Disallow uninitialised property overrides

src/compiler/checker.ts

@@ -29443,7 +29443,6 @@ namespace ts {
         }
 
         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.
@@ -29477,7 +29476,6 @@ namespace ts {
                 // 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.
@@ -29514,10 +29512,31 @@ namespace ts {
                         continue;
                     }
 
-                    if (isPrototypeProperty(base) || base.flags & SymbolFlags.PropertyOrAccessor && derived.flags & SymbolFlags.PropertyOrAccessor) {
+                    if (isPrototypeProperty(base)) {
                         // method is overridden with method or property/accessor is overridden with property/accessor - correct case
                         continue;
                     }
+                    if (base.flags & SymbolFlags.PropertyOrAccessor && derived.flags & SymbolFlags.PropertyOrAccessor) {
+                        const uninitialized = find(derived.declarations, d => d.kind === SyntaxKind.PropertyDeclaration && !(d as PropertyDeclaration).initializer);
+                        if (uninitialized
+                            && !(base.valueDeclaration && base.valueDeclaration.parent.kind === SyntaxKind.InterfaceDeclaration)
+                            && !(derived.flags & SymbolFlags.Transient)
+                            && !(baseDeclarationFlags & ModifierFlags.Abstract)
+                            && !(derivedDeclarationFlags & ModifierFlags.Abstract)
+                            && !derived.declarations.some(d => d.flags & NodeFlags.Ambient)) {
+                            const constructor = findConstructorDeclaration(getClassLikeDeclarationOfSymbol(type.symbol)!);
+                            const propName = (uninitialized as PropertyDeclaration).name;
+                            if ((uninitialized as PropertyDeclaration).exclamationToken
+                                || !constructor
+                                || !isIdentifier(propName)
+                                || !strictNullChecks
+                                || !isPropertyInitializedInConstructor(propName, type, constructor)) {
+                                const errorMessage = Diagnostics.Property_0_will_overwrite_the_base_property_in_1_Add_a_declare_modifier_or_an_initializer_to_avoid_this;
+                                error(getNameOfDeclaration(derived.valueDeclaration) || derived.valueDeclaration, errorMessage, symbolToString(base), typeToString(baseType));
+                            }
+                        }
+                        continue;
+                    }
 
                     let errorMessage: DiagnosticMessage;
                     if (isPrototypeProperty(base)) {
@@ -29583,6 +29602,9 @@ namespace ts {
             }
             const constructor = findConstructorDeclaration(node);
             for (const member of node.members) {
+                if (getModifierFlags(member) & ModifierFlags.Ambient) {
+                    continue;
+                }
                 if (isInstancePropertyWithoutInitializer(member)) {
                     const propName = (<PropertyDeclaration>member).name;
                     if (isIdentifier(propName)) {
@@ -32507,7 +32529,7 @@ namespace ts {
                         else if (flags & ModifierFlags.Async) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                         }
-                        else if (isClassLike(node.parent)) {
+                        else if (isClassLike(node.parent) && !isPropertyDeclaration(node)) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_class_element, "declare");
                         }
                         else if (node.kind === SyntaxKind.Parameter) {

src/compiler/diagnosticMessages.json

@@ -2204,6 +2204,10 @@
         "category": "Error",
         "code": 2609
     },
+    "Property '{0}' will overwrite the base property in '{1}'. Add a 'declare' modifier or an initializer to avoid this.": {
+        "category": "Error",
+        "code": 2610
+    },
     "Cannot augment module '{0}' with value exports because it resolves to a non-module entity.": {
         "category": "Error",
         "code": 2649
@@ -5144,6 +5148,14 @@
         "category": "Message",
         "code": 95093
     },
+    "Prefix with 'declare'": {
+        "category": "Message",
+        "code": 95094
+    },
+    "Prefix all incorrect property declarations with 'declare'": {
+        "category": "Message",
+        "code": 95095
+    },
 
     "No value exists in scope for the shorthand property '{0}'. Either declare one or provide an initializer.": {
         "category": "Error",

src/compiler/parser.ts

@@ -5982,8 +5982,16 @@ namespace ts {
                 token() === SyntaxKind.NumericLiteral ||
                 token() === SyntaxKind.AsteriskToken ||
                 token() === SyntaxKind.OpenBracketToken) {
-
-                return parsePropertyOrMethodDeclaration(<PropertyDeclaration | MethodDeclaration>node);
+                const isAmbient = node.modifiers && some(node.modifiers, isDeclareModifier);
+                if (isAmbient) {
+                    for (const m of node.modifiers!) {
+                        m.flags |= NodeFlags.Ambient;
+                    }
+                    return doInsideOfContext(NodeFlags.Ambient, () => parsePropertyOrMethodDeclaration(node as PropertyDeclaration | MethodDeclaration));
+                }
+                else {
+                    return parsePropertyOrMethodDeclaration(node as PropertyDeclaration | MethodDeclaration);
+                }
             }
 
             if (node.decorators || node.modifiers) {

src/services/codefixes/addMissingDeclareProperty.ts

@@ -0,0 +1,34 @@
+/* @internal */
+namespace ts.codefix {
+    const fixId = "addMissingDeclareProperty";
+    const errorCodes = [
+        Diagnostics.Property_0_will_overwrite_the_base_property_in_1_Add_a_declare_modifier_or_an_initializer_to_avoid_this.code,
+    ];
+
+    registerCodeFix({
+        errorCodes,
+        getCodeActions: (context) => {
+            const changes = textChanges.ChangeTracker.with(context, t => makeChange(t, context.sourceFile, context.span.start));
+            if (changes.length > 0) {
+                return [createCodeFixAction(fixId, changes, Diagnostics.Prefix_with_declare, fixId, Diagnostics.Prefix_all_incorrect_property_declarations_with_declare)];
+            }
+        },
+        fixIds: [fixId],
+        getAllCodeActions: context => {
+            const fixedNodes = new NodeSet();
+            return codeFixAll(context, errorCodes, (changes, diag) => makeChange(changes, diag.file, diag.start, fixedNodes));
+        },
+    });
+
+    function makeChange(changeTracker: textChanges.ChangeTracker, sourceFile: SourceFile, pos: number, fixedNodes?: NodeSet<Node>) {
+        const token = getTokenAtPosition(sourceFile, pos);
+        if (!isIdentifier(token)) {
+            return;
+        }
+        const declaration = token.parent;
+        if (declaration.kind === SyntaxKind.PropertyDeclaration &&
+            (!fixedNodes || fixedNodes.tryAdd(declaration))) {
+            changeTracker.insertModifierBefore(sourceFile, SyntaxKind.DeclareKeyword, declaration);
+        }
+    }
+}

src/services/services.ts

@@ -339,7 +339,6 @@ namespace ts {
     }
 
     class TokenObject<TKind extends SyntaxKind> extends TokenOrIdentifierObject implements Token<TKind> {
-        public symbol!: Symbol;
         public kind: TKind;
 
         constructor(kind: TKind, pos: number, end: number) {
@@ -349,9 +348,8 @@ namespace ts {
     }
 
     class IdentifierObject extends TokenOrIdentifierObject implements Identifier {
-        public kind!: SyntaxKind.Identifier;
+        public kind: SyntaxKind.Identifier = SyntaxKind.Identifier;
         public escapedText!: __String;
-        public symbol!: Symbol;
         public autoGenerateFlags!: GeneratedIdentifierFlags;
         _primaryExpressionBrand: any;
         _memberExpressionBrand: any;
@@ -541,7 +539,7 @@ namespace ts {
     }
 
     class SourceFileObject extends NodeObject implements SourceFile {
-        public kind!: SyntaxKind.SourceFile;
+        public kind: SyntaxKind.SourceFile = SyntaxKind.SourceFile;
         public _declarationBrand: any;
         public fileName!: string;
         public path!: Path;

src/services/tsconfig.json

@@ -47,6 +47,7 @@
         "codefixes/addConvertToUnknownForNonOverlappingTypes.ts",
         "codefixes/addMissingAwait.ts",
         "codefixes/addMissingConst.ts",
+        "codefixes/addMissingDeclareProperty.ts",
         "codefixes/addMissingInvocationForDecorator.ts",
         "codefixes/addNameToNamelessParameter.ts",
         "codefixes/annotateWithTypeFromJSDoc.ts",

tests/baselines/reference/apparentTypeSubtyping.errors.txt

@@ -1,9 +1,11 @@
 tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtyping.ts(10,5): error TS2416: Property 'x' in type 'Derived<U>' is not assignable to the same property in base type 'Base<string>'.
   Type 'String' is not assignable to type 'string'.
     'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.
+tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtyping.ts(10,5): error TS2610: Property 'x' will overwrite the base property in 'Base<string>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtyping.ts(20,5): error TS2610: Property 'x' will overwrite the base property in 'Base2'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtyping.ts (1 errors) ====
+==== tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtyping.ts (3 errors) ====
     // subtype checks use the apparent type of the target type
     // S is a subtype of a type T, and T is a supertype of S, if one of the following is true, where S' denotes the apparent type (section 3.8.1) of S:
 
@@ -18,6 +20,8 @@ tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtypi
 !!! error TS2416: Property 'x' in type 'Derived<U>' is not assignable to the same property in base type 'Base<string>'.
 !!! error TS2416:   Type 'String' is not assignable to type 'string'.
 !!! error TS2416:     'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Base<string>'. Add a 'declare' modifier or an initializer to avoid this.
     }
 
     class Base2 {
@@ -28,4 +32,6 @@ tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSubtypi
     // is U extends String (S) a subtype of String (T)? Apparent type of U is String so it succeeds
     class Derived2<U extends String> extends Base2 { // error because of the prototype's not matching, not because of the instance side
         x: U;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Base2'. Add a 'declare' modifier or an initializer to avoid this.
     }

tests/baselines/reference/apparentTypeSupertype.errors.txt

@@ -2,9 +2,10 @@ tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSuperty
   Type 'U' is not assignable to type 'string'.
     Type 'String' is not assignable to type 'string'.
       'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.
+tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSupertype.ts(10,5): error TS2610: Property 'x' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSupertype.ts (1 errors) ====
+==== tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSupertype.ts (2 errors) ====
     // subtype checks use the apparent type of the target type
     // S is a subtype of a type T, and T is a supertype of S, if one of the following is true, where S' denotes the apparent type (section 3.8.1) of S:
 
@@ -20,4 +21,6 @@ tests/cases/conformance/types/typeRelationships/apparentType/apparentTypeSuperty
 !!! error TS2416:   Type 'U' is not assignable to type 'string'.
 !!! error TS2416:     Type 'String' is not assignable to type 'string'.
 !!! error TS2416:       'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
     }

tests/baselines/reference/baseClassImprovedMismatchErrors.errors.txt

@@ -5,6 +5,7 @@ tests/cases/compiler/baseClassImprovedMismatchErrors.ts(8,5): error TS2416: Prop
         Types of property 'n' are incompatible.
           Type 'string | Derived' is not assignable to type 'string | Base'.
             Type 'Derived' is not assignable to type 'string | Base'.
+tests/cases/compiler/baseClassImprovedMismatchErrors.ts(8,5): error TS2610: Property 'n' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/compiler/baseClassImprovedMismatchErrors.ts(9,5): error TS2416: Property 'fn' in type 'Derived' is not assignable to the same property in base type 'Base'.
   Type '() => string | number' is not assignable to type '() => number'.
     Type 'string | number' is not assignable to type 'number'.
@@ -22,7 +23,7 @@ tests/cases/compiler/baseClassImprovedMismatchErrors.ts(15,5): error TS2416: Pro
       Type 'string' is not assignable to type 'number'.
 
 
-==== tests/cases/compiler/baseClassImprovedMismatchErrors.ts (4 errors) ====
+==== tests/cases/compiler/baseClassImprovedMismatchErrors.ts (5 errors) ====
     class Base {
         n: Base | string;
         fn() {
@@ -39,6 +40,8 @@ tests/cases/compiler/baseClassImprovedMismatchErrors.ts(15,5): error TS2416: Pro
 !!! error TS2416:         Types of property 'n' are incompatible.
 !!! error TS2416:           Type 'string | Derived' is not assignable to type 'string | Base'.
 !!! error TS2416:             Type 'Derived' is not assignable to type 'string | Base'.
+        ~
+!!! error TS2610: Property 'n' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
         fn() {
         ~~
 !!! error TS2416: Property 'fn' in type 'Derived' is not assignable to the same property in base type 'Base'.

tests/baselines/reference/classExpressionPropertyModifiers.errors.txt

@@ -1,12 +1,12 @@
-tests/cases/compiler/classExpressionPropertyModifiers.ts(2,5): error TS1031: 'declare' modifier cannot appear on a class element.
+tests/cases/compiler/classExpressionPropertyModifiers.ts(2,36): error TS1039: Initializers are not allowed in ambient contexts.
 tests/cases/compiler/classExpressionPropertyModifiers.ts(3,5): error TS1031: 'export' modifier cannot appear on a class element.
 
 
 ==== tests/cases/compiler/classExpressionPropertyModifiers.ts (2 errors) ====
     const a = class Cat {
         declare [Symbol.toStringTag] = "uh";
-        ~~~~~~~
-!!! error TS1031: 'declare' modifier cannot appear on a class element.
+                                       ~~~~
+!!! error TS1039: Initializers are not allowed in ambient contexts.
         export foo = 1;
         ~~~~~~
 !!! error TS1031: 'export' modifier cannot appear on a class element.

tests/baselines/reference/classIsSubtypeOfBaseType.errors.txt

@@ -1,15 +1,19 @@
+tests/cases/conformance/classes/classDeclarations/classHeritageSpecification/classIsSubtypeOfBaseType.ts(6,5): error TS2610: Property 'foo' will overwrite the base property in 'Base<{ bar: string; }>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/classDeclarations/classHeritageSpecification/classIsSubtypeOfBaseType.ts(12,5): error TS2416: Property 'foo' in type 'Derived2' is not assignable to the same property in base type 'Base<{ bar: string; }>'.
   Type '{ bar?: string; }' is not assignable to type '{ bar: string; }'.
     Property 'bar' is optional in type '{ bar?: string; }' but required in type '{ bar: string; }'.
+tests/cases/conformance/classes/classDeclarations/classHeritageSpecification/classIsSubtypeOfBaseType.ts(12,5): error TS2610: Property 'foo' will overwrite the base property in 'Base<{ bar: string; }>'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/classes/classDeclarations/classHeritageSpecification/classIsSubtypeOfBaseType.ts (1 errors) ====
+==== tests/cases/conformance/classes/classDeclarations/classHeritageSpecification/classIsSubtypeOfBaseType.ts (3 errors) ====
     class Base<T> {
         foo: T;
     }
 
     class Derived extends Base<{ bar: string; }> {
         foo: {
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<{ bar: string; }>'. Add a 'declare' modifier or an initializer to avoid this.
             bar: string; baz: number; // ok
         }
     }
@@ -20,6 +24,8 @@ tests/cases/conformance/classes/classDeclarations/classHeritageSpecification/cla
 !!! error TS2416: Property 'foo' in type 'Derived2' is not assignable to the same property in base type 'Base<{ bar: string; }>'.
 !!! error TS2416:   Type '{ bar?: string; }' is not assignable to type '{ bar: string; }'.
 !!! error TS2416:     Property 'bar' is optional in type '{ bar?: string; }' but required in type '{ bar: string; }'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<{ bar: string; }>'. Add a 'declare' modifier or an initializer to avoid this.
             bar?: string; // error
         }
     }