We will copy all symbol regardless of its reserved name because
                    // symbolsToArray will check whether the key is a reserved name and
                    // it will not copy symbol with reserved name to the array
                    if (!symbols.has(id)) {
                        symbols.set(id, symbol);
                    }
                }
            }

            function copySymbols(source: SymbolTable, meaning: SymbolFlags): void {
                if (meaning) {
                    source.forEach(symbol => {
                        copySymbol(symbol, meaning);
                    });
                }
            }
        }

        function isTypeDeclarationName(name: Node): boolean {
            return name.kind === SyntaxKind.Identifier &&
                isTypeDeclaration(name.parent) &&
                (<NamedDeclaration>name.parent).name === name;
        }

        function isTypeDeclaration(node: Node): node is TypeParameterDeclaration | ClassDeclaration | InterfaceDeclaration | TypeAliasDeclaration | EnumDeclaration {
            switch (node.kind) {
                case SyntaxKind.TypeParameter:
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.InterfaceDeclaration:
                case SyntaxKind.TypeAliasDeclaration:
                case SyntaxKind.EnumDeclaration:
                    return true;
                default:
                    return false;
            }
        }

        // True if the given identifier is part of a type reference
        function isTypeReferenceIdentifier(node: EntityName): boolean {
            while (node.parent.kind === SyntaxKind.QualifiedName) {
                node = node.parent as QualifiedName;
            }

            return node.parent.kind === SyntaxKind.TypeReference;
        }

        function isHeritageClauseElementIdentifier(node: Node): boolean {
            while (node.parent.kind === SyntaxKind.PropertyAccessExpression) {
                node = node.parent;
            }

            return node.parent.kind === SyntaxKind.ExpressionWithTypeArguments;
        }

        function forEachEnclosingClass<T>(node: Node, callback: (node: Node) => T | undefined): T | undefined {
            let result: T | undefined;

            while (true) {
                node = getContainingClass(node)!;
                if (!node) break;
                if (result = callback(node)) break;
            }

            return result;
        }

        function isNodeUsedDuringClassInitialization(node: Node, classDeclaration: ClassLikeDeclaration) {
            return !!findAncestor(node, element => {
                if ((isConstructorDeclaration(element) && nodeIsPresent(element.body) || isPropertyDeclaration(element)) && element.parent === classDeclaration) {
                    return true;
                }
                else if (element === classDeclaration || isFunctionLikeDeclaration(element)) {
                    return "quit";
                }

                return false;
            });
        }

        function isNodeWithinClass(node: Node, classDeclaration: ClassLikeDeclaration) {
            return !!forEachEnclosingClass(node, n => n === classDeclaration);
        }

        function getLeftSideOfImportEqualsOrExportAssignment(nodeOnRightSide: EntityName): ImportEqualsDeclaration | ExportAssignment | undefined {
            while (nodeOnRightSide.parent.kind === SyntaxKind.QualifiedName) {
                nodeOnRightSide = <QualifiedName>nodeOnRightSide.parent;
            }

            if (nodeOnRightSide.parent.kind === SyntaxKind.ImportEqualsDeclaration) {
                return (<ImportEqualsDeclaration>nodeOnRightSide.parent).moduleReference === nodeOnRightSide ? <ImportEqualsDeclaration>nodeOnRightSide.parent : undefined;
            }

            if (nodeOnRightSide.parent.kind === SyntaxKind.ExportAssignment) {
                return (<ExportAssignment>nodeOnRightSide.parent).expression === <Node>nodeOnRightSide ? <ExportAssignment>nodeOnRightSide.parent : undefined;
            }

            return undefined;
        }

        function isInRightSideOfImportOrExportAssignment(node: EntityName) {
            return getLeftSideOfImportEqualsOrExportAssignment(node) !== undefined;
        }

        function getSpecialPropertyAssignmentSymbolFromEntityName(entityName: EntityName | PropertyAccessExpression) {
            const specialPropertyAssignmentKind = getAssignmentDeclarationKind(entityName.parent.parent as BinaryExpression);
            switch (specialPropertyAssignmentKind) {
                case AssignmentDeclarationKind.ExportsProperty:
                case AssignmentDeclarationKind.PrototypeProperty:
                    return getSymbolOfNode(entityName.parent);
                case AssignmentDeclarationKind.ThisProperty:
                case AssignmentDeclarationKind.ModuleExports:
                case AssignmentDeclarationKind.Property:
                    return getSymbolOfNode(entityName.parent.parent);
            }
        }

        function isImportTypeQualifierPart(node: EntityName): ImportTypeNode | undefined {
            let parent = node.parent;
            while (isQualifiedName(parent)) {
                node = parent;
                parent = parent.parent;
            }
            if (parent && parent.kind === SyntaxKind.ImportType && (parent as ImportTypeNode).qualifier === node) {
                return parent as ImportTypeNode;
            }
            return undefined;
        }

        function getSymbolOfEntityNameOrPropertyAccessExpression(entityName: EntityName | PropertyAccessExpression): Symbol | undefined {
            if (isDeclarationName(entityName)) {
                return getSymbolOfNode(entityName.parent);
            }

            if (isInJSFile(entityName) &&
                entityName.parent.kind === SyntaxKind.PropertyAccessExpression &&
                entityName.parent === (entityName.parent.parent as BinaryExpression).left) {
                // Check if this is a special property assignment
                const specialPropertyAssignmentSymbol = getSpecialPropertyAssignmentSymbolFromEntityName(entityName);
                if (specialPropertyAssignmentSymbol) {
                    return specialPropertyAssignmentSymbol;
                }
            }

            if (entityName.parent.kind === SyntaxKind.ExportAssignment && isEntityNameExpression(entityName)) {
                // Even an entity name expression that doesn't resolve as an entityname may still typecheck as a property access expression
                const success = resolveEntityName(entityName,
                    /*all meanings*/ SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias, /*ignoreErrors*/ true);
                if (success && success !== unknownSymbol) {
                    return success;
                }
            }
            else if (!isPropertyAccessExpression(entityName) && isInRightSideOfImportOrExportAssignment(entityName)) {
                // Since we already checked for ExportAssignment, this really could only be an Import
                const importEqualsDeclaration = getAncestor(entityName, SyntaxKind.ImportEqualsDeclaration);
                Debug.assert(importEqualsDeclaration !== undefined);
                return getSymbolOfPartOfRightHandSideOfImportEquals(entityName, /*dontResolveAlias*/ true);
            }

            if (!isPropertyAccessExpression(entityName)) {
                const possibleImportNode = isImportTypeQualifierPart(entityName);
                if (possibleImportNode) {
                    getTypeFromTypeNode(possibleImportNode);
                    const sym = getNodeLinks(entityName).resolvedSymbol;
                    return sym === unknownSymbol ? undefined : sym;
                }
            }

            while (isRightSideOfQualifiedNameOrPropertyAccess(entityName)) {
                entityName = <QualifiedName | PropertyAccessEntityNameExpression>entityName.parent;
            }

            if (isHeritageClauseElementIdentifier(entityName)) {
                let meaning = SymbolFlags.None;
                // In an interface or class, we're definitely interested in a type.
                if (entityName.parent.kind === SyntaxKind.ExpressionWithTypeArguments) {
                    meaning = SymbolFlags.Type;

                    // In a class 'extends' clause we are also looking for a value.
                    if (isExpressionWithTypeArgumentsInClassExtendsClause(entityName.parent)) {
                        meaning |= SymbolFlags.Value;
                    }
                }
                else {
                    meaning = SymbolFlags.Namespace;
                }

                meaning |= SymbolFlags.Alias;
                const entityNameSymbol = isEntityNameExpression(entityName) ? resolveEntityName(entityName, meaning) : undefined;
                if (entityNameSymbol) {
                    return entityNameSymbol;
                }
            }

            if (entityName.parent.kind === SyntaxKind.JSDocParameterTag) {
                return getParameterSymbolFromJSDoc(entityName.parent as JSDocParameterTag);
            }

            if (entityName.parent.kind === SyntaxKind.TypeParameter && entityName.parent.parent.kind === SyntaxKind.JSDocTemplateTag) {
                Debug.assert(!isInJSFile(entityName)); // Otherwise `isDeclarationName` would have been true.
                const typeParameter = getTypeParameterFromJsDoc(entityName.parent as TypeParameterDeclaration & { parent: JSDocTemplateTag });
                return typeParameter && typeParameter.symbol;
            }

            if (isExpressionNode(entityName)) {
                if (nodeIsMissing(entityName)) {
                    // Missing entity name.
                    return undefined;
                }

                if (entityName.kind === SyntaxKind.Identifier) {
                    if (isJSXTagName(entityName) && isJsxIntrinsicIdentifier(entityName)) {
                        const symbol = getIntrinsicTagSymbol(<JsxOpeningLikeElement>entityName.parent);
                        return symbol === unknownSymbol ? undefined : symbol;
                    }

                    return resolveEntityName(entityName, SymbolFlags.Value, /*ignoreErrors*/ false, /*dontResolveAlias*/ true);
                }
                else if (entityName.kind === SyntaxKind.PropertyAccessExpression || entityName.kind === SyntaxKind.QualifiedName) {
                    const links = getNodeLinks(entityName);
                    if (links.resolvedSymbol) {
                        return links.resolvedSymbol;
                    }

                    if (entityName.kind === SyntaxKind.PropertyAccessExpression) {
                        checkPropertyAccessExpression(entityName);
                    }
                    else {
                        checkQualifiedName(entityName);
                    }
                    return links.resolvedSymbol;
                }
            }
            else if (isTypeReferenceIdentifier(<EntityName>entityName)) {
                const meaning = entityName.parent.kind === SyntaxKind.TypeReference ? SymbolFlags.Type : SymbolFlags.Namespace;
                return resolveEntityName(<EntityName>entityName, meaning, /*ignoreErrors*/ false, /*dontResolveAlias*/ true);
            }
            else if (entityName.parent.kind === SyntaxKind.JsxAttribute) {
                return getJsxAttributePropertySymbol(<JsxAttribute>entityName.parent);
            }

            if (entityName.parent.kind === SyntaxKind.TypePredicate) {
                return resolveEntityName(<Identifier>entityName, /*meaning*/ SymbolFlags.FunctionScopedVariable);
            }

            // Do we want to return undefined here?
            return undefined;
        }

        function getSymbolAtLocation(node: Node): Symbol | undefined {
            if (node.kind === SyntaxKind.SourceFile) {
                return isExternalModule(<SourceFile>node) ? getMergedSymbol(node.symbol) : undefined;
            }
            const { parent } = node;
            const grandParent = parent.parent;

            if (node.flags & NodeFlags.InWithStatement) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return undefined;
            }

            if (isDeclarationNameOrImportPropertyName(node)) {
                // This is a declaration, call getSymbolOfNode
                return getSymbolOfNode(parent);
            }
            else if (isLiteralComputedPropertyDeclarationName(node)) {
                return getSymbolOfNode(parent.parent);
            }

            if (node.kind === SyntaxKind.Identifier) {
                if (isInRightSideOfImportOrExportAssignment(<Identifier>node)) {
                    return getSymbolOfEntityNameOrPropertyAccessExpression(<Identifier>node);
                }
                else if (parent.kind === SyntaxKind.BindingElement &&
                    grandParent.kind === SyntaxKind.ObjectBindingPattern &&
                    node === (<BindingElement>parent).propertyName) {
                    const typeOfPattern = getTypeOfNode(grandParent);
                    const propertyDeclaration = getPropertyOfType(typeOfPattern, (<Identifier>node).escapedText);

                    if (propertyDeclaration) {
                        return propertyDeclaration;
                    }
                }
            }

            switch (node.kind) {
                case SyntaxKind.Identifier:
                case SyntaxKind.PropertyAccessExpression:
                case SyntaxKind.QualifiedName:
                    return getSymbolOfEntityNameOrPropertyAccessExpression(<EntityName | PropertyAccessExpression>node);

                case SyntaxKind.ThisKeyword:
                    const container = getThisContainer(node, /*includeArrowFunctions*/ false);
                    if (isFunctionLike(container)) {
                        const sig = getSignatureFromDeclaration(container);
                        if (sig.thisParameter) {
                            return sig.thisParameter;
                        }
                    }
                    if (isInExpressionContext(node)) {
                        return checkExpression(node as Expression).symbol;
                    }
                    // falls through

                case SyntaxKind.ThisType:
                    return getTypeFromThisTypeNode(node as ThisExpression | ThisTypeNode).symbol;

                case SyntaxKind.SuperKeyword:
                    return checkExpression(node as Expression).symbol;

                case SyntaxKind.ConstructorKeyword:
                    // constructor keyword for an overload, should take us to the definition if it exist
                    const constructorDeclaration = node.parent;
                    if (constructorDeclaration && constructorDeclaration.kind === SyntaxKind.Constructor) {
                        return (<ClassDeclaration>constructorDeclaration.parent).symbol;
                    }
                    return undefined;

                case SyntaxKind.StringLiteral:
                case SyntaxKind.NoSubstitutionTemplateLiteral:
                    // 1). import x = require("./mo/*gotToDefinitionHere*/d")
                    // 2). External module name in an import declaration
                    // 3). Dynamic import call or require in javascript
                    // 4). type A = import("./f/*gotToDefinitionHere*/oo")
                    if ((isExternalModuleImportEqualsDeclaration(node.parent.parent) && getExternalModuleImportEqualsDeclarationExpression(node.parent.parent) === node) ||
                        ((node.parent.kind === SyntaxKind.ImportDeclaration || node.parent.kind === SyntaxKind.ExportDeclaration) && (<ImportDeclaration>node.parent).moduleSpecifier === node) ||
                        ((isInJSFile(node) && isRequireCall(node.parent, /*checkArgumentIsStringLiteralLike*/ false)) || isImportCall(node.parent)) ||
                        (isLiteralTypeNode(node.parent) && isLiteralImportTypeNode(node.parent.parent) && node.parent.parent.argument === node.parent)
                    ) {
                        return resolveExternalModuleName(node, <LiteralExpression>node);
                    }
                    // falls through

                case SyntaxKind.NumericLiteral:
                    // index access
                    const objectType = isElementAccessExpression(parent)
                        ? parent.argumentExpression === node ? getTypeOfExpression(parent.expression) : undefined
                        : isLiteralTypeNode(parent) && isIndexedAccessTypeNode(grandParent)
                            ? getTypeFromTypeNode(grandParent.objectType)
                            : undefined;
                    return objectType && getPropertyOfType(objectType, escapeLeadingUnderscores((node as StringLiteral | NumericLiteral).text));

                case SyntaxKind.DefaultKeyword:
                case SyntaxKind.FunctionKeyword:
                case SyntaxKind.EqualsGreaterThanToken:
                case SyntaxKind.ClassKeyword:
                    return getSymbolOfNode(node.parent);
                case SyntaxKind.ImportType:
                    return isLiteralImportTypeNode(node) ? getSymbolAtLocation(node.argument.literal) : undefined;

                default:
                    return undefined;
            }
        }

        function getShorthandAssignmentValueSymbol(location: Node): Symbol | undefined {
            if (location && location.kind === SyntaxKind.ShorthandPropertyAssignment) {
                return resolveEntityName((<ShorthandPropertyAssignment>location).name, SymbolFlags.Value | SymbolFlags.Alias);
            }
            return undefined;
        }

        /** Returns the target of an export specifier without following aliases */
        function getExportSpecifierLocalTargetSymbol(node: ExportSpecifier): Symbol | undefined {
            return node.parent.parent.moduleSpecifier ?
                getExternalModuleMember(node.parent.parent, node) :
                resolveEntityName(node.propertyName || node.name, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias);
        }

        function getTypeOfNode(node: Node): Type {
            if (node.flags & NodeFlags.InWithStatement) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return errorType;
            }

            if (isPartOfTypeNode(node)) {
                const typeFromTypeNode = getTypeFromTypeNode(<TypeNode>node);

                if (isExpressionWithTypeArgumentsInClassImplementsClause(node)) {
                    return getTypeWithThisArgument(typeFromTypeNode, getTypeOfClassContainingHeritageClause(node).thisType);
                }

                return typeFromTypeNode;
            }

            if (isExpressionNode(node)) {
                return getRegularTypeOfExpression(<Expression>node);
            }

            if (isExpressionWithTypeArgumentsInClassExtendsClause(node)) {
                // A SyntaxKind.ExpressionWithTypeArguments is considered a type node, except when it occurs in the
                // extends clause of a class. We handle that case here.
                const classType = getTypeOfClassContainingHeritageClause(node);
                const baseType = firstOrUndefined(getBaseTypes(classType));
                return baseType ? getTypeWithThisArgument(baseType, classType.thisType) : errorType;
            }

            if (isTypeDeclaration(node)) {
                // In this case, we call getSymbolOfNode instead of getSymbolAtLocation because it is a declaration
                const symbol = getSymbolOfNode(node);
                return getDeclaredTypeOfSymbol(symbol);
            }

            if (isTypeDeclarationName(node)) {
                const symbol = getSymbolAtLocation(node);
                return symbol ? getDeclaredTypeOfSymbol(symbol) : errorType;
            }

            if (isDeclaration(node)) {
                // In this case, we call getSymbolOfNode instead of getSymbolAtLocation because it is a declaration
                const symbol = getSymbolOfNode(node);
                return getTypeOfSymbol(symbol);
            }

            if (isDeclarationNameOrImportPropertyName(node)) {
                const symbol = getSymbolAtLocation(node);
                return symbol ? getTypeOfSymbol(symbol) : errorType;
            }

            if (isBindingPattern(node)) {
                return getTypeForVariableLikeDeclaration(node.parent, /*includeOptionality*/ true) || errorType;
            }

            if (isInRightSideOfImportOrExportAssignment(<Identifier>node)) {
                const symbol = getSymbolAtLocation(node);
                if (symbol) {
                    const declaredType = getDeclaredTypeOfSymbol(symbol);
                    return declaredType !== errorType ? declaredType : getTypeOfSymbol(symbol);
                }
            }

            return errorType;
        }

        function getTypeOfClassContainingHeritageClause(node: ExpressionWithTypeArguments): InterfaceType {
            return getDeclaredTypeOfClassOrInterface(getSymbolOfNode(node.parent.parent));
        }

        // Gets the type of object literal or array literal of destructuring assignment.
        // { a } from
        //     for ( { a } of elems) {
        //     }
        // [ a ] from
        //     [a] = [ some array ...]
        function getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(expr: Expression): Type {
            Debug.assert(expr.kind === SyntaxKind.ObjectLiteralExpression || expr.kind === SyntaxKind.ArrayLiteralExpression);
            // If this is from "for of"
            //     for ( { a } of elems) {
            //     }
            if (expr.parent.kind === SyntaxKind.ForOfStatement) {
                const iteratedType = checkRightHandSideOfForOf((<ForOfStatement>expr.parent).expression, (<ForOfStatement>expr.parent).awaitModifier);
                return checkDestructuringAssignment(expr, iteratedType || errorType);
            }
            // If this is from "for" initializer
            //     for ({a } = elems[0];.....) { }
            if (expr.parent.kind === SyntaxKind.BinaryExpression) {
                const iteratedType = getTypeOfExpression((<BinaryExpression>expr.parent).right);
                return checkDestructuringAssignment(expr, iteratedType || errorType);
            }
            // If this is from nested object binding pattern
            //     for ({ skills: { primary, secondary } } = multiRobot, i = 0; i < 1; i++) {
            if (expr.parent.kind === SyntaxKind.PropertyAssignment) {
                const typeOfParentObjectLiteral = getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(<Expression>expr.parent.parent);
                return checkObjectLiteralDestructuringPropertyAssignment(typeOfParentObjectLiteral || errorType, <ObjectLiteralElementLike>expr.parent)!; // TODO: GH#18217
            }
            // Array literal assignment - array destructuring pattern
            Debug.assert(expr.parent.kind === SyntaxKind.ArrayLiteralExpression);
            //    [{ property1: p1, property2 }] = elems;
            const typeOfArrayLiteral = getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(<Expression>expr.parent);
            const elementType = checkIteratedTypeOrElementType(typeOfArrayLiteral || errorType, expr.parent, /*allowStringInput*/ false, /*allowAsyncIterables*/ false) || errorType;
            return checkArrayLiteralDestructuringElementAssignment(<ArrayLiteralExpression>expr.parent, typeOfArrayLiteral,
                (<ArrayLiteralExpression>expr.parent).elements.indexOf(expr), elementType || errorType)!; // TODO: GH#18217
        }

        // Gets the property symbol corresponding to the property in destructuring assignment
        // 'property1' from
        //     for ( { property1: a } of elems) {
        //     }
        // 'property1' at location 'a' from:
        //     [a] = [ property1, property2 ]
        function getPropertySymbolOfDestructuringAssignment(location: Identifier) {
            // Get the type of the object or array literal and then look for property of given name in the type
            const typeOfObjectLiteral = getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(<Expression>location.parent.parent);
            return typeOfObjectLiteral && getPropertyOfType(typeOfObjectLiteral, location.escapedText);
        }

        function getRegularTypeOfExpression(expr: Expression): Type {
            if (isRightSideOfQualifiedNameOrPropertyAccess(expr)) {
                expr = <Expression>expr.parent;
            }
            return getRegularTypeOfLiteralType(getTypeOfExpression(expr));
        }

        /**
         * Gets either the static or instance type of a class element, based on
         * whether the element is declared as "static".
         */
        function getParentTypeOfClassElement(node: ClassElement) {
            const classSymbol = getSymbolOfNode(node.parent)!;
            return hasModifier(node, ModifierFlags.Static)
                ? getTypeOfSymbol(classSymbol)
                : getDeclaredTypeOfSymbol(classSymbol);
        }

        function getClassElementPropertyKeyType(element: ClassElement) {
            const name = element.name!;
            switch (name.kind) {
                case SyntaxKind.Identifier:
                    return getLiteralType(idText(name));
                case SyntaxKind.NumericLiteral:
                case SyntaxKind.StringLiteral:
                    return getLiteralType(name.text);
                case SyntaxKind.ComputedPropertyName:
                    const nameType = checkComputedPropertyName(name);
                    return isTypeAssignableToKind(nameType, TypeFlags.ESSymbolLike) ? nameType : stringType;
                default:
                    Debug.fail("Unsupported property name.");
                    return errorType;
            }
        }

        // Return the list of properties of the given type, augmented with properties from Function
        // if the type has call or construct signatures
        function getAugmentedPropertiesOfType(type: Type): Symbol[] {
            type = getApparentType(type);
            const propsByName = createSymbolTable(getPropertiesOfType(type));
            if (typeHasCallOrConstructSignatures(type)) {
                forEach(getPropertiesOfType(globalFunctionType), p => {
                    if (!propsByName.has(p.escapedName)) {
                        propsByName.set(p.escapedName, p);
                    }
                });
            }
            return getNamedMembers(propsByName);
        }

        function typeHasCallOrConstructSignatures(type: Type): boolean {
            return ts.typeHasCallOrConstructSignatures(type, checker);
        }

        function getRootSymbols(symbol: Symbol): Symbol[] {
            const roots = getImmediateRootSymbols(symbol);
            return roots ? flatMap(roots, getRootSymbols) : [symbol];
        }
        function getImmediateRootSymbols(symbol: Symbol): ReadonlyArray<Symbol> | undefined {
            if (getCheckFlags(symbol) & CheckFlags.Synthetic) {
                return mapDefined(getSymbolLinks(symbol).containingType!.types, type => getPropertyOfType(type, symbol.escapedName));
            }
            else if (symbol.flags & SymbolFlags.Transient) {
                const { leftSpread, rightSpread, syntheticOrigin } = symbol as TransientSymbol;
                return leftSpread ? [leftSpread, rightSpread!]
                    : syntheticOrigin ? [syntheticOrigin]
                    : singleElementArray(tryGetAliasTarget(symbol));
            }
            return undefined;
        }
        function tryGetAliasTarget(symbol: Symbol): Symbol | undefined {
            let target: Symbol | undefined;
            let next: Symbol | undefined = symbol;
            while (next = getSymbolLinks(next).target) {
               target = next;
            }
            return target;
        }

        // Emitter support

        function isArgumentsLocalBinding(nodeIn: Identifier): boolean {
            if (!isGeneratedIdentifier(nodeIn)) {
                const node = getParseTreeNode(nodeIn, isIdentifier);
                if (node) {
                    const isPropertyName = node.parent.kind === SyntaxKind.PropertyAccessExpression && (<PropertyAccessExpression>node.parent).name === node;
                    return !isPropertyName && getReferencedValueSymbol(node) === argumentsSymbol;
                }
            }

            return false;
        }

        function moduleExportsSomeValue(moduleReferenceExpression: Expression): boolean {
            let moduleSymbol = resolveExternalModuleName(moduleReferenceExpression.parent, moduleReferenceExpression);
            if (!moduleSymbol || isShorthandAmbientModuleSymbol(moduleSymbol)) {
                // If the module is not found or is shorthand, assume that it may export a value.
                return true;
            }

            const hasExportAssignment = hasExportAssignmentSymbol(moduleSymbol);
            // if module has export assignment then 'resolveExternalModuleSymbol' will return resolved symbol for export assignment
            // otherwise it will return moduleSymbol itself
            moduleSymbol = resolveExternalModuleSymbol(moduleSymbol);

            const symbolLinks = getSymbolLinks(moduleSymbol);
            if (symbolLinks.exportsSomeValue === undefined) {
                // for export assignments - check if resolved symbol for RHS is itself a value
                // otherwise - check if at least one export is value
                symbolLinks.exportsSomeValue = hasExportAssignment
                    ? !!(moduleSymbol.flags & SymbolFlags.Value)
                    : forEachEntry(getExportsOfModule(moduleSymbol), isValue);
            }

            return symbolLinks.exportsSomeValue!;

            function isValue(s: Symbol): boolean {
                s = resolveSymbol(s);
                return s && !!(s.flags & SymbolFlags.Value);
            }
        }

        function isNameOfModuleOrEnumDeclaration(node: Identifier) {
            return isModuleOrEnumDeclaration(node.parent) && node === node.parent.name;
        }

        // When resolved as an expression identifier, if the given node references an exported entity, return the declaration
        // node of the exported entity's container. Otherwise, return undefined.
        function getReferencedExportContainer(nodeIn: Identifier, prefixLocals?: boolean): SourceFile | ModuleDeclaration | EnumDeclaration | undefined {
            const node = getParseTreeNode(nodeIn, isIdentifier);
            if (node) {
                // When resolving the export container for the name of a module or enum
                // declaration, we need to start resolution at the declaration's container.
                // Otherwise, we could incorrectly resolve the export container as the
                // declaration if it contains an exported member with the same name.
                let symbol = getReferencedValueSymbol(node, /*startInDeclarationContainer*/ isNameOfModuleOrEnumDeclaration(node));
                if (symbol) {
                    if (symbol.flags & SymbolFlags.ExportValue) {
                        // If we reference an exported entity within the same module declaration, then whether
                        // we prefix depends on the kind of entity. SymbolFlags.ExportHasLocal encompasses all the
                        // kinds that we do NOT prefix.
                        const exportSymbol = getMergedSymbol(symbol.exportSymbol!);
                        if (!prefixLocals && exportSymbol.flags & SymbolFlags.ExportHasLocal && !(exportSymbol.flags & SymbolFlags.Variable)) {
                            return undefined;
                        }
                        symbol = exportSymbol;
                    }
                    const parentSymbol = getParentOfSymbol(symbol);
                    if (parentSymbol) {
                        if (parentSymbol.flags & SymbolFlags.ValueModule && parentSymbol.valueDeclaration.kind === SyntaxKind.SourceFile) {
                            const symbolFile = <SourceFile>parentSymbol.valueDeclaration;
                            const referenceFile = getSourceFileOfNode(node);
                            // If `node` accesses an export and that export isn't in the same file, then symbol is a namespace export, so return undefined.
                            const symbolIsUmdExport = symbolFile !== referenceFile;
                            return symbolIsUmdExport ? undefined : symbolFile;
                        }
                        return findAncestor(node.parent, (n): n is ModuleDeclaration | EnumDeclaration => isModuleOrEnumDeclaration(n) && getSymbolOfNode(n) === parentSymbol);
                    }
                }
            }
        }

        // When resolved as an expression identifier, if the given node references an import, return the declaration of
        // that import. Otherwise, return undefined.
        function getReferencedImportDeclaration(nodeIn: Identifier): Declaration | undefined {
            const node = getParseTreeNode(nodeIn, isIdentifier);
            if (node) {
                const symbol = getReferencedValueSymbol(node);
                // We should only get the declaration of an alias if there isn't a local value
                // declaration for the symbol
                if (isNonLocalAlias(symbol, /*excludes*/ SymbolFlags.Value)) {
                    return getDeclarationOfAliasSymbol(symbol);
                }
            }

            return undefined;
        }

        function isSymbolOfDeclarationWithCollidingName(symbol: Symbol): boolean {
            if (symbol.flags & SymbolFlags.BlockScoped) {
                const links = getSymbolLinks(symbol);
                if (links.isDeclarationWithCollidingName === undefined) {
                    const container = getEnclosingBlockScopeContainer(symbol.valueDeclaration);
                    if (isStatementWithLocals(container)) {
                        const nodeLinks = getNodeLinks(symbol.valueDeclaration);
                        if (resolveName(container.parent, symbol.escapedName, SymbolFlags.Value, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ false)) {
                            // redeclaration - always should be renamed
                            links.isDeclarationWithCollidingName = true;
                        }
                        else if (nodeLinks.flags & NodeCheckFlags.CapturedBlockScopedBinding) {
                            // binding is captured in the function
                            // should be renamed if:
                            // - binding is not top level - top level bindings never collide with anything
                            // AND
                            //   - binding is not declared in loop, should be renamed to avoid name reuse across siblings
                            //     let a, b
                            //     { let x = 1; a = () => x; }
                            //     { let x = 100; b = () => x; }
                            //     console.log(a()); // should print '1'
                            //     console.log(b()); // should print '100'
                            //     OR
                            //   - binding is declared inside loop but not in inside initializer of iteration statement or directly inside loop body
                            //     * variables from initializer are passed to rewritten loop body as parameters so they are not captured directly
                            //     * variables that are declared immediately in loop body will become top level variable after loop is rewritten and thus
                            //       they will not collide with anything
                            const isDeclaredInLoop = nodeLinks.flags & NodeCheckFlags.BlockScopedBindingInLoop;
                            const inLoopInitializer = isIterationStatement(container, /*lookInLabeledStatements*/ false);
                            const inLoopBodyBlock = container.kind === SyntaxKind.Block && isIterationStatement(container.parent, /*lookInLabeledStatements*/ false);

                            links.isDeclarationWithCollidingName = !isBlockScopedContainerTopLevel(container) && (!isDeclaredInLoop || (!inLoopInitializer && !inLoopBodyBlock));
                        }
                        else {
                            links.isDeclarationWithCollidingName = false;
                        }
                    }
                }
                return links.isDeclarationWithCollidingName!;
            }
            return false;
        }

        // When resolved as an expression identifier, if the given node references a nested block scoped entity with
        // a name that either hides an existing name or might hide it when compiled downlevel,
        // return the declaration of that entity. Otherwise, return undefined.
        function getReferencedDeclarationWithCollidingName(nodeIn: Identifier): Declaration | undefined {
            if (!isGeneratedIdentifier(nodeIn)) {
                const node = getParseTreeNode(nodeIn, isIdentifier);
                if (node) {
                    const symbol = getReferencedValueSymbol(node);
                    if (symbol && isSymbolOfDeclarationWithCollidingName(symbol)) {
                        return symbol.valueDeclaration;
                    }
                }
            }

            return undefined;
        }

        // Return true if the given node is a declaration of a nested block scoped entity with a name that either hides an
        // existing name or might hide a name when compiled downlevel
        function isDeclarationWithCollidingName(nodeIn: Declaration): boolean {
            const node = getParseTreeNode(nodeIn, isDeclaration);
            if (node) {
                const symbol = getSymbolOfNode(node);
                if (symbol) {
                    return isSymbolOfDeclarationWithCollidingName(symbol);
                }
            }

            return false;
        }

        function isValueAliasDeclaration(node: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.ImportEqualsDeclaration:
                case SyntaxKind.ImportClause:
                case SyntaxKind.NamespaceImport:
                case SyntaxKind.ImportSpecifier:
                case SyntaxKind.ExportSpecifier:
                    return isAliasResolvedToValue(getSymbolOfNode(node) || unknownSymbol);
                case SyntaxKind.ExportDeclaration:
                    const exportClause = (<ExportDeclaration>node).exportClause;
                    return !!exportClause && some(exportClause.elements, isValueAliasDeclaration);
                case SyntaxKind.ExportAssignment:
                    return (<ExportAssignment>node).expression
                        && (<ExportAssignment>node).expression.kind === SyntaxKind.Identifier
                            ? isAliasResolvedToValue(getSymbolOfNode(node) || unknownSymbol)
                            : true;
            }
            return false;
        }

        function isTopLevelValueImportEqualsWithEntityName(nodeIn: ImportEqualsDeclaration): boolean {
            const node = getParseTreeNode(nodeIn, isImportEqualsDeclaration);
            if (node === undefined || node.parent.kind !== SyntaxKind.SourceFile || !isInternalModuleImportEqualsDeclaration(node)) {
                // parent is not source file or it is not reference to internal module
                return false;
            }

            const isValue = isAliasResolvedToValue(getSymbolOfNode(node));
            return isValue && node.moduleReference && !nodeIsMissing(node.moduleReference);
        }

        function isAliasResolvedToValue(symbol: Symbol): boolean {
            const target = resolveAlias(symbol);
            if (target === unknownSymbol) {
                return true;
            }
            // const enums and modules that contain only const enums are not considered values from the emit perspective
            // unless 'preserveConstEnums' option is set to true
            return !!(target.flags & SymbolFlags.Value) &&
                (compilerOptions.preserveConstEnums || !isConstEnumOrConstEnumOnlyModule(target));
        }

        function isConstEnumOrConstEnumOnlyModule(s: Symbol): boolean {
            return isConstEnumSymbol(s) || !!s.constEnumOnlyModule;
        }

        function isReferencedAliasDeclaration(node: Node, checkChildren?: boolean): boolean {
            if (isAliasSymbolDeclaration(node)) {
                const symbol = getSymbolOfNode(node);
                if (symbol && getSymbolLinks(symbol).referenced) {
                    return true;
                }
                const target = getSymbolLinks(symbol!).target; // TODO: GH#18217
                if (target && getModifierFlags(node) & ModifierFlags.Export && target.flags & SymbolFlags.Value) {
                    // An `export import ... =` of a value symbol is always considered referenced
                    return true;
                }
            }

            if (checkChildren) {
                return !!forEachChild(node, node => isReferencedAliasDeclaration(node, checkChildren));
            }
            return false;
        }

        function isImplementationOfOverload(node: SignatureDeclaration) {
            if (nodeIsPresent((node as FunctionLikeDeclaration).body)) {
                if (isGetAccessor(node) || isSetAccessor(node)) return false; // Get or set accessors can never be overload implementations, but can have up to 2 signatures
                const symbol = getSymbolOfNode(node);
                const signaturesOfSymbol = getSignaturesOfSymbol(symbol);
                // If this function body corresponds to function with multiple signature, it is implementation of overload
                // e.g.: function foo(a: string): string;
                //       function foo(a: number): number;
                //       function foo(a: any) { // This is implementation of the overloads
                //           return a;
                //       }
                return signaturesOfSymbol.length > 1 ||
                    // If there is single signature for the symbol, it is overload if that signature isn't coming from the node
                    // e.g.: function foo(a: string): string;
                    //       function foo(a: any) { // This is implementation of the overloads
                    //           return a;
                    //       }
                    (signaturesOfSymbol.length === 1 && signaturesOfSymbol[0].declaration !== node);
            }
            return false;
        }

        function isRequiredInitializedParameter(parameter: ParameterDeclaration | JSDocParameterTag): boolean {
            return !!strictNullChecks &&
                !isOptionalParameter(parameter) &&
                !isJSDocParameterTag(parameter) &&
                !!parameter.initializer &&
                !hasModifier(parameter, ModifierFlags.ParameterPropertyModifier);
        }

        function isOptionalUninitializedParameterProperty(parameter: ParameterDeclaration) {
            return strictNullChecks &&
                isOptionalParameter(parameter) &&
                !parameter.initializer &&
                hasModifier(parameter, ModifierFlags.ParameterPropertyModifier);
        }

        function isExpandoFunctionDeclaration(node: Declaration): boolean {
            const declaration = getParseTreeNode(node, isFunctionDeclaration);
            if (!declaration) {
                return false;
            }
            const symbol = getSymbolOfNode(declaration);
            if (!symbol || !(symbol.flags & SymbolFlags.Function)) {
                return false;
            }
            return !!forEachEntry(getExportsOfSymbol(symbol), p => p.flags & SymbolFlags.Value && isPropertyAccessExpression(p.valueDeclaration));
        }

        function getPropertiesOfContainerFunction(node: Declaration): Symbol[] {
            const declaration = getParseTreeNode(node, isFunctionDeclaration);
            if (!declaration) {
                return emptyArray;
            }
            const symbol = getSymbolOfNode(declaration);
            return symbol && getPropertiesOfType(getTypeOfSymbol(symbol)) || emptyArray;
        }

        function getNodeCheckFlags(node: Node): NodeCheckFlags {
            return getNodeLinks(node).flags || 0;
        }

        function getEnumMemberValue(node: EnumMember): string | number | undefined {
            computeEnumMemberValues(node.parent);
            return getNodeLinks(node).enumMemberValue;
        }

        function canHaveConstantValue(node: Node): node is EnumMember | PropertyAccessExpression | ElementAccessExpression {
            switch (node.kind) {
                case SyntaxKind.EnumMember:
                case SyntaxKind.PropertyAccessExpression:
                case SyntaxKind.ElementAccessExpression:
                    return true;
            }
            return false;
        }

        function getConstantValue(node: EnumMember | PropertyAccessExpression | ElementAccessExpression): string | number | undefined {
            if (node.kind === SyntaxKind.EnumMember) {
                return getEnumMemberValue(node);
            }

            const symbol = getNodeLinks(node).resolvedSymbol;
            if (symbol && (symbol.flags & SymbolFlags.EnumMember)) {
                // inline property\index accesses only for const enums
                const member = symbol.valueDeclaration as EnumMember;
                if (isEnumConst(member.parent)) {
                    return getEnumMemberValue(member);
                }
            }

            return undefined;
        }

        function isFunctionType(type: Type): boolean {
            return !!(type.flags & TypeFlags.Object) && getSignaturesOfType(type, SignatureKind.Call).length > 0;
        }

        function getTypeReferenceSerializationKind(typeNameIn: EntityName, location?: Node): TypeReferenceSerializationKind {
            // ensure both `typeName` and `location` are parse tree nodes.
            const typeName = getParseTreeNode(typeNameIn, isEntityName);
            if (!typeName) return TypeReferenceSerializationKind.Unknown;

            if (location) {
                location = getParseTreeNode(location);
                if (!location) return TypeReferenceSerializationKind.Unknown;
            }

            // Resolve the symbol as a value to ensure the type can be reached at runtime during emit.
            const valueSymbol = resolveEntityName(typeName, SymbolFlags.Value, /*ignoreErrors*/ true, /*dontResolveAlias*/ false, location);

            // Resolve the symbol as a type so that we can provide a more useful hint for the type serializer.
            const typeSymbol = resolveEntityName(typeName, SymbolFlags.Type, /*ignoreErrors*/ true, /*dontResolveAlias*/ false, location);
            if (valueSymbol && valueSymbol === typeSymbol) {
                const globalPromiseSymbol = getGlobalPromiseConstructorSymbol(/*reportErrors*/ false);
                if (globalPromiseSymbol && valueSymbol === globalPromiseSymbol) {
                    return TypeReferenceSerializationKind.Promise;
                }

                const constructorType = getTypeOfSymbol(valueSymbol);
                if (constructorType && isConstructorType(constructorType)) {
                    return TypeReferenceSerializationKind.TypeWithConstructSignatureAndValue;
                }
            }

            // We might not be able to resolve type symbol so use unknown type in that case (eg error case)
            if (!typeSymbol) {
                return TypeReferenceSerializationKind.Unknown;
            }
            const type = getDeclaredTypeOfSymbol(typeSymbol);
            if (type === errorType) {
                return TypeReferenceSerializationKind.Unknown;
            }
            else if (type.flags & TypeFlags.AnyOrUnknown) {
                return TypeReferenceSerializationKind.ObjectType;
            }
            else if (isTypeAssignableToKind(type, TypeFlags.Void | TypeFlags.Nullable | TypeFlags.Never)) {
                return TypeReferenceSerializationKind.VoidNullableOrNeverType;
            }
            else if (isTypeAssignableToKind(type, TypeFlags.BooleanLike)) {
                return TypeReferenceSerializationKind.BooleanType;
            }
            else if (isTypeAssignableToKind(type, TypeFlags.NumberLike)) {
                return TypeReferenceSerializationKind.NumberLikeType;
            }
            else if (isTypeAssignableToKind(type, TypeFlags.StringLike)) {
                return TypeReferenceSerializationKind.StringLikeType;
            }
            else if (isTupleType(type)) {
                return TypeReferenceSerializationKind.ArrayLikeType;
            }
            else if (isTypeAssignableToKind(type, TypeFlags.ESSymbolLike)) {
                return TypeReferenceSerializationKind.ESSymbolType;
            }
            else if (isFunctionType(type)) {
                return TypeReferenceSerializationKind.TypeWithCallSignature;
            }
            else if (isArrayType(type)) {
                return TypeReferenceSerializationKind.ArrayLikeType;
            }
            else {
                return TypeReferenceSerializationKind.ObjectType;
            }
        }

        function createTypeOfDeclaration(declarationIn: AccessorDeclaration | VariableLikeDeclaration | PropertyAccessExpression, enclosingDeclaration: Node, flags: NodeBuilderFlags, tracker: SymbolTracker, addUndefined?: boolean) {
            const declaration = getParseTreeNode(declarationIn, isVariableLikeOrAccessor);
            if (!declaration) {
                return createToken(SyntaxKind.AnyKeyword) as KeywordTypeNode;
            }
            // Get type of the symbol if this is the valid symbol otherwise get type at location
            const symbol = getSymbolOfNode(declaration);
            let type = symbol && !(symbol.flags & (SymbolFlags.TypeLiteral | SymbolFlags.Signature))
                ? getWidenedLiteralType(getTypeOfSymbol(symbol))
                : errorType;
            if (type.flags & TypeFlags.UniqueESSymbol &&
                type.symbol === symbol) {
                flags |= NodeBuilderFlags.AllowUniqueESSymbolType;
            }
            if (addUndefined) {
                type = getOptionalType(type);
            }
            return nodeBuilder.typeToTypeNode(type, enclosingDeclaration, flags | NodeBuilderFlags.MultilineObjectLiterals, tracker);
        }

        function createReturnTypeOfSignatureDeclaration(signatureDeclarationIn: SignatureDeclaration, enclosingDeclaration: Node, flags: NodeBuilderFlags, tracker: SymbolTracker) {
            const signatureDeclaration = getParseTreeNode(signatureDeclarationIn, isFunctionLike);
            if (!signatureDeclaration) {
                return createToken(SyntaxKind.AnyKeyword) as KeywordTypeNode;
            }
            const signature = getSignatureFromDeclaration(signatureDeclaration);
            return nodeBuilder.typeToTypeNode(getReturnTypeOfSignature(signature), enclosingDeclaration, flags | NodeBuilderFlags.MultilineObjectLiterals, tracker);
        }

        function createTypeOfExpression(exprIn: Expression, enclosingDeclaration: Node, flags: NodeBuilderFlags, tracker: SymbolTracker) {
            const expr = getParseTreeNode(exprIn, isExpression);
            if (!expr) {
                return createToken(SyntaxKind.AnyKeyword) as KeywordTypeNode;
            }
            const type = getWidenedType(getRegularTypeOfExpression(expr));
            return nodeBuilder.typeToTypeNode(type, enclosingDeclaration, flags | NodeBuilderFlags.MultilineObjectLiterals, tracker);
        }

        function hasGlobalName(name: string): boolean {
            return globals.has(escapeLeadingUnderscores(name));
        }

        function getReferencedValueSymbol(reference: Identifier, startInDeclarationContainer?: boolean): Symbol | undefined {
            const resolvedSymbol = getNodeLinks(reference).resolvedSymbol;
            if (resolvedSymbol) {
                return resolvedSymbol;
            }

            let location: Node = reference;
            if (startInDeclarationContainer) {
                // When resolving the name of a declaration as a value, we need to start resolution
                // at a point outside of the declaration.
                const parent = reference.parent;
                if (isDeclaration(parent) && reference === parent.name) {
                    location = getDeclarationContainer(parent);
                }
            }

            return resolveName(location, reference.escapedText, SymbolFlags.Value | SymbolFlags.ExportValue | SymbolFlags.Alias, /*nodeNotFoundMessage*/ undefined, /*nameArg*/ undefined, /*isUse*/ true);
        }

        function getReferencedValueDeclaration(referenceIn: Identifier): Declaration | undefined {
            if (!isGeneratedIdentifier(referenceIn)) {
                const reference = getParseTreeNode(referenceIn, isIdentifier);
                if (reference) {
                    const symbol = getReferencedValueSymbol(reference);
                    if (symbol) {
                        return getExportSymbolOfValueSymbolIfExported(symbol).valueDeclaration;
                    }
                }
            }

            return undefined;
        }

        function isLiteralConstDeclaration(node: VariableDeclaration | PropertyDeclaration | PropertySignature | ParameterDeclaration): boolean {
            if (isDeclarationReadonly(node) || isVariableDeclaration(node) && isVarConst(node)) {
                const type = getTypeOfSymbol(getSymbolOfNode(node));
                return !!(type.flags & TypeFlags.StringOrNumberLiteral && type.flags & TypeFlags.FreshLiteral);
            }
            return false;
        }

        function literalTypeToNode(type: LiteralType, enclosing: Node): Expression {
            const enumResult = type.flags & TypeFlags.EnumLiteral && nodeBuilder.symbolToExpression(type.symbol, SymbolFlags.Value, enclosing);
            return enumResult || createLiteral(type.value);
        }

        function createLiteralConstValue(node: VariableDeclaration | PropertyDeclaration | PropertySignature | ParameterDeclaration) {
            const type = getTypeOfSymbol(getSymbolOfNode(node));
            return literalTypeToNode(<LiteralType>type, node);
        }

        function createResolver(): EmitResolver {
            // this variable and functions that use it are deliberately moved here from the outer scope
            // to avoid scope pollution
            const resolvedTypeReferenceDirectives = host.getResolvedTypeReferenceDirectives();
            let fileToDirective: Map<string>;
            if (resolvedTypeReferenceDirectives) {
                // populate reverse mapping: file path -> type reference directive that was resolved to this file
                fileToDirective = createMap<string>();
                resolvedTypeReferenceDirectives.forEach((resolvedDirective, key) => {
                    if (!resolvedDirective || !resolvedDirective.resolvedFileName) {
                        return;
                    }
                    const file = host.getSourceFile(resolvedDirective.resolvedFileName)!;
                    fileToDirective.set(file.path, key);
                });
            }
            return {
                getReferencedExportContainer,
                getReferencedImportDeclaration,
                getReferencedDeclarationWithCollidingName,
                isDeclarationWithCollidingName,
                isValueAliasDeclaration: node => {
                    node = getParseTreeNode(node);
                    // Synthesized nodes are always treated like values.
                    return node ? isValueAliasDeclaration(node) : true;
                },
                hasGlobalName,
                isReferencedAliasDeclaration: (node, checkChildren?) => {
                    node = getParseTreeNode(node);
                    // Synthesized nodes are always treated as referenced.
                    return node ? isReferencedAliasDeclaration(node, checkChildren) : true;
                },
                getNodeCheckFlags: node => {
                    node = getParseTreeNode(node);
                    return node ? getNodeCheckFlags(node) : 0;
                },
                isTopLevelValueImportEqualsWithEntityName,
                isDeclarationVisible,
                isImplementationOfOverload,
                isRequiredInitializedParameter,
                isOptionalUninitializedParameterProperty,
                isExpandoFunctionDeclaration,
                getPropertiesOfContainerFunction,
                createTypeOfDeclaration,
                createReturnTypeOfSignatureDeclaration,
                createTypeOfExpression,
                createLiteralConstValue,
                isSymbolAccessible,
                isEntityNameVisible,
                getConstantValue: nodeIn => {
                    const node = getParseTreeNode(nodeIn, canHaveConstantValue);
                    return node ? getConstantValue(node) : undefined;
                },
                collectLinkedAliases,
                getReferencedValueDeclaration,
                getTypeReferenceSerializationKind,
                isOptionalParameter,
                moduleExportsSomeValue,
                isArgumentsLocalBinding,
                getExternalModuleFileFromDeclaration,
                getTypeReferenceDirectivesForEntityName,
                getTypeReferenceDirectivesForSymbol,
                isLiteralConstDeclaration,
                isLateBound: (nodeIn: Declaration): nodeIn is LateBoundDeclaration => {
                    const node = getParseTreeNode(nodeIn, isDeclaration);
                    const symbol = node && getSymbolOfNode(node);
                    return !!(symbol && getCheckFlags(symbol) & CheckFlags.Late);
                },
                getJsxFactoryEntity: location => location ? (getJsxNamespace(location), (getSourceFileOfNode(location).localJsxFactory || _jsxFactoryEntity)) : _jsxFactoryEntity,
                getAllAccessorDeclarations(accessor: AccessorDeclaration): AllAccessorDeclarations {
                    accessor = getParseTreeNode(accessor, isGetOrSetAccessorDeclaration)!; // TODO: GH#18217
                    const otherKind = accessor.kind === SyntaxKind.SetAccessor ? SyntaxKind.GetAccessor : SyntaxKind.SetAccessor;
                    const otherAccessor = getDeclarationOfKind<AccessorDeclaration>(getSymbolOfNode(accessor), otherKind);
                    const firstAccessor = otherAccessor && (otherAccessor.pos < accessor.pos) ? otherAccessor : accessor;
                    const secondAccessor = otherAccessor && (otherAccessor.pos < accessor.pos) ? accessor : otherAccessor;
                    const setAccessor = accessor.kind === SyntaxKind.SetAccessor ? accessor : otherAccessor as SetAccessorDeclaration;
                    const getAccessor = accessor.kind === SyntaxKind.GetAccessor ? accessor : otherAccessor as GetAccessorDeclaration;
                    return {
                        firstAccessor,
                        secondAccessor,
                        setAccessor,
                        getAccessor
                    };
                },
                getSymbolOfExternalModuleSpecifier: moduleName => resolveExternalModuleNameWorker(moduleName, moduleName, /*moduleNotFoundError*/ undefined)
            };

            function isInHeritageClause(node: PropertyAccessEntityNameExpression) {
                return node.parent && node.parent.kind === SyntaxKind.ExpressionWithTypeArguments && node.parent.parent && node.parent.parent.kind === SyntaxKind.HeritageClause;
            }

            // defined here to avoid outer scope pollution
            function getTypeReferenceDirectivesForEntityName(node: EntityNameOrEntityNameExpression): string[] | undefined {
                // program does not have any files with type reference directives - bail out
                if (!fileToDirective) {
                    return undefined;
                }
                // property access can only be used as values, or types when within an expression with type arguments inside a heritage clause
                // qualified names can only be used as types\namespaces
                // identifiers are treated as values only if they appear in type queries
                let meaning = SymbolFlags.Type | SymbolFlags.Namespace;
                if ((node.kind === SyntaxKind.Identifier && isInTypeQuery(node)) || (node.kind === SyntaxKind.PropertyAccessExpression && !isInHeritageClause(node))) {
                    meaning = SymbolFlags.Value | SymbolFlags.ExportValue;
                }

                const symbol = resolveEntityName(node, meaning, /*ignoreErrors*/ true);
                return symbol && symbol !== unknownSymbol ? getTypeReferenceDirectivesForSymbol(symbol, meaning) : undefined;
            }

            // defined here to avoid outer scope pollution
            function getTypeReferenceDirectivesForSymbol(symbol: Symbol, meaning?: SymbolFlags): string[] | undefined {
                // program does not have any files with type reference directives - bail out
                if (!fileToDirective) {
                    return undefined;
                }
                if (!isSymbolFromTypeDeclarationFile(symbol)) {
                    return undefined;
                }
                // check what declarations in the symbol can contribute to the target meaning
                let typeReferenceDirectives: string[] | undefined;
                for (const decl of symbol.declarations) {
                    // check meaning of the local symbol to see if declaration needs to be analyzed further
                    if (decl.symbol && decl.symbol.flags & meaning!) {
                        const file = getSourceFileOfNode(decl);
                        const typeReferenceDirective = fileToDirective.get(file.path);
                        if (typeReferenceDirective) {
                            (typeReferenceDirectives || (typeReferenceDirectives = [])).push(typeReferenceDirective);
                        }
                        else {
                            // found at least one entry that does not originate from type reference directive
                            return undefined;
                        }
                    }
                }
                return typeReferenceDirectives;
            }

            function isSymbolFromTypeDeclarationFile(symbol: Symbol): boolean {
                // bail out if symbol does not have associated declarations (i.e. this is transient symbol created for property in binding pattern)
                if (!symbol.declarations) {
                    return false;
                }

                // walk the parent chain for symbols to make sure that top level parent symbol is in the global scope
                // external modules cannot define or contribute to type declaration files
                let current = symbol;
                while (true) {
                    const parent = getParentOfSymbol(current);
                    if (parent) {
                        current = parent;
                    }
                    else {
                        break;
                    }
                }

                if (current.valueDeclaration && current.valueDeclaration.kind === SyntaxKind.SourceFile && current.flags & SymbolFlags.ValueModule) {
                    return false;
                }

                // check that at least one declaration of top level symbol originates from type declaration file
                for (const decl of symbol.declarations) {
                    const file = getSourceFileOfNode(decl);
                    if (fileToDirective.has(file.path)) {
                        return true;
                    }
                }
                return false;
            }
        }

        function getExternalModuleFileFromDeclaration(declaration: AnyImportOrReExport | ModuleDeclaration | ImportTypeNode): SourceFile | undefined {
            const specifier = declaration.kind === SyntaxKind.ModuleDeclaration ? tryCast(declaration.name, isStringLiteral) : getExternalModuleName(declaration);
            const moduleSymbol = resolveExternalModuleNameWorker(specifier!, specifier!, /*moduleNotFoundError*/ undefined); // TODO: GH#18217
            if (!moduleSymbol) {
                return undefined;
            }
            return getDeclarationOfKind(moduleSymbol, SyntaxKind.SourceFile);
        }

        function initializeTypeChecker() {
            // Bind all source files and propagate errors
            for (const file of host.getSourceFiles()) {
                bindSourceFile(file, compilerOptions);
            }

            amalgamatedDuplicates = createMap();

            // Initialize global symbol table
            let augmentations: ReadonlyArray<StringLiteral | Identifier>[] | undefined;
            for (const file of host.getSourceFiles()) {
                if (file.redirectInfo) {
                    continue;
                }
                if (!isExternalOrCommonJsModule(file)) {
                    mergeSymbolTable(globals, file.locals!);
                }
                if (file.patternAmbientModules && file.patternAmbientModules.length) {
                    patternAmbientModules = concatenate(patternAmbientModules, file.patternAmbientModules);
                }
                if (file.moduleAugmentations.length) {
                    (augmentations || (augmentations = [])).push(file.moduleAugmentations);
                }
                if (file.symbol && file.symbol.globalExports) {
                    // Merge in UMD exports with first-in-wins semantics (see #9771)
                    const source = file.symbol.globalExports;
                    source.forEach((sourceSymbol, id) => {
                        if (!globals.has(id)) {
                            globals.set(id, sourceSymbol);
                        }
                    });
                }
            }

            // We do global augmentations separately from module augmentations (and before creating global types) because they
            //  1. Affect global types. We won't have the correct global types until global augmentations are merged. Also,
            //  2. Module augmentation instantiation requires creating the type of a module, which, in turn, can require
            //       checking for an export or property on the module (if export=) which, in turn, can fall back to the
            //       apparent type of the module - either globalObjectType or globalFunctionType - which wouldn't exist if we
            //       did module augmentations prior to finalizing the global types.
            if (augmentations) {
                // merge _global_ module augmentations.
                // this needs to be done after global symbol table is initialized to make sure that all ambient modules are indexed
                for (const list of augmentations) {
                    for (const augmentation of list) {
                        if (!isGlobalScopeAugmentation(augmentation.parent as ModuleDeclaration)) continue;
                        mergeModuleAugmentation(augmentation);
                    }
                }
            }

            // Setup global builtins
            addToSymbolTable(globals, builtinGlobals, Diagnostics.Declaration_name_conflicts_with_built_in_global_identifier_0);

            getSymbolLinks(undefinedSymbol).type = undefinedWideningType;
            getSymbolLinks(argumentsSymbol).type = getGlobalType("IArguments" as __String, /*arity*/ 0, /*reportErrors*/ true);
            getSymbolLinks(unknownSymbol).type = errorType;

            // Initialize special types
            globalArrayType = getGlobalType("Array" as __String, /*arity*/ 1, /*reportErrors*/ true);
            globalObjectType = getGlobalType("Object" as __String, /*arity*/ 0, /*reportErrors*/ true);
            globalFunctionType = getGlobalType("Function" as __String, /*arity*/ 0, /*reportErrors*/ true);
            globalStringType = getGlobalType("String" as __String, /*arity*/ 0, /*reportErrors*/ true);
            globalNumberType = getGlobalType("Number" as __String, /*arity*/ 0, /*reportErrors*/ true);
            globalBooleanType = getGlobalType("Boolean" as __String, /*arity*/ 0, /*reportErrors*/ true);
            globalRegExpType = getGlobalType("RegExp" as __String, /*arity*/ 0, /*reportErrors*/ true);
            anyArrayType = createArrayType(anyType);

            autoArrayType = createArrayType(autoType);
            if (autoArrayType === emptyObjectType) {
                // autoArrayType is used as a marker, so even if global Array type is not defined, it needs to be a unique type
                autoArrayType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
            }

            globalReadonlyArrayType = <GenericType>getGlobalTypeOrUndefined("ReadonlyArray" as __String, /*arity*/ 1);
            anyReadonlyArrayType = globalReadonlyArrayType ? createTypeFromGenericGlobalType(globalReadonlyArrayType, [anyType]) : anyArrayType;
            globalThisType = <GenericType>getGlobalTypeOrUndefined("ThisType" as __String, /*arity*/ 1);

            if (augmentations) {
                // merge _nonglobal_ module augmentations.
                // this needs to be done after global symbol table is initialized to make sure that all ambient modules are indexed
                for (const list of augmentations) {
                    for (const augmentation of list) {
                        if (isGlobalScopeAugmentation(augmentation.parent as ModuleDeclaration)) continue;
                        mergeModuleAugmentation(augmentation);
                    }
                }
            }

            amalgamatedDuplicates.forEach(({ firstFile, secondFile, firstFileInstances, secondFileInstances }) => {
                const conflictingKeys = arrayFrom(firstFileInstances.keys());
                // If not many things conflict, issue individual errors
                if (conflictingKeys.length < 8) {
                    addErrorsForDuplicates(firstFileInstances, secondFileInstances);
                    addErrorsForDuplicates(secondFileInstances, firstFileInstances);
                    return;
                }
                // Otheriwse issue top-level error since the files appear very identical in terms of what they appear
                const list = conflictingKeys.join(", ");
                diagnostics.add(addRelatedInfo(
                    createDiagnosticForNode(firstFile, Diagnostics.Definitions_of_the_following_identifiers_conflict_with_those_in_another_file_Colon_0, list),
                    createDiagnosticForNode(secondFile, Diagnostics.Conflicts_are_in_this_file)
                ));
                diagnostics.add(addRelatedInfo(
                    createDiagnosticForNode(secondFile, Diagnostics.Definitions_of_the_following_identifiers_conflict_with_those_in_another_file_Colon_0, list),
                    createDiagnosticForNode(firstFile, Diagnostics.Conflicts_are_in_this_file)
                ));
            });
            amalgamatedDuplicates = undefined;

            function addErrorsForDuplicates(secondFileInstances: Map<{ instances: Node[]; blockScoped: boolean; }>, firstFileInstances: Map<{ instances: Node[]; blockScoped: boolean; }>) {
                secondFileInstances.forEach((locations, symbolName) => {
                    const firstFileEquivalent = firstFileInstances.get(symbolName)!;
                    const message = locations.blockScoped
                        ? Diagnostics.Cannot_redeclare_block_scoped_variable_0
                        : Diagnostics.Duplicate_identifier_0;
                    locations.instances.forEach(node => {
                        addDuplicateDeclarationError(node, message, symbolName, firstFileEquivalent.instances[0]);
                    });
                });
            }
        }

        function checkExternalEmitHelpers(location: Node, helpers: ExternalEmitHelpers) {
            if ((requestedExternalEmitHelpers & helpers) !== helpers && compilerOptions.importHelpers) {
                const sourceFile = getSourceFileOfNode(location);
                if (isEffectiveExternalModule(sourceFile, compilerOptions) && !(location.flags & NodeFlags.Ambient)) {
                    const helpersModule = resolveHelpersModule(sourceFile, location);
                    if (helpersModule !== unknownSymbol) {
                        const uncheckedHelpers = helpers & ~requestedExternalEmitHelpers;
                        for (let helper = ExternalEmitHelpers.FirstEmitHelper; helper <= ExternalEmitHelpers.LastEmitHelper; helper <<= 1) {
                            if (uncheckedHelpers & helper) {
                                const name = getHelperName(helper);
                                const symbol = getSymbol(helpersModule.exports!, escapeLeadingUnderscores(name), SymbolFlags.Value);
                                if (!symbol) {
                                    error(location, Diagnostics.This_syntax_requires_an_imported_helper_named_1_but_module_0_has_no_exported_member_1, externalHelpersModuleNameText, name);
                                }
                            }
                        }
                    }
                    requestedExternalEmitHelpers |= helpers;
                }
            }
        }

        function getHelperName(helper: ExternalEmitHelpers) {
            switch (helper) {
                case ExternalEmitHelpers.Extends: return "__extends";
                case ExternalEmitHelpers.Assign: return "__assign";
                case ExternalEmitHelpers.Rest: return "__rest";
                case ExternalEmitHelpers.Decorate: return "__decorate";
                case ExternalEmitHelpers.Metadata: return "__metadata";
                case ExternalEmitHelpers.Param: return "__param";
                case ExternalEmitHelpers.Awaiter: return "__awaiter";
                case ExternalEmitHelpers.Generator: return "__generator";
                case ExternalEmitHelpers.Values: return "__values";
                case ExternalEmitHelpers.Read: return "__read";
                case ExternalEmitHelpers.Spread: return "__spread";
                case ExternalEmitHelpers.Await: return "__await";
                case ExternalEmitHelpers.AsyncGenerator: return "__asyncGenerator";
                case ExternalEmitHelpers.AsyncDelegator: return "__asyncDelegator";
                case ExternalEmitHelpers.AsyncValues: return "__asyncValues";
                case ExternalEmitHelpers.ExportStar: return "__exportStar";
                case ExternalEmitHelpers.MakeTemplateObject: return "__makeTemplateObject";
                default: return Debug.fail("Unrecognized helper");
            }
        }

        function resolveHelpersModule(node: SourceFile, errorNode: Node) {
            if (!externalHelpersModule) {
                externalHelpersModule = resolveExternalModule(node, externalHelpersModuleNameText, Diagnostics.This_syntax_requires_an_imported_helper_but_module_0_cannot_be_found, errorNode) || unknownSymbol;
            }
            return externalHelpersModule;
        }

        // GRAMMAR CHECKING
        function checkGrammarDecoratorsAndModifiers(node: Node): boolean {
            return checkGrammarDecorators(node) || checkGrammarModifiers(node);
        }

        function checkGrammarDecorators(node: Node): boolean {
            if (!node.decorators) {
                return false;
            }
            if (!nodeCanBeDecorated(node, node.parent, node.parent.parent)) {
                if (node.kind === SyntaxKind.MethodDeclaration && !nodeIsPresent((<MethodDeclaration>node).body)) {
                    return grammarErrorOnFirstToken(node, Diagnostics.A_decorator_can_only_decorate_a_method_implementation_not_an_overload);
                }
                else {
                    return grammarErrorOnFirstToken(node, Diagnostics.Decorators_are_not_valid_here);
                }
            }
            else if (node.kind === SyntaxKind.GetAccessor || node.kind === SyntaxKind.SetAccessor) {
                const accessors = getAllAccessorDeclarations((<ClassDeclaration>node.parent).members, <AccessorDeclaration>node);
                if (accessors.firstAccessor.decorators && node === accessors.secondAccessor) {
                    return grammarErrorOnFirstToken(node, Diagnostics.Decorators_cannot_be_applied_to_multiple_get_Slashset_accessors_of_the_same_name);
                }
            }
            return false;
        }

        function checkGrammarModifiers(node: Node): boolean {
            const quickResult = reportObviousModifierErrors(node);
            if (quickResult !== undefined) {
                return quickResult;
            }

            let lastStatic: Node | undefined, lastDeclare: Node | undefined, lastAsync: Node | undefined, lastReadonly: Node | undefined;
            let flags = ModifierFlags.None;
            for (const modifier of node.modifiers!) {
                if (modifier.kind !== SyntaxKind.ReadonlyKeyword) {
                    if (node.kind === SyntaxKind.PropertySignature || node.kind === SyntaxKind.MethodSignature) {
                        return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_type_member, tokenToString(modifier.kind));
                    }
                    if (node.kind === SyntaxKind.IndexSignature) {
                        return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_an_index_signature, tokenToString(modifier.kind));
                    }
                }
                switch (modifier.kind) {
                    case SyntaxKind.ConstKeyword:
                        if (node.kind !== SyntaxKind.EnumDeclaration) {
                            return grammarErrorOnNode(node, Diagnostics.A_class_member_cannot_have_the_0_keyword, tokenToString(SyntaxKind.ConstKeyword));
                        }
                        break;
                    case SyntaxKind.PublicKeyword:
                    case SyntaxKind.ProtectedKeyword:
                    case SyntaxKind.PrivateKeyword:
                        const text = visibilityToString(modifierToFlag(modifier.kind));

                        if (flags & ModifierFlags.AccessibilityModifier) {
                            return grammarErrorOnNode(modifier, Diagnostics.Accessibility_modifier_already_seen);
                        }
                        else if (flags & ModifierFlags.Static) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "static");
                        }
                        else if (flags & ModifierFlags.Readonly) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "readonly");
                        }
                        else if (flags & ModifierFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_module_or_namespace_element, text);
                        }
                        else if (flags & ModifierFlags.Abstract) {
                            if (modifier.kind === SyntaxKind.PrivateKeyword) {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, text, "abstract");
                            }
                            else {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "abstract");
                            }
                        }
                        flags |= modifierToFlag(modifier.kind);
                        break;

                    case SyntaxKind.StaticKeyword:
                        if (flags & ModifierFlags.Static) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "static");
                        }
                        else if (flags & ModifierFlags.Readonly) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "static", "readonly");
                        }
                        else if (flags & ModifierFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "static", "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_module_or_namespace_element, "static");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "static");
                        }
                        else if (flags & ModifierFlags.Abstract) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "static", "abstract");
                        }
                        flags |= ModifierFlags.Static;
                        lastStatic = modifier;
                        break;

                    case SyntaxKind.ReadonlyKeyword:
                        if (flags & ModifierFlags.Readonly) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "readonly");
                        }
                        else if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature && node.kind !== SyntaxKind.IndexSignature && node.kind !== SyntaxKind.Parameter) {
                            // If node.kind === SyntaxKind.Parameter, checkParameter report an error if it's not a parameter property.
                            return grammarErrorOnNode(modifier, Diagnostics.readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature);
                        }
                        flags |= ModifierFlags.Readonly;
                        lastReadonly = modifier;
                        break;

                    case SyntaxKind.ExportKeyword:
                        if (flags & ModifierFlags.Export) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "export");
                        }
                        else if (flags & ModifierFlags.Ambient) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "export", "declare");
                        }
                        else if (flags & ModifierFlags.Abstract) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "export", "abstract");
                        }
                        else if (flags & ModifierFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "export", "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ClassDeclaration) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_class_element, "export");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "export");
                        }
                        flags |= ModifierFlags.Export;
                        break;
                    case SyntaxKind.DefaultKeyword:
                        const container = node.parent.kind === SyntaxKind.SourceFile ? node.parent : node.parent.parent;
                        if (container.kind === SyntaxKind.ModuleDeclaration && !isAmbientModule(container)) {
                            return grammarErrorOnNode(modifier, Diagnostics.A_default_export_can_only_be_used_in_an_ECMAScript_style_module);
                        }

                        flags |= ModifierFlags.Default;
                        break;
                    case SyntaxKind.DeclareKeyword:
                        if (flags & ModifierFlags.Ambient) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "declare");
                        }
                        else if (flags & ModifierFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ClassDeclaration) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_class_element, "declare");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "declare");
                        }
                        else if ((node.parent.flags & NodeFlags.Ambient) && node.parent.kind === SyntaxKind.ModuleBlock) {
                            return grammarErrorOnNode(modifier, Diagnostics.A_declare_modifier_cannot_be_used_in_an_already_ambient_context);
                        }
                        flags |= ModifierFlags.Ambient;
                        lastDeclare = modifier;
                        break;

                    case SyntaxKind.AbstractKeyword:
                        if (flags & ModifierFlags.Abstract) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "abstract");
                        }
                        if (node.kind !== SyntaxKind.ClassDeclaration) {
                            if (node.kind !== SyntaxKind.MethodDeclaration &&
                                node.kind !== SyntaxKind.PropertyDeclaration &&
                                node.kind !== SyntaxKind.GetAccessor &&
                                node.kind !== SyntaxKind.SetAccessor) {
                                return grammarErrorOnNode(modifier, Diagnostics.abstract_modifier_can_only_appear_on_a_class_method_or_property_declaration);
                            }
                            if (!(node.parent.kind === SyntaxKind.ClassDeclaration && hasModifier(node.parent, ModifierFlags.Abstract))) {
                                return grammarErrorOnNode(modifier, Diagnostics.Abstract_methods_can_only_appear_within_an_abstract_class);
                            }
                            if (flags & ModifierFlags.Static) {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "static", "abstract");
                            }
                            if (flags & ModifierFlags.Private) {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "private", "abstract");
                            }
                        }

                        flags |= ModifierFlags.Abstract;
                        break;

                    case SyntaxKind.AsyncKeyword:
                        if (flags & ModifierFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "async");
                        }
                        else if (flags & ModifierFlags.Ambient || node.parent.flags & NodeFlags.Ambient) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "async");
                        }
                        flags |= ModifierFlags.Async;
                        lastAsync = modifier;
                        break;
                }
            }

            if (node.kind === SyntaxKind.Constructor) {
                if (flags & ModifierFlags.Static) {
                    return grammarErrorOnNode(lastStatic!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "static");
                }
                if (flags & ModifierFlags.Abstract) {
                    return grammarErrorOnNode(lastStatic!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "abstract"); // TODO: GH#18217
                }
                else if (flags & ModifierFlags.Async) {
                    return grammarErrorOnNode(lastAsync!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "async");
                }
                else if (flags & ModifierFlags.Readonly) {
                    return grammarErrorOnNode(lastReadonly!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "readonly");
                }
                return false;
            }
            else if ((node.kind === SyntaxKind.ImportDeclaration || node.kind === SyntaxKind.ImportEqualsDeclaration) && flags & ModifierFlags.Ambient) {
                return grammarErrorOnNode(lastDeclare!, Diagnostics.A_0_modifier_cannot_be_used_with_an_import_declaration, "declare");
            }
            else if (node.kind === SyntaxKind.Parameter && (flags & ModifierFlags.ParameterPropertyModifier) && isBindingPattern((<ParameterDeclaration>node).name)) {
                return grammarErrorOnNode(node, Diagnostics.A_parameter_property_may_not_be_declared_using_a_binding_pattern);
            }
            else if (node.kind === SyntaxKind.Parameter && (flags & ModifierFlags.ParameterPropertyModifier) && (<ParameterDeclaration>node).dotDotDotToken) {
                return grammarErrorOnNode(node, Diagnostics.A_parameter_property_cannot_be_declared_using_a_rest_parameter);
            }
            if (flags & ModifierFlags.Async) {
                return checkGrammarAsyncModifier(node, lastAsync!);
            }
            return false;
        }

        /**
         * true | false: Early return this value from checkGrammarModifiers.
         * undefined: Need to do full checking on the modifiers.
         */
        function reportObviousModifierErrors(node: Node): boolean | undefined {
            return !node.modifiers
                ? false
                : shouldReportBadModifier(node)
                    ? grammarErrorOnFirstToken(node, Diagnostics.Modifiers_cannot_appear_here)
                    : undefined;
        }
        function shouldReportBadModifier(node: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                case SyntaxKind.Constructor:
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                case SyntaxKind.IndexSignature:
                case SyntaxKind.ModuleDeclaration:
                case SyntaxKind.ImportDeclaration:
                case SyntaxKind.ImportEqualsDeclaration:
                case SyntaxKind.ExportDeclaration:
                case SyntaxKind.ExportAssignment:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.Parameter:
                    return false;
                default:
                    if (node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                        return false;
                    }
                    switch (node.kind) {
                        case SyntaxKind.FunctionDeclaration:
                            return nodeHasAnyModifiersExcept(node, SyntaxKind.AsyncKeyword);
                        case SyntaxKind.ClassDeclaration:
                            return nodeHasAnyModifiersExcept(node, SyntaxKind.AbstractKeyword);
                        case SyntaxKind.InterfaceDeclaration:
                        case SyntaxKind.VariableStatement:
                        case SyntaxKind.TypeAliasDeclaration:
                            return true;
                        case SyntaxKind.EnumDeclaration:
                            return nodeHasAnyModifiersExcept(node, SyntaxKind.ConstKeyword);
                        default:
                            Debug.fail();
                            return false;
                    }
            }
        }
        function nodeHasAnyModifiersExcept(node: Node, allowedModifier: SyntaxKind): boolean {
            return node.modifiers!.length > 1 || node.modifiers![0].kind !== allowedModifier;
        }

        function checkGrammarAsyncModifier(node: Node, asyncModifier: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    return false;
            }

            return grammarErrorOnNode(asyncModifier, Diagnostics._0_modifier_cannot_be_used_here, "async");
        }

        function checkGrammarForDisallowedTrailingComma(list: NodeArray<Node> | undefined, diag = Diagnostics.Trailing_comma_not_allowed): boolean {
            if (list && list.hasTrailingComma) {
                return grammarErrorAtPos(list[0], list.end - ",".length, ",".length, diag);
            }
            return false;
        }

        function checkGrammarTypeParameterList(typeParameters: NodeArray<TypeParameterDeclaration> | undefined, file: SourceFile): boolean {
            if (typeParameters && typeParameters.length === 0) {
                const start = typeParameters.pos - "<".length;
                const end = skipTrivia(file.text, typeParameters.end) + ">".length;
                return grammarErrorAtPos(file, start, end - start, Diagnostics.Type_parameter_list_cannot_be_empty);
            }
            return false;
        }

        function checkGrammarParameterList(parameters: NodeArray<ParameterDeclaration>) {
            let seenOptionalParameter = false;
            const parameterCount = parameters.length;

            for (let i = 0; i < parameterCount; i++) {
                const parameter = parameters[i];
                if (parameter.dotDotDotToken) {
                    if (i !== (parameterCount - 1)) {
                        return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.A_rest_parameter_must_be_last_in_a_parameter_list);
                    }
                    if (!(parameter.flags & NodeFlags.Ambient)) { // Allow `...foo,` in ambient declarations; see GH#23070
                        checkGrammarForDisallowedTrailingComma(parameters, Diagnostics.A_rest_parameter_or_binding_pattern_may_not_have_a_trailing_comma);
                    }

                    if (isBindingPattern(parameter.name)) {
                        return grammarErrorOnNode(parameter.name, Diagnostics.A_rest_element_cannot_contain_a_binding_pattern);
                    }

                    if (parameter.questionToken) {
                        return grammarErrorOnNode(parameter.questionToken, Diagnostics.A_rest_parameter_cannot_be_optional);
                    }

                    if (parameter.initializer) {
                        return grammarErrorOnNode(parameter.name, Diagnostics.A_rest_parameter_cannot_have_an_initializer);
                    }
                }
                else if (parameter.questionToken) {
                    seenOptionalParameter = true;

                    if (parameter.initializer) {
                        return grammarErrorOnNode(parameter.name, Diagnostics.Parameter_cannot_have_question_mark_and_initializer);
                    }
                }
                else if (seenOptionalParameter && !parameter.initializer) {
                    return grammarErrorOnNode(parameter.name, Diagnostics.A_required_parameter_cannot_follow_an_optional_parameter);
                }
            }
        }

        function getNonSimpleParameters(parameters: ReadonlyArray<ParameterDeclaration>): ReadonlyArray<ParameterDeclaration> {
            return filter(parameters, parameter => !!parameter.initializer || isBindingPattern(parameter.name) || isRestParameter(parameter));
        }

        function checkGrammarForUseStrictSimpleParameterList(node: FunctionLikeDeclaration): boolean {
            if (languageVersion >= ScriptTarget.ES2016) {
                const useStrictDirective = node.body && isBlock(node.body) && findUseStrictPrologue(node.body.statements);
                if (useStrictDirective) {
                    const nonSimpleParameters = getNonSimpleParameters(node.parameters);
                    if (length(nonSimpleParameters)) {
                        forEach(nonSimpleParameters, parameter => {
                            addRelatedInfo(
                                error(parameter, Diagnostics.This_parameter_is_not_allowed_with_use_strict_directive),
                                createDiagnosticForNode(useStrictDirective, Diagnostics.use_strict_directive_used_here)
                            );
                        });

                        const diagnostics = nonSimpleParameters.map((parameter, index) => (
                            index === 0 ? createDiagnosticForNode(parameter, Diagnostics.Non_simple_parameter_declared_here) : createDiagnosticForNode(parameter, Diagnostics.and_here)
                        )) as [DiagnosticWithLocation, ...DiagnosticWithLocation[]];
                        addRelatedInfo(error(useStrictDirective, Diagnostics.use_strict_directive_cannot_be_used_with_non_simple_parameter_list), ...diagnostics);
                        return true;
                    }
                }
            }
            return false;
        }

        function checkGrammarFunctionLikeDeclaration(node: FunctionLikeDeclaration | MethodSignature): boolean {
            // Prevent cascading error by short-circuit
            const file = getSourceFileOfNode(node);
            return checkGrammarDecoratorsAndModifiers(node) || checkGrammarTypeParameterList(node.typeParameters, file) ||
                checkGrammarParameterList(node.parameters) || checkGrammarArrowFunction(node, file) ||
                (isFunctionLikeDeclaration(node) && checkGrammarForUseStrictSimpleParameterList(node));
        }

        function checkGrammarClassLikeDeclaration(node: ClassLikeDeclaration): boolean {
            const file = getSourceFileOfNode(node);
            return checkGrammarClassDeclarationHeritageClauses(node) || checkGrammarTypeParameterList(node.typeParameters, file);
        }

        function checkGrammarArrowFunction(node: Node, file: SourceFile): boolean {
            if (!isArrowFunction(node)) {
                return false;
            }

            const { equalsGreaterThanToken } = node;
            const startLine = getLineAndCharacterOfPosition(file, equalsGreaterThanToken.pos).line;
            const endLine = getLineAndCharacterOfPosition(file, equalsGreaterThanToken.end).line;
            return startLine !== endLine && grammarErrorOnNode(equalsGreaterThanToken, Diagnostics.Line_terminator_not_permitted_before_arrow);
        }

        function checkGrammarIndexSignatureParameters(node: SignatureDeclaration): boolean {
            const parameter = node.parameters[0];
            if (node.parameters.length !== 1) {
                if (parameter) {
                    return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_must_have_exactly_one_parameter);
                }
                else {
                    return grammarErrorOnNode(node, Diagnostics.An_index_signature_must_have_exactly_one_parameter);
                }
            }
            if (parameter.dotDotDotToken) {
                return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.An_index_signature_cannot_have_a_rest_parameter);
            }
            if (hasModifiers(parameter)) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_cannot_have_an_accessibility_modifier);
            }
            if (parameter.questionToken) {
                return grammarErrorOnNode(parameter.questionToken, Diagnostics.An_index_signature_parameter_cannot_have_a_question_mark);
            }
            if (parameter.initializer) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_cannot_have_an_initializer);
            }
            if (!parameter.type) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_must_have_a_type_annotation);
            }
            if (parameter.type.kind !== SyntaxKind.StringKeyword && parameter.type.kind !== SyntaxKind.NumberKeyword) {
                const type = getTypeFromTypeNode(parameter.type);

                if (type.flags & TypeFlags.String || type.flags & TypeFlags.Number) {
                    return grammarErrorOnNode(parameter.name,
                                              Diagnostics.An_index_signature_parameter_type_cannot_be_a_type_alias_Consider_writing_0_Colon_1_Colon_2_instead,
                                              getTextOfNode(parameter.name),
                                              typeToString(type),
                                              typeToString(getTypeFromTypeNode(node.type!)));
                }

                if (type.flags & TypeFlags.Union && allTypesAssignableToKind(type, TypeFlags.StringLiteral, /*strict*/ true)) {
                    return grammarErrorOnNode(parameter.name,
                                              Diagnostics.An_index_signature_parameter_type_cannot_be_a_union_type_Consider_using_a_mapped_object_type_instead);
                }

                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_must_be_string_or_number);
            }
            if (!node.type) {
                return grammarErrorOnNode(node, Diagnostics.An_index_signature_must_have_a_type_annotation);
            }
            return false;
        }

        function checkGrammarIndexSignature(node: SignatureDeclaration) {
            // Prevent cascading error by short-circuit
            return checkGrammarDecoratorsAndModifiers(node) || checkGrammarIndexSignatureParameters(node);
        }

        function checkGrammarForAtLeastOneTypeArgument(node: Node, typeArguments: NodeArray<TypeNode> | undefined): boolean {
            if (typeArguments && typeArguments.length === 0) {
                const sourceFile = getSourceFileOfNode(node);
                const start = typeArguments.pos - "<".length;
                const end = skipTrivia(sourceFile.text, typeArguments.end) + ">".length;
                return grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.Type_argument_list_cannot_be_empty);
            }
            return false;
        }

        function checkGrammarTypeArguments(node: Node, typeArguments: NodeArray<TypeNode> | undefined): boolean {
            return checkGrammarForDisallowedTrailingComma(typeArguments) ||
                checkGrammarForAtLeastOneTypeArgument(node, typeArguments);
        }

        function checkGrammarForOmittedArgument(args: NodeArray<Expression> | undefined): boolean {
            if (args) {
                for (const arg of args) {
                    if (arg.kind === SyntaxKind.OmittedExpression) {
                        return grammarErrorAtPos(arg, arg.pos, 0, Diagnostics.Argument_expression_expected);
                    }
                }
            }
            return false;
        }

        function checkGrammarArguments(args: NodeArray<Expression> | undefined): boolean {
            return checkGrammarForOmittedArgument(args);
        }

        function checkGrammarHeritageClause(node: HeritageClause): boolean {
            const types = node.types;
            if (checkGrammarForDisallowedTrailingComma(types)) {
                return true;
            }
            if (types && types.length === 0) {
                const listType = tokenToString(node.token);
                return grammarErrorAtPos(node, types.pos, 0, Diagnostics._0_list_cannot_be_empty, listType);
            }
            return some(types, checkGrammarExpressionWithTypeArguments);
        }

        function checkGrammarExpressionWithTypeArguments(node: ExpressionWithTypeArguments) {
            return checkGrammarTypeArguments(node, node.typeArguments);
        }

        function checkGrammarClassDeclarationHeritageClauses(node: ClassLikeDeclaration) {
            let seenExtendsClause = false;
            let seenImplementsClause = false;

            if (!checkGrammarDecoratorsAndModifiers(node) && node.heritageClauses) {
                for (const heritageClause of node.heritageClauses) {
                    if (heritageClause.token === SyntaxKind.ExtendsKeyword) {
                        if (seenExtendsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.extends_clause_already_seen);
                        }

                        if (seenImplementsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.extends_clause_must_precede_implements_clause);
                        }

                        if (heritageClause.types.length > 1) {
                            return grammarErrorOnFirstToken(heritageClause.types[1], Diagnostics.Classes_can_only_extend_a_single_class);
                        }

                        seenExtendsClause = true;
                    }
                    else {
                        Debug.assert(heritageClause.token === SyntaxKind.ImplementsKeyword);
                        if (seenImplementsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.implements_clause_already_seen);
                        }

                        seenImplementsClause = true;
                    }

                    // Grammar checking heritageClause inside class declaration
                    checkGrammarHeritageClause(heritageClause);
                }
            }
        }

        function checkGrammarInterfaceDeclaration(node: InterfaceDeclaration) {
            let seenExtendsClause = false;

            if (node.heritageClauses) {
                for (const heritageClause of node.heritageClauses) {
                    if (heritageClause.token === SyntaxKind.ExtendsKeyword) {
                        if (seenExtendsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.extends_clause_already_seen);
                        }

                        seenExtendsClause = true;
                    }
                    else {
                        Debug.assert(heritageClause.token === SyntaxKind.ImplementsKeyword);
                        return grammarErrorOnFirstToken(heritageClause, Diagnostics.Interface_declaration_cannot_have_implements_clause);
                    }

                    // Grammar checking heritageClause inside class declaration
                    checkGrammarHeritageClause(heritageClause);
                }
            }
            return false;
        }

        function checkGrammarComputedPropertyName(node: Node): boolean {
            // If node is not a computedPropertyName, just skip the grammar checking
            if (node.kind !== SyntaxKind.ComputedPropertyName) {
                return false;
            }

            const computedPropertyName = <ComputedPropertyName>node;
            if (computedPropertyName.expression.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>computedPropertyName.expression).operatorToken.kind === SyntaxKind.CommaToken) {
                return grammarErrorOnNode(computedPropertyName.expression, Diagnostics.A_comma_expression_is_not_allowed_in_a_computed_property_name);
            }
            return false;
        }

        function checkGrammarForGenerator(node: FunctionLikeDeclaration) {
            if (node.asteriskToken) {
                Debug.assert(
                    node.kind === SyntaxKind.FunctionDeclaration ||
                    node.kind === SyntaxKind.FunctionExpression ||
                    node.kind === SyntaxKind.MethodDeclaration);
                if (node.flags & NodeFlags.Ambient) {
                    return grammarErrorOnNode(node.asteriskToken!, Diagnostics.Generators_are_not_allowed_in_an_ambient_context);
                }
                if (!node.body) {
                    return grammarErrorOnNode(node.asteriskToken!, Diagnostics.An_overload_signature_cannot_be_declared_as_a_generator);
                }
            }
        }

        function checkGrammarForInvalidQuestionMark(questionToken: QuestionToken | undefined, message: DiagnosticMessage): boolean {
            return !!questionToken && grammarErrorOnNode(questionToken, message);
        }

        function checkGrammarForInvalidExclamationToken(exclamationToken: ExclamationToken | undefined, message: DiagnosticMessage): boolean {
            return !!exclamationToken && grammarErrorOnNode(exclamationToken, message);
        }

        function checkGrammarObjectLiteralExpression(node: ObjectLiteralExpression, inDestructuring: boolean) {
            const enum Flags {
                Property = 1,
                GetAccessor = 2,
                SetAccessor = 4,
                GetOrSetAccessor = GetAccessor | SetAccessor,
            }
            const seen = createUnderscoreEscapedMap<Flags>();

            for (const prop of node.properties) {
                if (prop.kind === SyntaxKind.SpreadAssignment) {
                    continue;
                }
                const name = prop.name;
                if (name.kind === SyntaxKind.ComputedPropertyName) {
                    // If the name is not a ComputedPropertyName, the grammar checking will skip it
                    checkGrammarComputedPropertyName(name);
                }

                if (prop.kind === SyntaxKind.ShorthandPropertyAssignment && !inDestructuring && prop.objectAssignmentInitializer) {
                    // having objectAssignmentInitializer is only valid in ObjectAssignmentPattern
                    // outside of destructuring it is a syntax error
                    return grammarErrorOnNode(prop.equalsToken!, Diagnostics.can_only_be_used_in_an_object_literal_property_inside_a_destructuring_assignment);
                }

                // Modifiers are never allowed on properties except for 'async' on a method declaration
                if (prop.modifiers) {
                    for (const mod of prop.modifiers!) { // TODO: GH#19955
                        if (mod.kind !== SyntaxKind.AsyncKeyword || prop.kind !== SyntaxKind.MethodDeclaration) {
                            grammarErrorOnNode(mod, Diagnostics._0_modifier_cannot_be_used_here, getTextOfNode(mod));
                        }
                    }
                }

                // ECMA-262 11.1.5 Object Initializer
                // If previous is not undefined then throw a SyntaxError exception if any of the following conditions are true
                // a.This production is contained in strict code and IsDataDescriptor(previous) is true and
                // IsDataDescriptor(propId.descriptor) is true.
                //    b.IsDataDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true.
                //    c.IsAccessorDescriptor(previous) is true and IsDataDescriptor(propId.descriptor) is true.
                //    d.IsAccessorDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true
                // and either both previous and propId.descriptor have[[Get]] fields or both previous and propId.descriptor have[[Set]] fields
                let currentKind: Flags;
                switch (prop.kind) {
                    case SyntaxKind.ShorthandPropertyAssignment:
                        checkGrammarForInvalidExclamationToken(prop.exclamationToken, Diagnostics.A_definite_assignment_assertion_is_not_permitted_in_this_context);
                    /* tslint:disable:no-switch-case-fall-through */
                    case SyntaxKind.PropertyAssignment:
                        // Grammar checking for computedPropertyName and shorthandPropertyAssignment
                        checkGrammarForInvalidQuestionMark(prop.questionToken, Diagnostics.An_object_member_cannot_be_declared_optional);
                        if (name.kind === SyntaxKind.NumericLiteral) {
                            checkGrammarNumericLiteral(name);
                        }
                        // falls through
                    case SyntaxKind.MethodDeclaration:
                        currentKind = Flags.Property;
                        break;
                    case SyntaxKind.GetAccessor:
                        currentKind = Flags.GetAccessor;
                        break;
                    case SyntaxKind.SetAccessor:
                        currentKind = Flags.SetAccessor;
                        break;
                    default:
                        throw Debug.assertNever(prop, "Unexpected syntax kind:" + (<Node>prop).kind);
                }

                const effectiveName = getPropertyNameForPropertyNameNode(name);
                if (effectiveName === undefined) {
                    continue;
                }

                const existingKind = seen.get(effectiveName);
                if (!existingKind) {
                    seen.set(effectiveName, currentKind);
                }
                else {
                    if (currentKind === Flags.Property && existingKind === Flags.Property) {
                        grammarErrorOnNode(name, Diagnostics.Duplicate_identifier_0, getTextOfNode(name));
                    }
                    else if ((currentKind & Flags.GetOrSetAccessor) && (existingKind & Flags.GetOrSetAccessor)) {
                        if (existingKind !== Flags.GetOrSetAccessor && currentKind !== existingKind) {
                            seen.set(effectiveName, currentKind | existingKind);
                        }
                        else {
                            return grammarErrorOnNode(name, Diagnostics.An_object_literal_cannot_have_multiple_get_Slashset_accessors_with_the_same_name);
                        }
                    }
                    else {
                        return grammarErrorOnNode(name, Diagnostics.An_object_literal_cannot_have_property_and_accessor_with_the_same_name);
                    }
                }
            }
        }

        function checkGrammarJsxElement(node: JsxOpeningLikeElement) {
            checkGrammarTypeArguments(node, node.typeArguments);
            const seen = createUnderscoreEscapedMap<boolean>();

            for (const attr of node.attributes.properties) {
                if (attr.kind === SyntaxKind.JsxSpreadAttribute) {
                    continue;
                }

                const { name, initializer } = attr;
                if (!seen.get(name.escapedText)) {
                    seen.set(name.escapedText, true);
                }
                else {
                    return grammarErrorOnNode(name, Diagnostics.JSX_elements_cannot_have_multiple_attributes_with_the_same_name);
                }

                if (initializer && initializer.kind === SyntaxKind.JsxExpression && !initializer.expression) {
                    return grammarErrorOnNode(initializer, Diagnostics.JSX_attributes_must_only_be_assigned_a_non_empty_expression);
                }
            }
        }

        function checkGrammarForInOrForOfStatement(forInOrOfStatement: ForInOrOfStatement): boolean {
            if (checkGrammarStatementInAmbientContext(forInOrOfStatement)) {
                return true;
            }

            if (forInOrOfStatement.kind === SyntaxKind.ForOfStatement && forInOrOfStatement.awaitModifier) {
                if ((forInOrOfStatement.flags & NodeFlags.AwaitContext) === NodeFlags.None) {
                    return grammarErrorOnNode(forInOrOfStatement.awaitModifier, Diagnostics.A_for_await_of_statement_is_only_allowed_within_an_async_function_or_async_generator);
                }
            }

            if (forInOrOfStatement.initializer.kind === SyntaxKind.VariableDeclarationList) {
                const variableList = <VariableDeclarationList>forInOrOfStatement.initializer;
                if (!checkGrammarVariableDeclarationList(variableList)) {
                    const declarations = variableList.declarations;

                    // declarations.length can be zero if there is an error in variable declaration in for-of or for-in
                    // See http://www.ecma-international.org/ecma-262/6.0/#sec-for-in-and-for-of-statements for details
                    // For example:
                    //      var let = 10;
                    //      for (let of [1,2,3]) {} // this is invalid ES6 syntax
                    //      for (let in [1,2,3]) {} // this is invalid ES6 syntax
                    // We will then want to skip on grammar checking on variableList declaration
                    if (!declarations.length) {
                        return false;
                    }

                    if (declarations.length > 1) {
                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
                            ? Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_in_statement
                            : Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_of_statement;
                        return grammarErrorOnFirstToken(variableList.declarations[1], diagnostic);
                    }
                    const firstDeclaration = declarations[0];

                    if (firstDeclaration.initializer) {
                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
                            ? Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer
                            : Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;
                        return grammarErrorOnNode(firstDeclaration.name, diagnostic);
                    }
                    if (firstDeclaration.type) {
                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
                            ? Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_use_a_type_annotation
                            : Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_use_a_type_annotation;
                        return grammarErrorOnNode(firstDeclaration, diagnostic);
                    }
                }
            }

            return false;
        }

        function checkGrammarAccessor(accessor: AccessorDeclaration): boolean {
            const kind = accessor.kind;
            if (languageVersion < ScriptTarget.ES5) {
                return grammarErrorOnNode(accessor.name, Diagnostics.Accessors_are_only_available_when_targeting_ECMAScript_5_and_higher);
            }
            else if (accessor.flags & NodeFlags.Ambient) {
                return grammarErrorOnNode(accessor.name, Diagnostics.An_accessor_cannot_be_declared_in_an_ambient_context);
            }
            else if (accessor.body === undefined && !hasModifier(accessor, ModifierFlags.Abstract)) {
                return grammarErrorAtPos(accessor, accessor.end - 1, ";".length, Diagnostics._0_expected, "{");
            }
            else if (accessor.body && hasModifier(accessor, ModifierFlags.Abstract)) {
                return grammarErrorOnNode(accessor, Diagnostics.An_abstract_accessor_cannot_have_an_implementation);
            }
            else if (accessor.typeParameters) {
                return grammarErrorOnNode(accessor.name, Diagnostics.An_accessor_cannot_have_type_parameters);
            }
            else if (!doesAccessorHaveCorrectParameterCount(accessor)) {
                return grammarErrorOnNode(accessor.name,
                                          kind === SyntaxKind.GetAccessor ?
                                          Diagnostics.A_get_accessor_cannot_have_parameters :
                                          Diagnostics.A_set_accessor_must_have_exactly_one_parameter);
            }
            else if (kind === SyntaxKind.SetAccessor) {
                if (accessor.type) {
                    return grammarErrorOnNode(accessor.name, Diagnostics.A_set_accessor_cannot_have_a_return_type_annotation);
                }
                else {
                    const parameter = accessor.parameters[0];
                    if (parameter.dotDotDotToken) {
                        return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.A_set_accessor_cannot_have_rest_parameter);
                    }
                    else if (parameter.questionToken) {
                        return grammarErrorOnNode(parameter.questionToken, Diagnostics.A_set_accessor_cannot_have_an_optional_parameter);
                    }
                    else if (parameter.initializer) {
                        return grammarErrorOnNode(accessor.name, Diagnostics.A_set_accessor_parameter_cannot_have_an_initializer);
                    }
                }
            }
            return false;
        }

        /** Does the accessor have the right number of parameters?
         * A get accessor has no parameters or a single `this` parameter.
         * A set accessor has one parameter or a `this` parameter and one more parameter.
         */
        function doesAccessorHaveCorrectParameterCount(accessor: AccessorDeclaration) {
            return getAccessorThisParameter(accessor) || accessor.parameters.length === (accessor.kind === SyntaxKind.GetAccessor ? 0 : 1);
        }

        function getAccessorThisParameter(accessor: AccessorDeclaration): ParameterDeclaration | undefined {
            if (accessor.parameters.length === (accessor.kind === SyntaxKind.GetAccessor ? 1 : 2)) {
                return getThisParameter(accessor);
            }
        }

        function checkGrammarTypeOperatorNode(node: TypeOperatorNode) {
            if (node.operator === SyntaxKind.UniqueKeyword) {
                if (node.type.kind !== SyntaxKind.SymbolKeyword) {
                    return grammarErrorOnNode(node.type, Diagnostics._0_expected, tokenToString(SyntaxKind.SymbolKeyword));
                }

                const parent = walkUpParenthesizedTypes(node.parent);
                switch (parent.kind) {
                    case SyntaxKind.VariableDeclaration:
                        const decl = parent as VariableDeclaration;
                        if (decl.name.kind !== SyntaxKind.Identifier) {
                            return grammarErrorOnNode(node, Diagnostics.unique_symbol_types_may_not_be_used_on_a_variable_declaration_with_a_binding_name);
                        }
                        if (!isVariableDeclarationInVariableStatement(decl)) {
                            return grammarErrorOnNode(node, Diagnostics.unique_symbol_types_are_only_allowed_on_variables_in_a_variable_statement);
                        }
                        if (!(decl.parent.flags & NodeFlags.Const)) {
                            return grammarErrorOnNode((<VariableDeclaration>parent).name, Diagnostics.A_variable_whose_type_is_a_unique_symbol_type_must_be_const);
                        }
                        break;

                    case SyntaxKind.PropertyDeclaration:
                        if (!hasModifier(parent, ModifierFlags.Static) ||
                            !hasModifier(parent, ModifierFlags.Readonly)) {
                            return grammarErrorOnNode((<PropertyDeclaration>parent).name, Diagnostics.A_property_of_a_class_whose_type_is_a_unique_symbol_type_must_be_both_static_and_readonly);
                        }
                        break;

                    case SyntaxKind.PropertySignature:
                        if (!hasModifier(parent, ModifierFlags.Readonly)) {
                            return grammarErrorOnNode((<PropertySignature>parent).name, Diagnostics.A_property_of_an_interface_or_type_literal_whose_type_is_a_unique_symbol_type_must_be_readonly);
                        }
                        break;

                    default:
                        return grammarErrorOnNode(node, Diagnostics.unique_symbol_types_are_not_allowed_here);
                }
            }
        }

        function checkGrammarForInvalidDynamicName(node: DeclarationName, message: DiagnosticMessage) {
            if (isNonBindableDynamicName(node)) {
                return grammarErrorOnNode(node, message);
            }
        }

        function checkGrammarMethod(node: MethodDeclaration | MethodSignature) {
            if (checkGrammarFunctionLikeDeclaration(node)) {
                return true;
            }

            if (node.kind === SyntaxKind.MethodDeclaration) {
                if (node.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                    // We only disallow modifier on a method declaration if it is a property of object-literal-expression
                    if (node.modifiers && !(node.modifiers.length === 1 && first(node.modifiers).kind === SyntaxKind.AsyncKeyword)) {
                        return grammarErrorOnFirstToken(node, Diagnostics.Modifiers_cannot_appear_here);
                    }
                    else if (checkGrammarForInvalidQuestionMark(node.questionToken, Diagnostics.An_object_member_cannot_be_declared_optional)) {
                        return true;
                    }
                    else if (checkGrammarForInvalidExclamationToken(node.exclamationToken, Diagnostics.A_definite_assignment_assertion_is_not_permitted_in_this_context)) {
                        return true;
                    }
                    else if (node.body === undefined) {
                        return grammarErrorAtPos(node, node.end - 1, ";".length, Diagnostics._0_expected, "{");
                    }
                }
                if (checkGrammarForGenerator(node)) {
                    return true;
                }
            }

            if (isClassLike(node.parent)) {
                // Technically, computed properties in ambient contexts is disallowed
                // for property declarations and accessors too, not just methods.
                // However, property declarations disallow computed names in general,
                // and accessors are not allowed in ambient contexts in general,
                // so this error only really matters for methods.
                if (node.flags & NodeFlags.Ambient) {
                    return checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_an_ambient_context_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type);
                }
                else if (node.kind === SyntaxKind.MethodDeclaration && !node.body) {
                    return checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_a_method_overload_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type);
                }
            }
            else if (node.parent.kind === SyntaxKind.InterfaceDeclaration) {
                return checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_an_interface_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type);
            }
            else if (node.parent.kind === SyntaxKind.TypeLiteral) {
                return checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_a_type_literal_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type);
            }
        }

        function checkGrammarBreakOrContinueStatement(node: BreakOrContinueStatement): boolean {
            let current: Node = node;
            while (current) {
                if (isFunctionLike(current)) {
                    return grammarErrorOnNode(node, Diagnostics.Jump_target_cannot_cross_function_boundary);
                }

                switch (current.kind) {
                    case SyntaxKind.LabeledStatement:
                        if (node.label && (<LabeledStatement>current).label.escapedText === node.label.escapedText) {
                            // found matching label - verify that label usage is correct
                            // continue can only target labels that are on iteration statements
                            const isMisplacedContinueLabel = node.kind === SyntaxKind.ContinueStatement
                                && !isIterationStatement((<LabeledStatement>current).statement, /*lookInLabeledStatement*/ true);

                            if (isMisplacedContinueLabel) {
                                return grammarErrorOnNode(node, Diagnostics.A_continue_statement_can_only_jump_to_a_label_of_an_enclosing_iteration_statement);
                            }

                            return false;
                        }
                        break;
                    case SyntaxKind.SwitchStatement:
                        if (node.kind === SyntaxKind.BreakStatement && !node.label) {
                            // unlabeled break within switch statement - ok
                            return false;
                        }
                        break;
                    default:
                        if (isIterationStatement(current, /*lookInLabeledStatement*/ false) && !node.label) {
                            // unlabeled break or continue within iteration statement - ok
                            return false;
                        }
                        break;
                }

                current = current.parent;
            }

            if (node.label) {
                const message = node.kind === SyntaxKind.BreakStatement
                    ? Diagnostics.A_break_statement_can_only_jump_to_a_label_of_an_enclosing_statement
                    : Diagnostics.A_continue_statement_can_only_jump_to_a_label_of_an_enclosing_iteration_statement;

                return grammarErrorOnNode(node, message);
            }
            else {
                const message = node.kind === SyntaxKind.BreakStatement
                    ? Diagnostics.A_break_statement_can_only_be_used_within_an_enclosing_iteration_or_switch_statement
                    : Diagnostics.A_continue_statement_can_only_be_used_within_an_enclosing_iteration_statement;
                return grammarErrorOnNode(node, message);
            }
        }

        function checkGrammarBindingElement(node: BindingElement) {
            if (node.dotDotDotToken) {
                const elements = node.parent.elements;
                if (node !== last(elements)) {
                    return grammarErrorOnNode(node, Diagnostics.A_rest_element_must_be_last_in_a_destructuring_pattern);
                }
                checkGrammarForDisallowedTrailingComma(elements, Diagnostics.A_rest_parameter_or_binding_pattern_may_not_have_a_trailing_comma);

                if (node.propertyName) {
                    return grammarErrorOnNode(node.name, Diagnostics.A_rest_element_cannot_have_a_property_name);
                }

                if (node.initializer) {
                    // Error on equals token which immediately precedes the initializer
                    return grammarErrorAtPos(node, node.initializer.pos - 1, 1, Diagnostics.A_rest_element_cannot_have_an_initializer);
                }
            }
        }

        function isStringOrNumberLiteralExpression(expr: Expression) {
            return expr.kind === SyntaxKind.StringLiteral || expr.kind === SyntaxKind.NumericLiteral ||
                expr.kind === SyntaxKind.PrefixUnaryExpression && (<PrefixUnaryExpression>expr).operator === SyntaxKind.MinusToken &&
                (<PrefixUnaryExpression>expr).operand.kind === SyntaxKind.NumericLiteral;
        }

        function isSimpleLiteralEnumReference(expr: Expression) {
            if (
                (isPropertyAccessExpression(expr) || (isElementAccessExpression(expr) && isStringOrNumberLiteralExpression(expr.argumentExpression))) &&
                isEntityNameExpression(expr.expression)
            ) return !!(checkExpressionCached(expr).flags & TypeFlags.EnumLiteral);
        }

        function checkAmbientInitializer(node: VariableDeclaration | PropertyDeclaration | PropertySignature) {
            if (node.initializer) {
                const isInvalidInitializer = !(isStringOrNumberLiteralExpression(node.initializer) || isSimpleLiteralEnumReference(node.initializer));
                const isConstOrReadonly = isDeclarationReadonly(node) || isVariableDeclaration(node) && isVarConst(node);
                if (isConstOrReadonly && !node.type) {
                    if (isInvalidInitializer) {
                        return grammarErrorOnNode(node.initializer!, Diagnostics.A_const_initializer_in_an_ambient_context_must_be_a_string_or_numeric_literal_or_literal_enum_reference);
                    }
                }
                else {
                    return grammarErrorOnNode(node.initializer!, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
                }
                if (!isConstOrReadonly || isInvalidInitializer) {
                    return grammarErrorOnNode(node.initializer!, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
                }
            }
        }

        function checkGrammarVariableDeclaration(node: VariableDeclaration) {
            if (node.parent.parent.kind !== SyntaxKind.ForInStatement && node.parent.parent.kind !== SyntaxKind.ForOfStatement) {
                if (node.flags & NodeFlags.Ambient) {
                    checkAmbientInitializer(node);
                }
                else if (!node.initializer) {
                    if (isBindingPattern(node.name) && !isBindingPattern(node.parent)) {
                        return grammarErrorOnNode(node, Diagnostics.A_destructuring_declaration_must_have_an_initializer);
                    }
                    if (isVarConst(node)) {
                        return grammarErrorOnNode(node, Diagnostics.const_declarations_must_be_initialized);
                    }
                }
            }

            if (node.exclamationToken && (node.parent.parent.kind !== SyntaxKind.VariableStatement || !node.type || node.initializer || node.flags & NodeFlags.Ambient)) {
                return grammarErrorOnNode(node.exclamationToken, Diagnostics.A_definite_assignment_assertion_is_not_permitted_in_this_context);
            }

            if (compilerOptions.module !== ModuleKind.ES2015 && compilerOptions.module !== ModuleKind.ESNext && compilerOptions.module !== ModuleKind.System && !compilerOptions.noEmit &&
                !(node.parent.parent.flags & NodeFlags.Ambient) && hasModifier(node.parent.parent, ModifierFlags.Export)) {
                checkESModuleMarker(node.name);
            }

            const checkLetConstNames = (isLet(node) || isVarConst(node));

            // 1. LexicalDeclaration : LetOrConst BindingList ;
            // It is a Syntax Error if the BoundNames of BindingList contains "let".
            // 2. ForDeclaration: ForDeclaration : LetOrConst ForBinding
            // It is a Syntax Error if the BoundNames of ForDeclaration contains "let".

            // It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code
            // and its Identifier is eval or arguments
            return checkLetConstNames && checkGrammarNameInLetOrConstDeclarations(node.name);
        }

        function checkESModuleMarker(name: Identifier | BindingPattern): boolean {
            if (name.kind === SyntaxKind.Identifier) {
                if (idText(name) === "__esModule") {
                    return grammarErrorOnNode(name, Diagnostics.Identifier_expected_esModule_is_reserved_as_an_exported_marker_when_transforming_ECMAScript_modules);
                }
            }
            else {
                const elements = name.elements;
                for (const element of elements) {
                    if (!isOmittedExpression(element)) {
                        return checkESModuleMarker(element.name);
                    }
                }
            }
            return false;
        }

        function checkGrammarNameInLetOrConstDeclarations(name: Identifier | BindingPattern): boolean {
            if (name.kind === SyntaxKind.Identifier) {
                if (name.originalKeywordKind === SyntaxKind.LetKeyword) {
                    return grammarErrorOnNode(name, Diagnostics.let_is_not_allowed_to_be_used_as_a_name_in_let_or_const_declarations);
                }
            }
            else {
                const elements = name.elements;
                for (const element of elements) {
                    if (!isOmittedExpression(element)) {
                        checkGrammarNameInLetOrConstDeclarations(element.name);
                    }
                }
            }
            return false;
        }

        function checkGrammarVariableDeclarationList(declarationList: VariableDeclarationList): boolean {
            const declarations = declarationList.declarations;
            if (checkGrammarForDisallowedTrailingComma(declarationList.declarations)) {
                return true;
            }

            if (!declarationList.declarations.length) {
                return grammarErrorAtPos(declarationList, declarations.pos, declarations.end - declarations.pos, Diagnostics.Variable_declaration_list_cannot_be_empty);
            }
            return false;
        }

        function allowLetAndConstDeclarations(parent: Node): boolean {
            switch (parent.kind) {
                case SyntaxKind.IfStatement:
                case SyntaxKind.DoStatement:
                case SyntaxKind.WhileStatement:
                case SyntaxKind.WithStatement:
                case SyntaxKind.ForStatement:
                case SyntaxKind.ForInStatement:
                case SyntaxKind.ForOfStatement:
                    return false;
                case SyntaxKind.LabeledStatement:
                    return allowLetAndConstDeclarations(parent.parent);
            }

            return true;
        }

        function checkGrammarForDisallowedLetOrConstStatement(node: VariableStatement) {
            if (!allowLetAndConstDeclarations(node.parent)) {
                if (isLet(node.declarationList)) {
                    return grammarErrorOnNode(node, Diagnostics.let_declarations_can_only_be_declared_inside_a_block);
                }
                else if (isVarConst(node.declarationList)) {
                    return grammarErrorOnNode(node, Diagnostics.const_declarations_can_only_be_declared_inside_a_block);
                }
            }
        }

        function checkGrammarMetaProperty(node: MetaProperty) {
            const escapedText = node.name.escapedText;
            switch (node.keywordToken) {
                case SyntaxKind.NewKeyword:
                    if (escapedText !== "target") {
                        return grammarErrorOnNode(node.name, Diagnostics._0_is_not_a_valid_meta_property_for_keyword_1_Did_you_mean_2, node.name.escapedText, tokenToString(node.keywordToken), "target");
                    }
                    break;
                case SyntaxKind.ImportKeyword:
                    if (escapedText !== "meta") {
                        return grammarErrorOnNode(node.name, Diagnostics._0_is_not_a_valid_meta_property_for_keyword_1_Did_you_mean_2, node.name.escapedText, tokenToString(node.keywordToken), "meta");
                    }
                    break;
            }
        }

        function hasParseDiagnostics(sourceFile: SourceFile): boolean {
            return sourceFile.parseDiagnostics.length > 0;
        }

        function grammarErrorOnFirstToken(node: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
                diagnostics.add(createFileDiagnostic(sourceFile, span.start, span.length, message, arg0, arg1, arg2));
                return true;
            }
            return false;
        }

        function grammarErrorAtPos(nodeForSourceFile: Node, start: number, length: number, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(nodeForSourceFile);
            if (!hasParseDiagnostics(sourceFile)) {
                diagnostics.add(createFileDiagnostic(sourceFile, start, length, message, arg0, arg1, arg2));
                return true;
            }
            return false;
        }

        function grammarErrorOnNode(node: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                diagnostics.add(createDiagnosticForNode(node, message, arg0, arg1, arg2));
                return true;
            }
            return false;
        }

        function checkGrammarConstructorTypeParameters(node: ConstructorDeclaration) {
            const jsdocTypeParameters = isInJSFile(node) && getJSDocTypeParameterDeclarations(node);
            if (node.typeParameters || jsdocTypeParameters && jsdocTypeParameters.length) {
                const { pos, end } = node.typeParameters || jsdocTypeParameters && jsdocTypeParameters[0] || node;
                return grammarErrorAtPos(node, pos, end - pos, Diagnostics.Type_parameters_cannot_appear_on_a_constructor_declaration);
            }
        }

        function checkGrammarConstructorTypeAnnotation(node: ConstructorDeclaration) {
            const type = getEffectiveReturnTypeNode(node);
            if (type) {
                return grammarErrorOnNode(type, Diagnostics.Type_annotation_cannot_appear_on_a_constructor_declaration);
            }
        }

        function checkGrammarProperty(node: PropertyDeclaration | PropertySignature) {
            if (isClassLike(node.parent)) {
                if (checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_a_class_property_declaration_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type)) {
                    return true;
                }
            }
            else if (node.parent.kind === SyntaxKind.InterfaceDeclaration) {
                if (checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_an_interface_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type)) {
                    return true;
                }
                if (node.initializer) {
                    return grammarErrorOnNode(node.initializer, Diagnostics.An_interface_property_cannot_have_an_initializer);
                }
            }
            else if (node.parent.kind === SyntaxKind.TypeLiteral) {
                if (checkGrammarForInvalidDynamicName(node.name, Diagnostics.A_computed_property_name_in_a_type_literal_must_refer_to_an_expression_whose_type_is_a_literal_type_or_a_unique_symbol_type)) {
                    return true;
                }
                if (node.initializer) {
                    return grammarErrorOnNode(node.initializer, Diagnostics.A_type_literal_property_cannot_have_an_initializer);
                }
            }

            if (node.flags & NodeFlags.Ambient) {
                checkAmbientInitializer(node);
            }

            if (isPropertyDeclaration(node) && node.exclamationToken && (!isClassLike(node.parent) || !node.type || node.initializer ||
                node.flags & NodeFlags.Ambient || hasModifier(node, ModifierFlags.Static | ModifierFlags.Abstract))) {
                return grammarErrorOnNode(node.exclamationToken, Diagnostics.A_definite_assignment_assertion_is_not_permitted_in_this_context);
            }
        }

        function checkGrammarTopLevelElementForRequiredDeclareModifier(node: Node): boolean {
            // A declare modifier is required for any top level .d.ts declaration except export=, export default, export as namespace
            // interfaces and imports categories:
            //
            //  DeclarationElement:
            //     ExportAssignment
            //     export_opt   InterfaceDeclaration
            //     export_opt   TypeAliasDeclaration
            //     export_opt   ImportDeclaration
            //     export_opt   ExternalImportDeclaration
            //     export_opt   AmbientDeclaration
            //
            // TODO: The spec needs to be amended to reflect this grammar.
            if (node.kind === SyntaxKind.InterfaceDeclaration ||
                node.kind === SyntaxKind.TypeAliasDeclaration ||
                node.kind === SyntaxKind.ImportDeclaration ||
                node.kind === SyntaxKind.ImportEqualsDeclaration ||
                node.kind === SyntaxKind.ExportDeclaration ||
                node.kind === SyntaxKind.ExportAssignment ||
                node.kind === SyntaxKind.NamespaceExportDeclaration ||
                hasModifier(node, ModifierFlags.Ambient | ModifierFlags.Export | ModifierFlags.Default)) {
                    return false;
            }

            return grammarErrorOnFirstToken(node, Diagnostics.A_declare_modifier_is_required_for_a_top_level_declaration_in_a_d_ts_file);
        }

        function checkGrammarTopLevelElementsForRequiredDeclareModifier(file: SourceFile): boolean {
            for (const decl of file.statements) {
                if (isDeclaration(decl) || decl.kind === SyntaxKind.VariableStatement) {
                    if (checkGrammarTopLevelElementForRequiredDeclareModifier(decl)) {
                        return true;
                    }
                }
            }
            return false;
        }

        function checkGrammarSourceFile(node: SourceFile): boolean {
            return !!(node.flags & NodeFlags.Ambient) && checkGrammarTopLevelElementsForRequiredDeclareModifier(node);
        }

        function checkGrammarStatementInAmbientContext(node: Node): boolean {
            if (node.flags & NodeFlags.Ambient) {
                // An accessors is already reported about the ambient context
                if (isAccessor(node.parent)) {
                    return getNodeLinks(node).hasReportedStatementInAmbientContext = true;
                }

                // Find containing block which is either Block, ModuleBlock, SourceFile
                const links = getNodeLinks(node);
                if (!links.hasReportedStatementInAmbientContext && isFunctionLike(node.parent)) {
                    return getNodeLinks(node).hasReportedStatementInAmbientContext = grammarErrorOnFirstToken(node, Diagnostics.An_implementation_cannot_be_declared_in_ambient_contexts);
                }

                // We are either parented by another statement, or some sort of block.
                // If we're in a block, we only want to really report an error once
                // to prevent noisiness.  So use a bit on the block to indicate if
                // this has already been reported, and don't report if it has.
                //
                if (node.parent.kind === SyntaxKind.Block || node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                    const links = getNodeLinks(node.parent);
                    // Check if the containing block ever report this error
                    if (!links.hasReportedStatementInAmbientContext) {
                        return links.hasReportedStatementInAmbientContext = grammarErrorOnFirstToken(node, Diagnostics.Statements_are_not_allowed_in_ambient_contexts);
                    }
                }
                else {
                    // We must be parented by a statement.  If so, there's no need
                    // to report the error as our parent will have already done it.
                    // Debug.assert(isStatement(node.parent));
                }
            }
            return false;
        }

        function checkGrammarNumericLiteral(node: NumericLiteral): boolean {
            // Grammar checking
            if (node.numericLiteralFlags & TokenFlags.Octal) {
                let diagnosticMessage: DiagnosticMessage | undefined;
                if (languageVersion >= ScriptTarget.ES5) {
                    diagnosticMessage = Diagnostics.Octal_literals_are_not_available_when_targeting_ECMAScript_5_and_higher_Use_the_syntax_0;
                }
                else if (isChildOfNodeWithKind(node, SyntaxKind.LiteralType)) {
                    diagnosticMessage = Diagnostics.Octal_literal_types_must_use_ES2015_syntax_Use_the_syntax_0;
                }
                else if (isChildOfNodeWithKind(node, SyntaxKind.EnumMember)) {
                    diagnosticMessage = Diagnostics.Octal_literals_are_not_allowed_in_enums_members_initializer_Use_the_syntax_0;
                }
                if (diagnosticMessage) {
                    const withMinus = isPrefixUnaryExpression(node.parent) && node.parent.operator === SyntaxKind.MinusToken;
                    const literal = (withMinus ? "-" : "") + "0o" + node.text;
                    return grammarErrorOnNode(withMinus ? node.parent : node, diagnosticMessage, literal);
                }
            }
            return false;
        }

        function grammarErrorAfterFirstToken(node: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
                diagnostics.add(createFileDiagnostic(sourceFile, textSpanEnd(span), /*length*/ 0, message, arg0, arg1, arg2));
                return true;
            }
            return false;
        }

        function getAmbientModules(): Symbol[] {
            if (!ambientModulesCache) {
                ambientModulesCache = [];
                globals.forEach((global, sym) => {
                    // No need to `unescapeLeadingUnderscores`, an escaped symbol is never an ambient module.
                    if (ambientModuleSymbolRegex.test(sym as string)) {
                        ambientModulesCache!.push(global);
                    }
                });
            }
            return ambientModulesCache;
        }

        function checkGrammarImportCallExpression(node: ImportCall): boolean {
            if (moduleKind === ModuleKind.ES2015) {
                return grammarErrorOnNode(node, Diagnostics.Dynamic_import_is_only_supported_when_module_flag_is_commonjs_or_esNext);
            }

            if (node.typeArguments) {
                return grammarErrorOnNode(node, Diagnostics.Dynamic_import_cannot_have_type_arguments);
            }

            const nodeArguments = node.arguments;
            if (nodeArguments.length !== 1) {
                return grammarErrorOnNode(node, Diagnostics.Dynamic_import_must_have_one_specifier_as_an_argument);
            }

            // see: parseArgumentOrArrayLiteralElement...we use this function which parse arguments of callExpression to parse specifier for dynamic import.
            // parseArgumentOrArrayLiteralElement allows spread element to be in an argument list which is not allowed as specifier in dynamic import.
            if (isSpreadElement(nodeArguments[0])) {
                return grammarErrorOnNode(nodeArguments[0], Diagnostics.Specifier_of_dynamic_import_cannot_be_spread_element);
            }
            return false;
        }
    }

    /** Like 'isDeclarationName', but returns true for LHS of `import { x as y }` or `export { x as y }`. */
    function isDeclarationNameOrImportPropertyName(name: Node): boolean {
        switch (name.parent.kind) {
            case SyntaxKind.ImportSpecifier:
            case SyntaxKind.ExportSpecifier:
                return isIdentifier(name);
            default:
                return isDeclarationName(name);
        }
    }

    function isSomeImportDeclaration(decl: Node): boolean {
        switch (decl.kind) {
            case SyntaxKind.ImportClause: // For default import
            case SyntaxKind.ImportEqualsDeclaration:
            case SyntaxKind.NamespaceImport:
            case SyntaxKind.ImportSpecifier: // For rename import `x as y`
                return true;
            case SyntaxKind.Identifier:
                // For regular import, `decl` is an Identifier under the ImportSpecifier.
                return decl.parent.kind === SyntaxKind.ImportSpecifier;
            default:
                return false;
        }
    }

    namespace JsxNames {
        // tslint:disable variable-name
        export const JSX = "JSX" as __String;
        export const IntrinsicElements = "IntrinsicElements" as __String;
        export const ElementClass = "ElementClass" as __String;
        export const ElementAttributesPropertyNameContainer = "ElementAttributesProperty" as __String; // TODO: Deprecate and remove support
        export const ElementChildrenAttributeNameContainer = "ElementChildrenAttribute" as __String;
        export const Element = "Element" as __String;
        export const IntrinsicAttributes = "IntrinsicAttributes" as __String;
        export const IntrinsicClassAttributes = "IntrinsicClassAttributes" as __String;
        export const LibraryManagedAttributes = "LibraryManagedAttributes" as __String;
        // tslint:enable variable-name
    }

    function typeIsLiteralType(type: Type): type is LiteralType {
        return !!(type.flags & TypeFlags.Literal);
    }
}