diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index f8c8817888591..30d671748c74c 100644
--- a/src/compiler/checker.ts
+++ b/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) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 44bba361bbc55..957e1911ec3fa 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/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",
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index f437ca697a854..669c41094a697 100644
--- a/src/compiler/parser.ts
+++ b/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) {
diff --git a/src/services/codefixes/addMissingDeclareProperty.ts b/src/services/codefixes/addMissingDeclareProperty.ts
new file mode 100644
index 0000000000000..002357239ead2
--- /dev/null
+++ b/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);
+        }
+    }
+}
diff --git a/src/services/services.ts b/src/services/services.ts
index 644629ce2af4f..ebf63d246a6e2 100644
--- a/src/services/services.ts
+++ b/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;
diff --git a/src/services/tsconfig.json b/src/services/tsconfig.json
index b90607b0a85a3..5c0e39f9f6564 100644
--- a/src/services/tsconfig.json
+++ b/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",
diff --git a/tests/baselines/reference/apparentTypeSubtyping.errors.txt b/tests/baselines/reference/apparentTypeSubtyping.errors.txt
index ca2f0765d6211..9f43225eb1594 100644
--- a/tests/baselines/reference/apparentTypeSubtyping.errors.txt
+++ b/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.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/apparentTypeSupertype.errors.txt b/tests/baselines/reference/apparentTypeSupertype.errors.txt
index a4a8ccc0a8bdd..1dde3b891f812 100644
--- a/tests/baselines/reference/apparentTypeSupertype.errors.txt
+++ b/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.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/baseClassImprovedMismatchErrors.errors.txt b/tests/baselines/reference/baseClassImprovedMismatchErrors.errors.txt
index 5f46ee70d2aaa..4bdf83b31c789 100644
--- a/tests/baselines/reference/baseClassImprovedMismatchErrors.errors.txt
+++ b/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'.
diff --git a/tests/baselines/reference/classExpressionPropertyModifiers.errors.txt b/tests/baselines/reference/classExpressionPropertyModifiers.errors.txt
index f4b3fed7e5458..cb01d6561260f 100644
--- a/tests/baselines/reference/classExpressionPropertyModifiers.errors.txt
+++ b/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.
diff --git a/tests/baselines/reference/classIsSubtypeOfBaseType.errors.txt b/tests/baselines/reference/classIsSubtypeOfBaseType.errors.txt
index 8b84602b807c5..f8369af4ad403 100644
--- a/tests/baselines/reference/classIsSubtypeOfBaseType.errors.txt
+++ b/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
         }
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/commentsInheritance.errors.txt b/tests/baselines/reference/commentsInheritance.errors.txt
new file mode 100644
index 0000000000000..2a9a5eff5dca7
--- /dev/null
+++ b/tests/baselines/reference/commentsInheritance.errors.txt
@@ -0,0 +1,159 @@
+tests/cases/compiler/commentsInheritance.ts(90,12): error TS2610: Property 'p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/compiler/commentsInheritance.ts(98,12): error TS2610: Property 'nc_p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/commentsInheritance.ts (2 errors) ====
+    /** i1 is interface with properties*/
+    interface i1 {
+        /** i1_p1*/
+        i1_p1: number;
+        /** i1_f1*/
+        i1_f1(): void;
+        /** i1_l1*/
+        i1_l1: () => void;
+        // il_nc_p1
+        i1_nc_p1: number;
+        i1_nc_f1(): void;
+        i1_nc_l1: () => void;
+        p1: number;
+        f1(): void;
+        l1: () => void;
+        nc_p1: number;
+        nc_f1(): void;
+        nc_l1: () => void;
+    }
+    class c1 implements i1 {
+        public i1_p1: number;
+        // i1_f1
+        public i1_f1() {
+        }
+        public i1_l1: () => void;
+        public i1_nc_p1: number;
+        public i1_nc_f1() {
+        }
+        public i1_nc_l1: () => void;
+        /** c1_p1*/
+        public p1: number;
+        /** c1_f1*/
+        public f1() {
+        }
+        /** c1_l1*/
+        public l1: () => void;
+        /** c1_nc_p1*/
+        public nc_p1: number;
+        /** c1_nc_f1*/
+        public nc_f1() {
+        }
+        /** c1_nc_l1*/
+        public nc_l1: () => void;
+    }
+    var i1_i: i1;
+    var c1_i = new c1();
+    // assign to interface
+    i1_i = c1_i;
+    class c2 {
+        /** c2 c2_p1*/
+        public c2_p1: number;
+        /** c2 c2_f1*/
+        public c2_f1() {
+        }
+        /** c2 c2_prop*/
+        public get c2_prop() {
+            return 10;
+        }
+        public c2_nc_p1: number;
+        public c2_nc_f1() {
+        }
+        public get c2_nc_prop() {
+            return 10;
+        }
+        /** c2 p1*/
+        public p1: number;
+        /** c2 f1*/
+        public f1() {
+        }
+        /** c2 prop*/
+        public get prop() {
+            return 10;
+        }
+        public nc_p1: number;
+        public nc_f1() {
+        }
+        public get nc_prop() {
+            return 10;
+        }
+        /** c2 constructor*/
+        constructor(a: number) {
+            this.c2_p1 = a;
+        }
+    }
+    class c3 extends c2 {
+        constructor() {
+            super(10);
+        }
+        /** c3 p1*/
+        public p1: number;
+               ~~
+!!! error TS2610: Property 'p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
+        /** c3 f1*/
+        public f1() {
+        }
+        /** c3 prop*/
+        public get prop() {
+            return 10;
+        }
+        public nc_p1: number;
+               ~~~~~
+!!! error TS2610: Property 'nc_p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
+        public nc_f1() {
+        }
+        public get nc_prop() {
+            return 10;
+        }
+    }
+    var c2_i = new c2(10);
+    var c3_i = new c3();
+    // assign
+    c2_i = c3_i;
+    class c4 extends c2 {
+    }
+    var c4_i = new c4(10);
+    interface i2 {
+        /** i2_p1*/
+        i2_p1: number;
+        /** i2_f1*/
+        i2_f1(): void;
+        /** i2_l1*/
+        i2_l1: () => void;
+        // i2_nc_p1
+        i2_nc_p1: number;
+        i2_nc_f1(): void;
+        i2_nc_l1: () => void;
+        /** i2 p1*/
+        p1: number;
+        /** i2 f1*/
+        f1(): void;
+        /** i2 l1*/
+        l1: () => void;
+        nc_p1: number;
+        nc_f1(): void;
+        nc_l1: () => void;
+    }
+    interface i3 extends i2 {
+        /** i3 p1 */
+        p1: number;
+        /**
+        * i3 f1
+        */
+        f1(): void;
+        /** i3 l1*/
+        l1: () => void;
+        nc_p1: number;
+        nc_f1(): void;
+        nc_l1: () => void;
+    }
+    var i2_i: i2;
+    var i3_i: i3;
+    // assign to interface
+    i2_i = i3_i;
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/declarationEmitProtectedMembers.errors.txt b/tests/baselines/reference/declarationEmitProtectedMembers.errors.txt
new file mode 100644
index 0000000000000..5adfa76f4f627
--- /dev/null
+++ b/tests/baselines/reference/declarationEmitProtectedMembers.errors.txt
@@ -0,0 +1,55 @@
+tests/cases/compiler/declarationEmitProtectedMembers.ts(34,5): error TS2610: Property 'x' will overwrite the base property in 'C2'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/declarationEmitProtectedMembers.ts (1 errors) ====
+    // Class with protected members
+    class C1 {
+        protected x: number;
+    
+        protected f() {
+            return this.x;
+        }
+    
+        protected set accessor(a: number) { }
+        protected get accessor() { return 0; }
+    
+        protected static sx: number;
+    
+        protected static sf() {
+            return this.sx;
+        }
+    
+        protected static set staticSetter(a: number) { }
+        protected static get staticGetter() { return 0; }
+    }
+    
+    // Derived class overriding protected members
+    class C2 extends C1 {
+        protected f() {
+            return super.f() + this.x;
+        }
+        protected static sf() {
+            return super.sf() + this.sx;
+        }
+    }
+    
+    // Derived class making protected members public
+    class C3 extends C2 {
+        x: number;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'C2'. Add a 'declare' modifier or an initializer to avoid this.
+        static sx: number;
+        f() {
+            return super.f();
+        }
+        static sf() {
+            return super.sf();
+        }
+    
+        static get staticGetter() { return 1; }
+    }
+    
+    // Protected properties in constructors
+    class C4 {
+        constructor(protected a: number, protected b) { }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/derivedClassOverridesProtectedMembers.errors.txt b/tests/baselines/reference/derivedClassOverridesProtectedMembers.errors.txt
new file mode 100644
index 0000000000000..a7c03264131bb
--- /dev/null
+++ b/tests/baselines/reference/derivedClassOverridesProtectedMembers.errors.txt
@@ -0,0 +1,44 @@
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers.ts(21,15): error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers.ts(25,15): error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers.ts (2 errors) ====
+    var x: { foo: string; }
+    var y: { foo: string; bar: string; }
+    
+    class Base {
+        protected a: typeof x;
+        protected b(a: typeof x) { }
+        protected get c() { return x; }
+        protected set c(v: typeof x) { }
+        protected d: (a: typeof x) => void;
+    
+        protected static r: typeof x;
+        protected static s(a: typeof x) { }
+        protected static get t() { return x; }
+        protected static set t(v: typeof x) { }
+        protected static u: (a: typeof x) => void;
+    
+        constructor(a: typeof x) { }
+    }
+    
+    class Derived extends Base {
+        protected a: typeof y;
+                  ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+        protected b(a: typeof y) { }
+        protected get c() { return y; }
+        protected set c(v: typeof y) { }
+        protected d: (a: typeof y) => void;
+                  ~
+!!! error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    
+        protected static r: typeof y;
+        protected static s(a: typeof y) { }
+        protected static get t() { return y; }
+        protected static set t(a: typeof y) { }
+        protected static u: (a: typeof y) => void;
+    
+        constructor(a: typeof y) { super(x) }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/derivedClassOverridesProtectedMembers2.errors.txt b/tests/baselines/reference/derivedClassOverridesProtectedMembers2.errors.txt
new file mode 100644
index 0000000000000..816f3b4df5d25
--- /dev/null
+++ b/tests/baselines/reference/derivedClassOverridesProtectedMembers2.errors.txt
@@ -0,0 +1,72 @@
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers2.ts(22,5): error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers2.ts(26,5): error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers2.ts (2 errors) ====
+    var x: { foo: string; }
+    var y: { foo: string; bar: string; }
+    
+    class Base {
+        protected a: typeof x;
+        protected b(a: typeof x) { }
+        protected get c() { return x; }
+        protected set c(v: typeof x) { }
+        protected d: (a: typeof x) => void ;
+    
+        protected static r: typeof x;
+        protected static s(a: typeof x) { }
+        protected static get t() { return x; }
+        protected static set t(v: typeof x) { }
+        protected static u: (a: typeof x) => void ;
+    
+    constructor(a: typeof x) { }
+    }
+    
+    // Increase visibility of all protected members to public
+    class Derived extends Base {
+        a: typeof y;
+        ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+        b(a: typeof y) { }
+        get c() { return y; }
+        set c(v: typeof y) { }
+        d: (a: typeof y) => void;
+        ~
+!!! error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    
+        static r: typeof y;
+        static s(a: typeof y) { }
+        static get t() { return y; }
+        static set t(a: typeof y) { }
+        static u: (a: typeof y) => void;
+    
+        constructor(a: typeof y) { super(a); }
+    }
+    
+    var d: Derived = new Derived(y);
+    var r1 = d.a;
+    var r2 = d.b(y);
+    var r3 = d.c;
+    var r3a = d.d;
+    d.c = y;
+    var r4 = Derived.r;
+    var r5 = Derived.s(y);
+    var r6 = Derived.t;
+    var r6a = Derived.u;
+    Derived.t = y;
+    
+    class Base2 {
+        [i: string]: Object;
+        [i: number]: typeof x;
+    }
+    
+    class Derived2 extends Base2 {
+        [i: string]: typeof x;
+        [i: number]: typeof y;
+    }
+    
+    var d2: Derived2;
+    var r7 = d2[''];
+    var r8 = d2[1];
+    
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/derivedClassOverridesProtectedMembers3.errors.txt b/tests/baselines/reference/derivedClassOverridesProtectedMembers3.errors.txt
index faf228e8d55c2..1be0509e6524d 100644
--- a/tests/baselines/reference/derivedClassOverridesProtectedMembers3.errors.txt
+++ b/tests/baselines/reference/derivedClassOverridesProtectedMembers3.errors.txt
@@ -1,5 +1,6 @@
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(22,7): error TS2415: Class 'Derived1' incorrectly extends base class 'Base'.
   Property 'a' is protected in type 'Derived1' but public in type 'Base'.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(23,15): error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(27,7): error TS2415: Class 'Derived2' incorrectly extends base class 'Base'.
   Property 'b' is protected in type 'Derived2' but public in type 'Base'.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(32,7): error TS2415: Class 'Derived3' incorrectly extends base class 'Base'.
@@ -8,6 +9,7 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
   Property 'c' is protected in type 'Derived4' but public in type 'Base'.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(42,7): error TS2415: Class 'Derived5' incorrectly extends base class 'Base'.
   Property 'd' is protected in type 'Derived5' but public in type 'Base'.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(43,15): error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(47,7): error TS2417: Class static side 'typeof Derived6' incorrectly extends base class static side 'typeof Base'.
   Property 'r' is protected in type 'typeof Derived6' but public in type 'typeof Base'.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts(52,7): error TS2417: Class static side 'typeof Derived7' incorrectly extends base class static side 'typeof Base'.
@@ -20,7 +22,7 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
   Property 'u' is protected in type 'typeof Derived10' but public in type 'typeof Base'.
 
 
-==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts (10 errors) ====
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers3.ts (12 errors) ====
     var x: { foo: string; }
     var y: { foo: string; bar: string; }
     
@@ -47,6 +49,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
 !!! error TS2415: Class 'Derived1' incorrectly extends base class 'Base'.
 !!! error TS2415:   Property 'a' is protected in type 'Derived1' but public in type 'Base'.
         protected a: typeof x;
+                  ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
         constructor(a: typeof x) { super(a); }
     }
     
@@ -79,6 +83,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
 !!! error TS2415: Class 'Derived5' incorrectly extends base class 'Base'.
 !!! error TS2415:   Property 'd' is protected in type 'Derived5' but public in type 'Base'.
         protected d: (a: typeof x) => void ;
+                  ~
+!!! error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
         constructor(a: typeof x) { super(a); }
     }
     
diff --git a/tests/baselines/reference/derivedClassOverridesProtectedMembers4.errors.txt b/tests/baselines/reference/derivedClassOverridesProtectedMembers4.errors.txt
index 064e2e8d43c2f..2d11066559826 100644
--- a/tests/baselines/reference/derivedClassOverridesProtectedMembers4.errors.txt
+++ b/tests/baselines/reference/derivedClassOverridesProtectedMembers4.errors.txt
@@ -1,8 +1,10 @@
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers4.ts(9,12): error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers4.ts(12,7): error TS2415: Class 'Derived2' incorrectly extends base class 'Derived1'.
   Property 'a' is protected in type 'Derived2' but public in type 'Derived1'.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers4.ts(13,15): error TS2610: Property 'a' will overwrite the base property in 'Derived1'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers4.ts (1 errors) ====
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesProtectedMembers4.ts (3 errors) ====
     var x: { foo: string; }
     var y: { foo: string; bar: string; }
     
@@ -12,6 +14,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
     
     class Derived1 extends Base {
         public a: typeof x;
+               ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class Derived2 extends Derived1 {
@@ -19,4 +23,6 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
 !!! error TS2415: Class 'Derived2' incorrectly extends base class 'Derived1'.
 !!! error TS2415:   Property 'a' is protected in type 'Derived2' but public in type 'Derived1'.
         protected a: typeof x; // Error, parent was public
+                  ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'Derived1'. Add a 'declare' modifier or an initializer to avoid this.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/derivedClassOverridesPublicMembers.errors.txt b/tests/baselines/reference/derivedClassOverridesPublicMembers.errors.txt
index 9292ff64411d5..45511c1a2c013 100644
--- a/tests/baselines/reference/derivedClassOverridesPublicMembers.errors.txt
+++ b/tests/baselines/reference/derivedClassOverridesPublicMembers.errors.txt
@@ -2,13 +2,15 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(8,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(13,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(14,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(21,5): error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(23,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(24,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(25,5): error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(29,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts(30,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 
 
-==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts (8 errors) ====
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesPublicMembers.ts (10 errors) ====
     var x: { foo: string; }
     var y: { foo: string; bar: string; }
     
@@ -38,6 +40,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
     
     class Derived extends Base {
         a: typeof y;
+        ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
         b(a: typeof y) { }
         get c() { return y; }
             ~
@@ -46,6 +50,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOve
             ~
 !!! error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
         d: (a: typeof y) => void;
+        ~
+!!! error TS2610: Property 'd' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
     
         static r: typeof y;
         static s(a: typeof y) { }
diff --git a/tests/baselines/reference/derivedClassOverridesWithoutSubtype.errors.txt b/tests/baselines/reference/derivedClassOverridesWithoutSubtype.errors.txt
new file mode 100644
index 0000000000000..8f65f436dcac8
--- /dev/null
+++ b/tests/baselines/reference/derivedClassOverridesWithoutSubtype.errors.txt
@@ -0,0 +1,29 @@
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesWithoutSubtype.ts(8,5): error TS2610: Property 'x' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassOverridesWithoutSubtype.ts (1 errors) ====
+    class Base {
+        x: {
+            foo: string;
+        }
+    }
+    
+    class Derived extends Base {
+        x: {
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+            foo: any;
+        }
+    }
+    
+    class Base2 {
+        static y: {
+            foo: string;
+        }
+    }
+    
+    class Derived2 extends Base2 {
+        static y: {
+            foo: any;
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/derivedClassWithAny.errors.txt b/tests/baselines/reference/derivedClassWithAny.errors.txt
index aa91979706b51..5f8a219b58eb2 100644
--- a/tests/baselines/reference/derivedClassWithAny.errors.txt
+++ b/tests/baselines/reference/derivedClassWithAny.errors.txt
@@ -1,7 +1,9 @@
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(3,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(9,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(18,5): error TS2610: Property 'x' will overwrite the base property in 'C'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(19,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(27,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(37,5): error TS2610: Property 'x' will overwrite the base property in 'D'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(38,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(44,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts(57,1): error TS2322: Type 'E' is not assignable to type 'C'.
@@ -9,7 +11,7 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWit
     Type 'string' is not assignable to type 'number'.
 
 
-==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts (7 errors) ====
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWithAny.ts (9 errors) ====
     class C {
         x: number;
         get X(): number { return 1; }
@@ -32,6 +34,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWit
     
     class D extends C {
         x: any;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'C'. Add a 'declare' modifier or an initializer to avoid this.
         get X(): any {
             ~
 !!! error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
@@ -55,6 +59,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedClassWit
     // if D is a valid class definition than E is now not safe tranisitively through C
     class E extends D {
         x: string;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'D'. Add a 'declare' modifier or an initializer to avoid this.
         get X(): string{ return ''; }
             ~
 !!! error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
diff --git a/tests/baselines/reference/derivedGenericClassWithAny.errors.txt b/tests/baselines/reference/derivedGenericClassWithAny.errors.txt
index e63849e4aa887..6d32d3f814739 100644
--- a/tests/baselines/reference/derivedGenericClassWithAny.errors.txt
+++ b/tests/baselines/reference/derivedGenericClassWithAny.errors.txt
@@ -1,6 +1,8 @@
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(3,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(10,5): error TS2610: Property 'x' will overwrite the base property in 'C<number>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(11,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(19,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(29,5): error TS2610: Property 'x' will overwrite the base property in 'D'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(30,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts(30,18): error TS2322: Type '""' is not assignable to type 'T'.
   '""' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
@@ -11,7 +13,7 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericC
     Type 'string' is not assignable to type 'number'.
 
 
-==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts (7 errors) ====
+==== tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericClassWithAny.ts (9 errors) ====
     class C<T extends number> {
         x: T;
         get X(): T { return null; }
@@ -24,6 +26,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericC
     
     class D extends C<number> {
         x: any;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'C<number>'. Add a 'declare' modifier or an initializer to avoid this.
         get X(): any {
             ~
 !!! error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
@@ -47,6 +51,8 @@ tests/cases/conformance/classes/members/inheritanceAndOverriding/derivedGenericC
     // if D is a valid class definition than E is now not safe tranisitively through C
     class E<T extends string> extends D {
         x: T;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'D'. Add a 'declare' modifier or an initializer to avoid this.
         get X(): T { return ''; } // error
             ~
 !!! error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
diff --git a/tests/baselines/reference/derivedUninitializedPropertyDeclaration.errors.txt b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.errors.txt
new file mode 100644
index 0000000000000..0c0fda3415616
--- /dev/null
+++ b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.errors.txt
@@ -0,0 +1,100 @@
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(6,5): error TS2610: Property 'property' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(12,21): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(15,5): error TS1031: 'declare' modifier cannot appear on a class element.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(15,17): error TS1183: An implementation cannot be declared in ambient contexts.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(17,24): error TS1039: Initializers are not allowed in ambient contexts.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(24,5): error TS2564: Property 'p' has no initializer and is not definitely assigned in the constructor.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(27,5): error TS2564: Property 'p' has no initializer and is not definitely assigned in the constructor.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(27,5): error TS2610: Property 'p' will overwrite the base property in 'C'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(39,5): error TS2610: Property 'p1' will overwrite the base property in 'E'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts(65,5): error TS2610: Property 'r' will overwrite the base property in 'J'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts (10 errors) ====
+    class A {
+        property = 'x';
+        m() { return 1 }
+    }
+    class B extends A {
+        property: any; // error
+        ~~~~~~~~
+!!! error TS2610: Property 'property' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    class BD extends A {
+        declare property: any; // ok because it's implicitly initialised
+    }
+    class BDBang extends A {
+        declare property!: any; // ! is not allowed, this is an ambient declaration
+                        ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+    }
+    class BOther extends A {
+        declare m() { return 2 } // not allowed on methods
+        ~~~~~~~
+!!! error TS1031: 'declare' modifier cannot appear on a class element.
+                    ~
+!!! error TS1183: An implementation cannot be declared in ambient contexts.
+        declare nonce: any; // ok, even though it's not in the base
+        declare property = 'y' // initialiser not allowed with declare
+                           ~~~
+!!! error TS1039: Initializers are not allowed in ambient contexts.
+    }
+    class U {
+        declare nonce: any; // ok, even though there's no base
+    }
+    
+    class C {
+        p: string;
+        ~
+!!! error TS2564: Property 'p' has no initializer and is not definitely assigned in the constructor.
+    }
+    class D extends C {
+        p: 'hi'; // error
+        ~
+!!! error TS2564: Property 'p' has no initializer and is not definitely assigned in the constructor.
+        ~
+!!! error TS2610: Property 'p' will overwrite the base property in 'C'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    class DD extends C {
+        declare p: 'bye'; // ok
+    }
+    
+    
+    declare class E {
+        p1: string
+        p2: string
+    }
+    class F extends E {
+        p1!: 'z'
+        ~~
+!!! error TS2610: Property 'p1' will overwrite the base property in 'E'. Add a 'declare' modifier or an initializer to avoid this.
+        declare p2: 'alpha'
+    }
+    
+    class G extends E {
+        p1: 'z'
+        constructor() {
+            super()
+            this.p1 = 'z'
+        }
+    }
+    
+    abstract class H extends E {
+        abstract p1: 'a' | 'b' | 'c'
+        declare abstract p2: 'a' | 'b' | 'c'
+    }
+    
+    interface I {
+        q: number
+    }
+    interface J extends I { }
+    class J {
+        r = 5
+    }
+    class K extends J {
+        q!: 1 | 2 | 3 // ok, extends a property from an interface
+        r!: 4 | 5 // error, from class
+        ~
+!!! error TS2610: Property 'r' will overwrite the base property in 'J'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/derivedUninitializedPropertyDeclaration.js b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.js
new file mode 100644
index 0000000000000..471cd5bbf1d18
--- /dev/null
+++ b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.js
@@ -0,0 +1,182 @@
+//// [derivedUninitializedPropertyDeclaration.ts]
+class A {
+    property = 'x';
+    m() { return 1 }
+}
+class B extends A {
+    property: any; // error
+}
+class BD extends A {
+    declare property: any; // ok because it's implicitly initialised
+}
+class BDBang extends A {
+    declare property!: any; // ! is not allowed, this is an ambient declaration
+}
+class BOther extends A {
+    declare m() { return 2 } // not allowed on methods
+    declare nonce: any; // ok, even though it's not in the base
+    declare property = 'y' // initialiser not allowed with declare
+}
+class U {
+    declare nonce: any; // ok, even though there's no base
+}
+
+class C {
+    p: string;
+}
+class D extends C {
+    p: 'hi'; // error
+}
+class DD extends C {
+    declare p: 'bye'; // ok
+}
+
+
+declare class E {
+    p1: string
+    p2: string
+}
+class F extends E {
+    p1!: 'z'
+    declare p2: 'alpha'
+}
+
+class G extends E {
+    p1: 'z'
+    constructor() {
+        super()
+        this.p1 = 'z'
+    }
+}
+
+abstract class H extends E {
+    abstract p1: 'a' | 'b' | 'c'
+    declare abstract p2: 'a' | 'b' | 'c'
+}
+
+interface I {
+    q: number
+}
+interface J extends I { }
+class J {
+    r = 5
+}
+class K extends J {
+    q!: 1 | 2 | 3 // ok, extends a property from an interface
+    r!: 4 | 5 // error, from class
+}
+
+
+//// [derivedUninitializedPropertyDeclaration.js]
+"use strict";
+var __extends = (this && this.__extends) || (function () {
+    var extendStatics = function (d, b) {
+        extendStatics = Object.setPrototypeOf ||
+            ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+            function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+        return extendStatics(d, b);
+    };
+    return function (d, b) {
+        extendStatics(d, b);
+        function __() { this.constructor = d; }
+        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    };
+})();
+var A = /** @class */ (function () {
+    function A() {
+        this.property = 'x';
+    }
+    A.prototype.m = function () { return 1; };
+    return A;
+}());
+var B = /** @class */ (function (_super) {
+    __extends(B, _super);
+    function B() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return B;
+}(A));
+var BD = /** @class */ (function (_super) {
+    __extends(BD, _super);
+    function BD() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return BD;
+}(A));
+var BDBang = /** @class */ (function (_super) {
+    __extends(BDBang, _super);
+    function BDBang() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return BDBang;
+}(A));
+var BOther = /** @class */ (function (_super) {
+    __extends(BOther, _super);
+    function BOther() {
+        var _this = _super !== null && _super.apply(this, arguments) || this;
+        _this.property = 'y'; // initialiser not allowed with declare
+        return _this;
+    }
+    BOther.prototype.m = function () { return 2; }; // not allowed on methods
+    return BOther;
+}(A));
+var U = /** @class */ (function () {
+    function U() {
+    }
+    return U;
+}());
+var C = /** @class */ (function () {
+    function C() {
+    }
+    return C;
+}());
+var D = /** @class */ (function (_super) {
+    __extends(D, _super);
+    function D() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return D;
+}(C));
+var DD = /** @class */ (function (_super) {
+    __extends(DD, _super);
+    function DD() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return DD;
+}(C));
+var F = /** @class */ (function (_super) {
+    __extends(F, _super);
+    function F() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return F;
+}(E));
+var G = /** @class */ (function (_super) {
+    __extends(G, _super);
+    function G() {
+        var _this = _super.call(this) || this;
+        _this.p1 = 'z';
+        return _this;
+    }
+    return G;
+}(E));
+var H = /** @class */ (function (_super) {
+    __extends(H, _super);
+    function H() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return H;
+}(E));
+var J = /** @class */ (function () {
+    function J() {
+        this.r = 5;
+    }
+    return J;
+}());
+var K = /** @class */ (function (_super) {
+    __extends(K, _super);
+    function K() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return K;
+}(J));
diff --git a/tests/baselines/reference/derivedUninitializedPropertyDeclaration.symbols b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.symbols
new file mode 100644
index 0000000000000..87e912887042a
--- /dev/null
+++ b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.symbols
@@ -0,0 +1,149 @@
+=== tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts ===
+class A {
+>A : Symbol(A, Decl(derivedUninitializedPropertyDeclaration.ts, 0, 0))
+
+    property = 'x';
+>property : Symbol(A.property, Decl(derivedUninitializedPropertyDeclaration.ts, 0, 9))
+
+    m() { return 1 }
+>m : Symbol(A.m, Decl(derivedUninitializedPropertyDeclaration.ts, 1, 19))
+}
+class B extends A {
+>B : Symbol(B, Decl(derivedUninitializedPropertyDeclaration.ts, 3, 1))
+>A : Symbol(A, Decl(derivedUninitializedPropertyDeclaration.ts, 0, 0))
+
+    property: any; // error
+>property : Symbol(B.property, Decl(derivedUninitializedPropertyDeclaration.ts, 4, 19))
+}
+class BD extends A {
+>BD : Symbol(BD, Decl(derivedUninitializedPropertyDeclaration.ts, 6, 1))
+>A : Symbol(A, Decl(derivedUninitializedPropertyDeclaration.ts, 0, 0))
+
+    declare property: any; // ok because it's implicitly initialised
+>property : Symbol(BD.property, Decl(derivedUninitializedPropertyDeclaration.ts, 7, 20))
+}
+class BDBang extends A {
+>BDBang : Symbol(BDBang, Decl(derivedUninitializedPropertyDeclaration.ts, 9, 1))
+>A : Symbol(A, Decl(derivedUninitializedPropertyDeclaration.ts, 0, 0))
+
+    declare property!: any; // ! is not allowed, this is an ambient declaration
+>property : Symbol(BDBang.property, Decl(derivedUninitializedPropertyDeclaration.ts, 10, 24))
+}
+class BOther extends A {
+>BOther : Symbol(BOther, Decl(derivedUninitializedPropertyDeclaration.ts, 12, 1))
+>A : Symbol(A, Decl(derivedUninitializedPropertyDeclaration.ts, 0, 0))
+
+    declare m() { return 2 } // not allowed on methods
+>m : Symbol(BOther.m, Decl(derivedUninitializedPropertyDeclaration.ts, 13, 24))
+
+    declare nonce: any; // ok, even though it's not in the base
+>nonce : Symbol(BOther.nonce, Decl(derivedUninitializedPropertyDeclaration.ts, 14, 28))
+
+    declare property = 'y' // initialiser not allowed with declare
+>property : Symbol(BOther.property, Decl(derivedUninitializedPropertyDeclaration.ts, 15, 23))
+}
+class U {
+>U : Symbol(U, Decl(derivedUninitializedPropertyDeclaration.ts, 17, 1))
+
+    declare nonce: any; // ok, even though there's no base
+>nonce : Symbol(U.nonce, Decl(derivedUninitializedPropertyDeclaration.ts, 18, 9))
+}
+
+class C {
+>C : Symbol(C, Decl(derivedUninitializedPropertyDeclaration.ts, 20, 1))
+
+    p: string;
+>p : Symbol(C.p, Decl(derivedUninitializedPropertyDeclaration.ts, 22, 9))
+}
+class D extends C {
+>D : Symbol(D, Decl(derivedUninitializedPropertyDeclaration.ts, 24, 1))
+>C : Symbol(C, Decl(derivedUninitializedPropertyDeclaration.ts, 20, 1))
+
+    p: 'hi'; // error
+>p : Symbol(D.p, Decl(derivedUninitializedPropertyDeclaration.ts, 25, 19))
+}
+class DD extends C {
+>DD : Symbol(DD, Decl(derivedUninitializedPropertyDeclaration.ts, 27, 1))
+>C : Symbol(C, Decl(derivedUninitializedPropertyDeclaration.ts, 20, 1))
+
+    declare p: 'bye'; // ok
+>p : Symbol(DD.p, Decl(derivedUninitializedPropertyDeclaration.ts, 28, 20))
+}
+
+
+declare class E {
+>E : Symbol(E, Decl(derivedUninitializedPropertyDeclaration.ts, 30, 1))
+
+    p1: string
+>p1 : Symbol(E.p1, Decl(derivedUninitializedPropertyDeclaration.ts, 33, 17))
+
+    p2: string
+>p2 : Symbol(E.p2, Decl(derivedUninitializedPropertyDeclaration.ts, 34, 14))
+}
+class F extends E {
+>F : Symbol(F, Decl(derivedUninitializedPropertyDeclaration.ts, 36, 1))
+>E : Symbol(E, Decl(derivedUninitializedPropertyDeclaration.ts, 30, 1))
+
+    p1!: 'z'
+>p1 : Symbol(F.p1, Decl(derivedUninitializedPropertyDeclaration.ts, 37, 19))
+
+    declare p2: 'alpha'
+>p2 : Symbol(F.p2, Decl(derivedUninitializedPropertyDeclaration.ts, 38, 12))
+}
+
+class G extends E {
+>G : Symbol(G, Decl(derivedUninitializedPropertyDeclaration.ts, 40, 1))
+>E : Symbol(E, Decl(derivedUninitializedPropertyDeclaration.ts, 30, 1))
+
+    p1: 'z'
+>p1 : Symbol(G.p1, Decl(derivedUninitializedPropertyDeclaration.ts, 42, 19))
+
+    constructor() {
+        super()
+>super : Symbol(E, Decl(derivedUninitializedPropertyDeclaration.ts, 30, 1))
+
+        this.p1 = 'z'
+>this.p1 : Symbol(G.p1, Decl(derivedUninitializedPropertyDeclaration.ts, 42, 19))
+>this : Symbol(G, Decl(derivedUninitializedPropertyDeclaration.ts, 40, 1))
+>p1 : Symbol(G.p1, Decl(derivedUninitializedPropertyDeclaration.ts, 42, 19))
+    }
+}
+
+abstract class H extends E {
+>H : Symbol(H, Decl(derivedUninitializedPropertyDeclaration.ts, 48, 1))
+>E : Symbol(E, Decl(derivedUninitializedPropertyDeclaration.ts, 30, 1))
+
+    abstract p1: 'a' | 'b' | 'c'
+>p1 : Symbol(H.p1, Decl(derivedUninitializedPropertyDeclaration.ts, 50, 28))
+
+    declare abstract p2: 'a' | 'b' | 'c'
+>p2 : Symbol(H.p2, Decl(derivedUninitializedPropertyDeclaration.ts, 51, 32))
+}
+
+interface I {
+>I : Symbol(I, Decl(derivedUninitializedPropertyDeclaration.ts, 53, 1))
+
+    q: number
+>q : Symbol(I.q, Decl(derivedUninitializedPropertyDeclaration.ts, 55, 13))
+}
+interface J extends I { }
+>J : Symbol(J, Decl(derivedUninitializedPropertyDeclaration.ts, 57, 1), Decl(derivedUninitializedPropertyDeclaration.ts, 58, 25))
+>I : Symbol(I, Decl(derivedUninitializedPropertyDeclaration.ts, 53, 1))
+
+class J {
+>J : Symbol(J, Decl(derivedUninitializedPropertyDeclaration.ts, 57, 1), Decl(derivedUninitializedPropertyDeclaration.ts, 58, 25))
+
+    r = 5
+>r : Symbol(J.r, Decl(derivedUninitializedPropertyDeclaration.ts, 59, 9))
+}
+class K extends J {
+>K : Symbol(K, Decl(derivedUninitializedPropertyDeclaration.ts, 61, 1))
+>J : Symbol(J, Decl(derivedUninitializedPropertyDeclaration.ts, 57, 1), Decl(derivedUninitializedPropertyDeclaration.ts, 58, 25))
+
+    q!: 1 | 2 | 3 // ok, extends a property from an interface
+>q : Symbol(K.q, Decl(derivedUninitializedPropertyDeclaration.ts, 62, 19))
+
+    r!: 4 | 5 // error, from class
+>r : Symbol(K.r, Decl(derivedUninitializedPropertyDeclaration.ts, 63, 17))
+}
+
diff --git a/tests/baselines/reference/derivedUninitializedPropertyDeclaration.types b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.types
new file mode 100644
index 0000000000000..d9c96f0c9fc07
--- /dev/null
+++ b/tests/baselines/reference/derivedUninitializedPropertyDeclaration.types
@@ -0,0 +1,152 @@
+=== tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts ===
+class A {
+>A : A
+
+    property = 'x';
+>property : string
+>'x' : "x"
+
+    m() { return 1 }
+>m : () => number
+>1 : 1
+}
+class B extends A {
+>B : B
+>A : A
+
+    property: any; // error
+>property : any
+}
+class BD extends A {
+>BD : BD
+>A : A
+
+    declare property: any; // ok because it's implicitly initialised
+>property : any
+}
+class BDBang extends A {
+>BDBang : BDBang
+>A : A
+
+    declare property!: any; // ! is not allowed, this is an ambient declaration
+>property : any
+}
+class BOther extends A {
+>BOther : BOther
+>A : A
+
+    declare m() { return 2 } // not allowed on methods
+>m : () => number
+>2 : 2
+
+    declare nonce: any; // ok, even though it's not in the base
+>nonce : any
+
+    declare property = 'y' // initialiser not allowed with declare
+>property : string
+>'y' : "y"
+}
+class U {
+>U : U
+
+    declare nonce: any; // ok, even though there's no base
+>nonce : any
+}
+
+class C {
+>C : C
+
+    p: string;
+>p : string
+}
+class D extends C {
+>D : D
+>C : C
+
+    p: 'hi'; // error
+>p : "hi"
+}
+class DD extends C {
+>DD : DD
+>C : C
+
+    declare p: 'bye'; // ok
+>p : "bye"
+}
+
+
+declare class E {
+>E : E
+
+    p1: string
+>p1 : string
+
+    p2: string
+>p2 : string
+}
+class F extends E {
+>F : F
+>E : E
+
+    p1!: 'z'
+>p1 : "z"
+
+    declare p2: 'alpha'
+>p2 : "alpha"
+}
+
+class G extends E {
+>G : G
+>E : E
+
+    p1: 'z'
+>p1 : "z"
+
+    constructor() {
+        super()
+>super() : void
+>super : typeof E
+
+        this.p1 = 'z'
+>this.p1 = 'z' : "z"
+>this.p1 : "z"
+>this : this
+>p1 : "z"
+>'z' : "z"
+    }
+}
+
+abstract class H extends E {
+>H : H
+>E : E
+
+    abstract p1: 'a' | 'b' | 'c'
+>p1 : "a" | "b" | "c"
+
+    declare abstract p2: 'a' | 'b' | 'c'
+>p2 : "a" | "b" | "c"
+}
+
+interface I {
+    q: number
+>q : number
+}
+interface J extends I { }
+class J {
+>J : J
+
+    r = 5
+>r : number
+>5 : 5
+}
+class K extends J {
+>K : K
+>J : J
+
+    q!: 1 | 2 | 3 // ok, extends a property from an interface
+>q : 1 | 2 | 3
+
+    r!: 4 | 5 // error, from class
+>r : 5 | 4
+}
+
diff --git a/tests/baselines/reference/genericPrototypeProperty2.errors.txt b/tests/baselines/reference/genericPrototypeProperty2.errors.txt
new file mode 100644
index 0000000000000..2d27cf0fc4e72
--- /dev/null
+++ b/tests/baselines/reference/genericPrototypeProperty2.errors.txt
@@ -0,0 +1,24 @@
+tests/cases/compiler/genericPrototypeProperty2.ts(7,5): error TS2610: Property 'target' will overwrite the base property in 'BaseEvent'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/compiler/genericPrototypeProperty2.ts(14,5): error TS2610: Property 't' will overwrite the base property in 'BaseEventWrapper'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/genericPrototypeProperty2.ts (2 errors) ====
+    interface EventTarget { x } 
+    class BaseEvent {
+        target: EventTarget;
+    }
+    
+    class MyEvent<T extends EventTarget> extends BaseEvent {
+        target: T;
+        ~~~~~~
+!!! error TS2610: Property 'target' will overwrite the base property in 'BaseEvent'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    class BaseEventWrapper {
+        t: BaseEvent;
+    }
+    
+    class MyEventWrapper extends BaseEventWrapper {
+        t: MyEvent<any>; // any satisfies constraint and passes assignability check between 'target' properties
+        ~
+!!! error TS2610: Property 't' will overwrite the base property in 'BaseEventWrapper'. Add a 'declare' modifier or an initializer to avoid this.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/genericPrototypeProperty3.errors.txt b/tests/baselines/reference/genericPrototypeProperty3.errors.txt
new file mode 100644
index 0000000000000..c12e29d3da0cc
--- /dev/null
+++ b/tests/baselines/reference/genericPrototypeProperty3.errors.txt
@@ -0,0 +1,23 @@
+tests/cases/compiler/genericPrototypeProperty3.ts(6,5): error TS2610: Property 'target' will overwrite the base property in 'BaseEvent'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/compiler/genericPrototypeProperty3.ts(13,5): error TS2610: Property 't' will overwrite the base property in 'BaseEventWrapper'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/genericPrototypeProperty3.ts (2 errors) ====
+    class BaseEvent {
+        target: {};
+    }
+    
+    class MyEvent<T> extends BaseEvent { // T is instantiated to any in the prototype, which is assignable to {}
+        target: T;
+        ~~~~~~
+!!! error TS2610: Property 'target' will overwrite the base property in 'BaseEvent'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    class BaseEventWrapper {
+        t: BaseEvent;
+    }
+    
+    class MyEventWrapper extends BaseEventWrapper {
+        t: MyEvent<any>;
+        ~
+!!! error TS2610: Property 't' will overwrite the base property in 'BaseEventWrapper'. Add a 'declare' modifier or an initializer to avoid this.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/illegalModifiersOnClassElements.errors.txt b/tests/baselines/reference/illegalModifiersOnClassElements.errors.txt
index eb373ad2fae07..99ba043b52a80 100644
--- a/tests/baselines/reference/illegalModifiersOnClassElements.errors.txt
+++ b/tests/baselines/reference/illegalModifiersOnClassElements.errors.txt
@@ -1,12 +1,12 @@
-tests/cases/compiler/illegalModifiersOnClassElements.ts(2,5): error TS1031: 'declare' modifier cannot appear on a class element.
+tests/cases/compiler/illegalModifiersOnClassElements.ts(2,19): error TS1039: Initializers are not allowed in ambient contexts.
 tests/cases/compiler/illegalModifiersOnClassElements.ts(3,5): error TS1031: 'export' modifier cannot appear on a class element.
 
 
 ==== tests/cases/compiler/illegalModifiersOnClassElements.ts (2 errors) ====
     class C {
         declare foo = 1;
-        ~~~~~~~
-!!! error TS1031: 'declare' modifier cannot appear on a class element.
+                      ~
+!!! error TS1039: Initializers are not allowed in ambient contexts.
         export bar = 1;
         ~~~~~~
 !!! error TS1031: 'export' modifier cannot appear on a class element.
diff --git a/tests/baselines/reference/implementingAnInterfaceExtendingClassWithProtecteds.errors.txt b/tests/baselines/reference/implementingAnInterfaceExtendingClassWithProtecteds.errors.txt
index ecb3e4664a470..d8cb657aa4305 100644
--- a/tests/baselines/reference/implementingAnInterfaceExtendingClassWithProtecteds.errors.txt
+++ b/tests/baselines/reference/implementingAnInterfaceExtendingClassWithProtecteds.errors.txt
@@ -10,9 +10,10 @@ tests/cases/conformance/interfaces/interfacesExtendingClasses/implementingAnInte
   Property 'y' is missing in type 'Bar5' but required in type 'I'.
 tests/cases/conformance/interfaces/interfacesExtendingClasses/implementingAnInterfaceExtendingClassWithProtecteds.ts(29,7): error TS2420: Class 'Bar6' incorrectly implements interface 'I'.
   Property 'y' is protected in type 'Bar6' but public in type 'I'.
+tests/cases/conformance/interfaces/interfacesExtendingClasses/implementingAnInterfaceExtendingClassWithProtecteds.ts(38,5): error TS2610: Property 'x' will overwrite the base property in 'Foo'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/interfaces/interfacesExtendingClasses/implementingAnInterfaceExtendingClassWithProtecteds.ts (6 errors) ====
+==== tests/cases/conformance/interfaces/interfacesExtendingClasses/implementingAnInterfaceExtendingClassWithProtecteds.ts (7 errors) ====
     class Foo {
         protected x: string;
     }
@@ -71,6 +72,8 @@ tests/cases/conformance/interfaces/interfacesExtendingClasses/implementingAnInte
     
     class Bar8 extends Foo implements I {
         x: string;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Foo'. Add a 'declare' modifier or an initializer to avoid this.
         y: number;
     }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/inheritance.errors.txt b/tests/baselines/reference/inheritance.errors.txt
index b476f1399b445..45a1dbfc5666e 100644
--- a/tests/baselines/reference/inheritance.errors.txt
+++ b/tests/baselines/reference/inheritance.errors.txt
@@ -1,9 +1,10 @@
+tests/cases/compiler/inheritance.ts(22,12): error TS2610: Property 'y' will overwrite the base property in 'N'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/compiler/inheritance.ts(31,12): error TS2425: Class 'Good' defines instance member property 'f', but extended class 'Baad' defines it as instance member function.
 tests/cases/compiler/inheritance.ts(32,12): error TS2416: Property 'g' in type 'Baad' is not assignable to the same property in base type 'Good'.
   Type '(n: number) => number' is not assignable to type '() => number'.
 
 
-==== tests/cases/compiler/inheritance.ts (2 errors) ====
+==== tests/cases/compiler/inheritance.ts (3 errors) ====
     class B1 {
         public x;
     }
@@ -26,6 +27,8 @@ tests/cases/compiler/inheritance.ts(32,12): error TS2416: Property 'g' in type '
     
     class ND extends N { // any is assignable to number
         public y;
+               ~
+!!! error TS2610: Property 'y' will overwrite the base property in 'N'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class Good {
diff --git a/tests/baselines/reference/inheritanceMemberPropertyOverridingAccessor.errors.txt b/tests/baselines/reference/inheritanceMemberPropertyOverridingAccessor.errors.txt
index ec8ec05a8aeb5..3996eaf15c34b 100644
--- a/tests/baselines/reference/inheritanceMemberPropertyOverridingAccessor.errors.txt
+++ b/tests/baselines/reference/inheritanceMemberPropertyOverridingAccessor.errors.txt
@@ -1,8 +1,9 @@
 tests/cases/compiler/inheritanceMemberPropertyOverridingAccessor.ts(3,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/compiler/inheritanceMemberPropertyOverridingAccessor.ts(6,9): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/compiler/inheritanceMemberPropertyOverridingAccessor.ts(12,5): error TS2610: Property 'x' will overwrite the base property in 'a'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/compiler/inheritanceMemberPropertyOverridingAccessor.ts (2 errors) ====
+==== tests/cases/compiler/inheritanceMemberPropertyOverridingAccessor.ts (3 errors) ====
     class a {
         private __x: () => string;
         get x() {
@@ -19,4 +20,6 @@ tests/cases/compiler/inheritanceMemberPropertyOverridingAccessor.ts(6,9): error
     
     class b extends a {
         x: () => string;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'a'. Add a 'declare' modifier or an initializer to avoid this.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/inheritanceMemberPropertyOverridingProperty.errors.txt b/tests/baselines/reference/inheritanceMemberPropertyOverridingProperty.errors.txt
new file mode 100644
index 0000000000000..79b3e6a8f1ea8
--- /dev/null
+++ b/tests/baselines/reference/inheritanceMemberPropertyOverridingProperty.errors.txt
@@ -0,0 +1,13 @@
+tests/cases/compiler/inheritanceMemberPropertyOverridingProperty.ts(6,5): error TS2610: Property 'x' will overwrite the base property in 'a'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/inheritanceMemberPropertyOverridingProperty.ts (1 errors) ====
+    class a {
+        x: () => string;
+    }
+    
+    class b extends a {
+        x: () => string;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'a'. Add a 'declare' modifier or an initializer to avoid this.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/instanceSubtypeCheck2.errors.txt b/tests/baselines/reference/instanceSubtypeCheck2.errors.txt
index 84b454d7ea0a6..1d3622f49e79f 100644
--- a/tests/baselines/reference/instanceSubtypeCheck2.errors.txt
+++ b/tests/baselines/reference/instanceSubtypeCheck2.errors.txt
@@ -1,8 +1,9 @@
 tests/cases/compiler/instanceSubtypeCheck2.ts(6,5): error TS2416: Property 'x' in type 'C2<T>' is not assignable to the same property in base type 'C1<T>'.
   Type 'string' is not assignable to type 'C2<T>'.
+tests/cases/compiler/instanceSubtypeCheck2.ts(6,5): error TS2610: Property 'x' will overwrite the base property in 'C1<T>'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/compiler/instanceSubtypeCheck2.ts (1 errors) ====
+==== tests/cases/compiler/instanceSubtypeCheck2.ts (2 errors) ====
     class C1<T> {
         x: C2<T>;
     }
@@ -12,4 +13,6 @@ tests/cases/compiler/instanceSubtypeCheck2.ts(6,5): error TS2416: Property 'x' i
         ~
 !!! error TS2416: Property 'x' in type 'C2<T>' is not assignable to the same property in base type 'C1<T>'.
 !!! error TS2416:   Type 'string' is not assignable to type 'C2<T>'.
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'C1<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/interfaceExtendsObjectIntersectionErrors.errors.txt b/tests/baselines/reference/interfaceExtendsObjectIntersectionErrors.errors.txt
index 17cb5585cf87e..cf04e9cbc4221 100644
--- a/tests/baselines/reference/interfaceExtendsObjectIntersectionErrors.errors.txt
+++ b/tests/baselines/reference/interfaceExtendsObjectIntersectionErrors.errors.txt
@@ -16,14 +16,18 @@ tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectI
     Type 'number' is not assignable to type 'string'.
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(16,38): error TS2416: Property 'a' in type 'C1' is not assignable to the same property in base type 'T1'.
   Type 'string' is not assignable to type 'number'.
+tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(16,38): error TS2610: Property 'a' will overwrite the base property in 'T1'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(17,38): error TS2416: Property 'b' in type 'C2' is not assignable to the same property in base type 'T2'.
   Type 'string' is not assignable to type 'number'.
+tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(17,38): error TS2610: Property 'b' will overwrite the base property in 'T2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(18,38): error TS2416: Property 'length' in type 'C3' is not assignable to the same property in base type 'number[]'.
   Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(19,38): error TS2416: Property '0' in type 'C4' is not assignable to the same property in base type '[string, number]'.
   Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(19,38): error TS2610: Property '0' will overwrite the base property in '[string, number]'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(20,38): error TS2416: Property 'c' in type 'C5' is not assignable to the same property in base type 'T5'.
   Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(20,38): error TS2610: Property 'c' will overwrite the base property in 'T5'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(30,11): error TS2430: Interface 'I10' incorrectly extends interface 'typeof CX'.
   Types of property 'a' are incompatible.
     Type 'number' is not assignable to type 'string'.
@@ -55,7 +59,7 @@ tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectI
 tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts(47,26): error TS2312: An interface can only extend an object type or intersection of object types with statically known members.
 
 
-==== tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts (23 errors) ====
+==== tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectIntersectionErrors.ts (27 errors) ====
     type T1 = { a: number };
     type T2 = T1 & { b: number };
     type T3 = number[];
@@ -96,10 +100,14 @@ tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectI
                                          ~
 !!! error TS2416: Property 'a' in type 'C1' is not assignable to the same property in base type 'T1'.
 !!! error TS2416:   Type 'string' is not assignable to type 'number'.
+                                         ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'T1'. Add a 'declare' modifier or an initializer to avoid this.
     class C2 extends Constructor<T2>() { b: string }
                                          ~
 !!! error TS2416: Property 'b' in type 'C2' is not assignable to the same property in base type 'T2'.
 !!! error TS2416:   Type 'string' is not assignable to type 'number'.
+                                         ~
+!!! error TS2610: Property 'b' will overwrite the base property in 'T2'. Add a 'declare' modifier or an initializer to avoid this.
     class C3 extends Constructor<T3>() { length: string }
                                          ~~~~~~
 !!! error TS2416: Property 'length' in type 'C3' is not assignable to the same property in base type 'number[]'.
@@ -108,10 +116,14 @@ tests/cases/conformance/interfaces/interfaceDeclarations/interfaceExtendsObjectI
                                          ~
 !!! error TS2416: Property '0' in type 'C4' is not assignable to the same property in base type '[string, number]'.
 !!! error TS2416:   Type 'number' is not assignable to type 'string'.
+                                         ~
+!!! error TS2610: Property '0' will overwrite the base property in '[string, number]'. Add a 'declare' modifier or an initializer to avoid this.
     class C5 extends Constructor<T5>() { c: number }
                                          ~
 !!! error TS2416: Property 'c' in type 'C5' is not assignable to the same property in base type 'T5'.
 !!! error TS2416:   Type 'number' is not assignable to type 'string'.
+                                         ~
+!!! error TS2610: Property 'c' will overwrite the base property in 'T5'. Add a 'declare' modifier or an initializer to avoid this.
     
     declare class CX { static a: string }
     declare enum EX { A, B, C }
diff --git a/tests/baselines/reference/multipleInheritance.errors.txt b/tests/baselines/reference/multipleInheritance.errors.txt
index c4a440d9a0ea5..2e3788c895de9 100644
--- a/tests/baselines/reference/multipleInheritance.errors.txt
+++ b/tests/baselines/reference/multipleInheritance.errors.txt
@@ -1,11 +1,12 @@
 tests/cases/compiler/multipleInheritance.ts(9,21): error TS1174: Classes can only extend a single class.
 tests/cases/compiler/multipleInheritance.ts(18,21): error TS1174: Classes can only extend a single class.
+tests/cases/compiler/multipleInheritance.ts(26,12): error TS2610: Property 'y' will overwrite the base property in 'N'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/compiler/multipleInheritance.ts(35,12): error TS2425: Class 'Good' defines instance member property 'f', but extended class 'Baad' defines it as instance member function.
 tests/cases/compiler/multipleInheritance.ts(36,12): error TS2416: Property 'g' in type 'Baad' is not assignable to the same property in base type 'Good'.
   Type '(n: number) => number' is not assignable to type '() => number'.
 
 
-==== tests/cases/compiler/multipleInheritance.ts (4 errors) ====
+==== tests/cases/compiler/multipleInheritance.ts (5 errors) ====
     class B1 {
         public x;
     }
@@ -36,6 +37,8 @@ tests/cases/compiler/multipleInheritance.ts(36,12): error TS2416: Property 'g' i
     
     class ND extends N { // any is assignable to number
         public y;
+               ~
+!!! error TS2610: Property 'y' will overwrite the base property in 'N'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class Good {
diff --git a/tests/baselines/reference/mutuallyRecursiveInference.errors.txt b/tests/baselines/reference/mutuallyRecursiveInference.errors.txt
new file mode 100644
index 0000000000000..cdb3abea28fee
--- /dev/null
+++ b/tests/baselines/reference/mutuallyRecursiveInference.errors.txt
@@ -0,0 +1,24 @@
+tests/cases/compiler/mutuallyRecursiveInference.ts(9,5): error TS2610: Property 'a' will overwrite the base property in 'L<X>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/compiler/mutuallyRecursiveInference.ts(10,5): error TS2610: Property 'b' will overwrite the base property in 'L<X>'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/mutuallyRecursiveInference.ts (2 errors) ====
+    class T<A> {
+        a: A;
+        b: any
+    }
+    class L<RT extends { a: 'a' | 'b', b: any }> extends T<RT[RT['a']]> {
+        m() { this.a }
+    }
+    class X extends L<X> {
+        a: 'a' | 'b'
+        ~
+!!! error TS2610: Property 'a' will overwrite the base property in 'L<X>'. Add a 'declare' modifier or an initializer to avoid this.
+        b: number
+        ~
+!!! error TS2610: Property 'b' will overwrite the base property in 'L<X>'. Add a 'declare' modifier or an initializer to avoid this.
+        m2() {
+            this.a
+        }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/parserAstSpans1.errors.txt b/tests/baselines/reference/parserAstSpans1.errors.txt
index 648929e974e3e..6fa8715a8f6ea 100644
--- a/tests/baselines/reference/parserAstSpans1.errors.txt
+++ b/tests/baselines/reference/parserAstSpans1.errors.txt
@@ -3,12 +3,14 @@ tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(85,16): error TS10
 tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(94,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(100,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(111,25): error TS2340: Only public and protected methods of the base class are accessible via the 'super' keyword.
+tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(114,12): error TS2610: Property 'p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(119,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
+tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(122,12): error TS2610: Property 'nc_p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(125,16): error TS1056: Accessors are only available when targeting ECMAScript 5 and higher.
 tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(217,24): error TS2340: Only public and protected methods of the base class are accessible via the 'super' keyword.
 
 
-==== tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts (8 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts (10 errors) ====
     /** i1 is interface with properties*/
     interface i1 {
         /** i1_p1*/
@@ -133,6 +135,8 @@ tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(217,24): error TS2
         }
         /** c3 p1*/
         public p1: number;
+               ~~
+!!! error TS2610: Property 'p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
         /** c3 f1*/
         public f1() {
         }
@@ -143,6 +147,8 @@ tests/cases/conformance/parser/ecmascript5/parserAstSpans1.ts(217,24): error TS2
             return 10;
         }
         public nc_p1: number;
+               ~~~~~
+!!! error TS2610: Property 'nc_p1' will overwrite the base property in 'c2'. Add a 'declare' modifier or an initializer to avoid this.
         public nc_f1() {
         }
         public get nc_prop() {
diff --git a/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt b/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt
index dc19cb0f8550a..402355919a031 100644
--- a/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt
+++ b/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt
@@ -1,9 +1,12 @@
 tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts(2,5): error TS1031: 'declare' modifier cannot appear on a class element.
+tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts(2,19): error TS1183: An implementation cannot be declared in ambient contexts.
 
 
-==== tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts (1 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts (2 errors) ====
     class C {
         declare Foo() { }
         ~~~~~~~
 !!! error TS1031: 'declare' modifier cannot appear on a class element.
+                      ~
+!!! error TS1183: An implementation cannot be declared in ambient contexts.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/parserMemberVariableDeclaration5.errors.txt b/tests/baselines/reference/parserMemberVariableDeclaration5.errors.txt
deleted file mode 100644
index 2b2ea9664ceb4..0000000000000
--- a/tests/baselines/reference/parserMemberVariableDeclaration5.errors.txt
+++ /dev/null
@@ -1,9 +0,0 @@
-tests/cases/conformance/parser/ecmascript5/MemberVariableDeclarations/parserMemberVariableDeclaration5.ts(2,3): error TS1031: 'declare' modifier cannot appear on a class element.
-
-
-==== tests/cases/conformance/parser/ecmascript5/MemberVariableDeclarations/parserMemberVariableDeclaration5.ts (1 errors) ====
-    class C {
-      declare Foo;
-      ~~~~~~~
-!!! error TS1031: 'declare' modifier cannot appear on a class element.
-    }
\ No newline at end of file
diff --git a/tests/baselines/reference/parserRealSource11.errors.txt b/tests/baselines/reference/parserRealSource11.errors.txt
index bb0a52e0a9997..fdfb15759460e 100644
--- a/tests/baselines/reference/parserRealSource11.errors.txt
+++ b/tests/baselines/reference/parserRealSource11.errors.txt
@@ -288,6 +288,8 @@ tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1095,29): error
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1098,56): error TS2304: Cannot find name 'FncFlags'.
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1098,79): error TS2304: Cannot find name 'FncFlags'.
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1111,33): error TS2304: Cannot find name 'IFileReference'.
+tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1120,16): error TS2610: Property 'vars' will overwrite the base property in 'FuncDecl'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1121,16): error TS2610: Property 'scopes' will overwrite the base property in 'FuncDecl'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1127,84): error TS2304: Cannot find name 'NodeType'.
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1132,36): error TS2304: Cannot find name 'TypeFlow'.
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(1144,42): error TS2304: Cannot find name 'NodeType'.
@@ -511,7 +513,7 @@ tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(2356,30): error
 tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(2356,48): error TS2304: Cannot find name 'TokenID'.
 
 
-==== tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts (511 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts (513 errors) ====
     // Copyright (c) Microsoft. All rights reserved. Licensed under the Apache License, Version 2.0. 
     // See LICENSE.txt in the project root for complete license information.
     
@@ -2212,7 +2214,11 @@ tests/cases/conformance/parser/ecmascript5/parserRealSource11.ts(2356,48): error
             public leftCurlyCount = 0;
             public rightCurlyCount = 0;
             public vars: ASTList;
+                   ~~~~
+!!! error TS2610: Property 'vars' will overwrite the base property in 'FuncDecl'. Add a 'declare' modifier or an initializer to avoid this.
             public scopes: ASTList;
+                   ~~~~~~
+!!! error TS2610: Property 'scopes' will overwrite the base property in 'FuncDecl'. Add a 'declare' modifier or an initializer to avoid this.
             // Remember if the script contains Unicode chars, that is needed when generating code for this script object to decide the output file correct encoding.
             public containsUnicodeChar = false;
             public containsUnicodeCharInComment = false;
diff --git a/tests/baselines/reference/parserharness.errors.txt b/tests/baselines/reference/parserharness.errors.txt
index 990fa5816c787..9c00868b16d22 100644
--- a/tests/baselines/reference/parserharness.errors.txt
+++ b/tests/baselines/reference/parserharness.errors.txt
@@ -12,6 +12,10 @@ tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(347,13): e
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(351,17): error TS2662: Cannot find name 'errorHandlerStack'. Did you mean the static member 'Runnable.errorHandlerStack'?
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(354,17): error TS2662: Cannot find name 'errorHandlerStack'. Did you mean the static member 'Runnable.errorHandlerStack'?
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(354,35): error TS2662: Cannot find name 'errorHandlerStack'. Did you mean the static member 'Runnable.errorHandlerStack'?
+tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(359,16): error TS2610: Property 'description' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(360,16): error TS2610: Property 'block' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(402,16): error TS2610: Property 'description' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(403,16): error TS2610: Property 'block' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(691,50): error TS2304: Cannot find name 'ITextWriter'.
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(716,47): error TS2503: Cannot find namespace 'TypeScript'.
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(721,62): error TS2304: Cannot find name 'ITextWriter'.
@@ -110,7 +114,7 @@ tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(1787,68):
 tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(2030,32): error TS2304: Cannot find name 'Diff'.
 
 
-==== tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts (110 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts (114 errors) ====
     //
     // Copyright (c) Microsoft Corporation.  All rights reserved.
     // 
@@ -498,7 +502,11 @@ tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(2030,32):
         }
         export class TestCase extends Runnable {
             public description: string;
+                   ~~~~~~~~~~~
+!!! error TS2610: Property 'description' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
             public block;
+                   ~~~~~
+!!! error TS2610: Property 'block' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
     
             constructor(description: string, block: any) {
                 super(description, block);
@@ -541,7 +549,11 @@ tests/cases/conformance/parser/ecmascript5/RealWorld/parserharness.ts(2030,32):
     
         export class Scenario extends Runnable {
             public description: string;
+                   ~~~~~~~~~~~
+!!! error TS2610: Property 'description' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
             public block;
+                   ~~~~~
+!!! error TS2610: Property 'block' will overwrite the base property in 'Runnable'. Add a 'declare' modifier or an initializer to avoid this.
     
             constructor(description: string, block: any) {
                 super(description, block);
diff --git a/tests/baselines/reference/protectedClassPropertyAccessibleWithinNestedSubclass1.errors.txt b/tests/baselines/reference/protectedClassPropertyAccessibleWithinNestedSubclass1.errors.txt
index 2d882681d7269..dd395b2a2a4a2 100644
--- a/tests/baselines/reference/protectedClassPropertyAccessibleWithinNestedSubclass1.errors.txt
+++ b/tests/baselines/reference/protectedClassPropertyAccessibleWithinNestedSubclass1.errors.txt
@@ -6,6 +6,7 @@ tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAcce
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(52,19): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived2'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(53,20): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived2'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(55,20): error TS2445: Property 'x' is protected and only accessible within class 'Derived3' and its subclasses.
+tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(63,15): error TS2610: Property 'x' will overwrite the base property in 'Derived1'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(73,19): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived3'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(74,20): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived3'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(75,20): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived3'.
@@ -21,7 +22,7 @@ tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAcce
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts(114,4): error TS2445: Property 'x' is protected and only accessible within class 'Base' and its subclasses.
 
 
-==== tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts (21 errors) ====
+==== tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinNestedSubclass1.ts (22 errors) ====
     class Base {
         protected x: string;
         method() {
@@ -101,6 +102,8 @@ tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAcce
     
     class Derived3 extends Derived1 {
         protected x: string;
+                  ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Derived1'. Add a 'declare' modifier or an initializer to avoid this.
         method3() {
             class D {
                 method3d() {
diff --git a/tests/baselines/reference/protectedClassPropertyAccessibleWithinSubclass2.errors.txt b/tests/baselines/reference/protectedClassPropertyAccessibleWithinSubclass2.errors.txt
index 996d62c165212..2d43267ed419e 100644
--- a/tests/baselines/reference/protectedClassPropertyAccessibleWithinSubclass2.errors.txt
+++ b/tests/baselines/reference/protectedClassPropertyAccessibleWithinSubclass2.errors.txt
@@ -6,6 +6,7 @@ tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAcce
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(42,11): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived2'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(43,12): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived2'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(45,12): error TS2445: Property 'x' is protected and only accessible within class 'Derived3' and its subclasses.
+tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(51,15): error TS2610: Property 'x' will overwrite the base property in 'Derived1'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(59,11): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived3'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(60,12): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived3'.
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(61,12): error TS2446: Property 'x' is protected and only accessible through an instance of class 'Derived3'.
@@ -21,7 +22,7 @@ tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAcce
 tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts(94,4): error TS2445: Property 'x' is protected and only accessible within class 'Base' and its subclasses.
 
 
-==== tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts (21 errors) ====
+==== tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAccessibleWithinSubclass2.ts (22 errors) ====
     class Base {
         protected x: string;
         method() {
@@ -89,6 +90,8 @@ tests/cases/conformance/classes/members/accessibility/protectedClassPropertyAcce
     
     class Derived3 extends Derived1 {
         protected x: string;
+                  ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'Derived1'. Add a 'declare' modifier or an initializer to avoid this.
         method3() {
             var b: Base;
             var d1: Derived1;
diff --git a/tests/baselines/reference/protectedMembers.errors.txt b/tests/baselines/reference/protectedMembers.errors.txt
index 9171df2a485b6..563a4a757fc6c 100644
--- a/tests/baselines/reference/protectedMembers.errors.txt
+++ b/tests/baselines/reference/protectedMembers.errors.txt
@@ -1,3 +1,4 @@
+tests/cases/compiler/protectedMembers.ts(25,5): error TS2610: Property 'x' will overwrite the base property in 'C2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/compiler/protectedMembers.ts(40,4): error TS2445: Property 'x' is protected and only accessible within class 'C1' and its subclasses.
 tests/cases/compiler/protectedMembers.ts(41,4): error TS2445: Property 'f' is protected and only accessible within class 'C1' and its subclasses.
 tests/cases/compiler/protectedMembers.ts(42,4): error TS2445: Property 'sx' is protected and only accessible within class 'C1' and its subclasses.
@@ -12,11 +13,13 @@ tests/cases/compiler/protectedMembers.ts(97,1): error TS2322: Type 'B1' is not a
   Property 'x' is protected but type 'B1' is not a class derived from 'A1'.
 tests/cases/compiler/protectedMembers.ts(98,1): error TS2322: Type 'A1' is not assignable to type 'B1'.
   Property 'x' is protected in type 'A1' but public in type 'B1'.
+tests/cases/compiler/protectedMembers.ts(104,5): error TS2610: Property 'x' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/compiler/protectedMembers.ts(111,7): error TS2415: Class 'B3' incorrectly extends base class 'A3'.
   Property 'x' is protected in type 'B3' but public in type 'A3'.
+tests/cases/compiler/protectedMembers.ts(112,15): error TS2610: Property 'x' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/compiler/protectedMembers.ts (13 errors) ====
+==== tests/cases/compiler/protectedMembers.ts (16 errors) ====
     // Class with protected members
     class C1 {
         protected x: number;
@@ -42,6 +45,8 @@ tests/cases/compiler/protectedMembers.ts(111,7): error TS2415: Class 'B3' incorr
     // Derived class making protected members public
     class C3 extends C2 {
         x: number;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'C2'. Add a 'declare' modifier or an initializer to avoid this.
         static sx: number;
         f() {
             return super.f();
@@ -147,6 +152,8 @@ tests/cases/compiler/protectedMembers.ts(111,7): error TS2415: Class 'B3' incorr
     }
     class B2 extends A2 {
         x;
+        ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class A3 {
@@ -158,6 +165,8 @@ tests/cases/compiler/protectedMembers.ts(111,7): error TS2415: Class 'B3' incorr
 !!! error TS2415: Class 'B3' incorrectly extends base class 'A3'.
 !!! error TS2415:   Property 'x' is protected in type 'B3' but public in type 'A3'.
         protected x;
+                  ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     
\ No newline at end of file
diff --git a/tests/baselines/reference/scopeTests.errors.txt b/tests/baselines/reference/scopeTests.errors.txt
index ce24301efc2bf..362da88655557 100644
--- a/tests/baselines/reference/scopeTests.errors.txt
+++ b/tests/baselines/reference/scopeTests.errors.txt
@@ -1,8 +1,9 @@
 tests/cases/compiler/scopeTests.ts(2,7): error TS2415: Class 'D' incorrectly extends base class 'C'.
   Property 'v' is private in type 'C' but not in type 'D'.
+tests/cases/compiler/scopeTests.ts(4,10): error TS2610: Property 'p' will overwrite the base property in 'C'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/compiler/scopeTests.ts (1 errors) ====
+==== tests/cases/compiler/scopeTests.ts (2 errors) ====
     class C { private v; public p; static s; }
     class D extends C {
           ~
@@ -10,6 +11,8 @@ tests/cases/compiler/scopeTests.ts(2,7): error TS2415: Class 'D' incorrectly ext
 !!! error TS2415:   Property 'v' is private in type 'C' but not in type 'D'.
       public v: number;
       public p: number
+             ~
+!!! error TS2610: Property 'p' will overwrite the base property in 'C'. Add a 'declare' modifier or an initializer to avoid this.
       constructor() {
        super()
        this.v = 1;
diff --git a/tests/baselines/reference/subtypesOfTypeParameter.errors.txt b/tests/baselines/reference/subtypesOfTypeParameter.errors.txt
index 0ec8951fe9391..33f7c1d48ec2d 100644
--- a/tests/baselines/reference/subtypesOfTypeParameter.errors.txt
+++ b/tests/baselines/reference/subtypesOfTypeParameter.errors.txt
@@ -1,9 +1,10 @@
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameter.ts(8,5): error TS2416: Property 'foo' in type 'D1<T, U>' is not assignable to the same property in base type 'C3<T>'.
   Type 'U' is not assignable to type 'T'.
     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameter.ts(8,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameter.ts (1 errors) ====
+==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameter.ts (2 errors) ====
     // checking whether other types are subtypes of type parameters
     
     class C3<T> {
@@ -16,6 +17,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D1<T, U>' is not assignable to the same property in base type 'C3<T>'.
 !!! error TS2416:   Type 'U' is not assignable to type 'T'.
 !!! error TS2416:     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     function f1<T, U>(x: T, y: U) {
diff --git a/tests/baselines/reference/subtypesOfTypeParameterWithConstraints.errors.txt b/tests/baselines/reference/subtypesOfTypeParameterWithConstraints.errors.txt
index 5fe0186e04a1b..37ae8137c5a2b 100644
--- a/tests/baselines/reference/subtypesOfTypeParameterWithConstraints.errors.txt
+++ b/tests/baselines/reference/subtypesOfTypeParameterWithConstraints.errors.txt
@@ -1,21 +1,39 @@
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(9,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(14,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(19,5): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(19,5): error TS2416: Property 'foo' in type 'D3<T, U>' is not assignable to the same property in base type 'C3<T>'.
   Type 'U' is not assignable to type 'T'.
     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(19,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(24,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(33,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(38,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(43,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(50,5): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(50,5): error TS2416: Property 'foo' in type 'D8<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
   Type 'U' is not assignable to type 'T'.
     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
       Type 'V' is not assignable to type 'T'.
         'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(50,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(55,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(60,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(67,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(67,5): error TS2416: Property 'foo' in type 'D11<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
   Type 'V' is not assignable to type 'T'.
     'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(67,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(72,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(72,5): error TS2416: Property 'foo' in type 'D12<T, U, V>' is not assignable to the same property in base type 'C3<U>'.
   Type 'V' is not assignable to type 'U'.
     'V' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '{}'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(72,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(77,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(85,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(90,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(95,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(100,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(107,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(112,5): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(112,5): error TS2416: Property 'foo' in type 'D19<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
   Type 'U' is not assignable to type 'T'.
@@ -24,33 +42,44 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
         'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
           Type 'Date' is not assignable to type 'T'.
             'Date' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(112,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(117,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(122,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(129,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(134,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(134,5): error TS2416: Property 'foo' in type 'D23<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
   Type 'V' is not assignable to type 'T'.
     'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
       Type 'Date' is not assignable to type 'T'.
         'Date' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(134,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(139,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(139,5): error TS2416: Property 'foo' in type 'D24<T, U, V>' is not assignable to the same property in base type 'C3<U>'.
   Type 'V' is not assignable to type 'U'.
     'V' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Date'.
       Type 'Date' is not assignable to type 'U'.
         'Date' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Date'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(139,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(144,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(151,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(156,5): error TS2411: Property 'foo' of type 'Date' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(156,5): error TS2416: Property 'foo' in type 'D27<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
   Type 'Date' is not assignable to type 'T'.
     'Date' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(156,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(161,5): error TS2411: Property 'foo' of type 'Date' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(161,5): error TS2416: Property 'foo' in type 'D28<T, U, V>' is not assignable to the same property in base type 'C3<U>'.
   Type 'Date' is not assignable to type 'U'.
     'Date' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Date'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(161,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(166,5): error TS2411: Property 'foo' of type 'Date' is not assignable to string index type 'V'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(166,5): error TS2416: Property 'foo' in type 'D29<T, U, V>' is not assignable to the same property in base type 'C3<V>'.
   Type 'Date' is not assignable to type 'V'.
     'Date' is assignable to the constraint of type 'V', but 'V' could be instantiated with a different subtype of constraint 'Date'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts(166,5): error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts (20 errors) ====
+==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints.ts (49 errors) ====
     // checking whether other types are subtypes of type parameters with constraints
     
     class C3<T> {
@@ -60,11 +89,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D1<T extends U, U> extends C3<T> {
         [x: string]: T;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D2<T extends U, U> extends C3<U> {
         [x: string]: U;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D3<T extends U, U> extends C3<T> {
@@ -76,11 +109,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D3<T, U>' is not assignable to the same property in base type 'C3<T>'.
 !!! error TS2416:   Type 'U' is not assignable to type 'T'.
 !!! error TS2416:     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D4<T extends U, U> extends C3<U> {
         [x: string]: U;
         foo: U; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     
@@ -90,16 +127,22 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D5<T extends U, U extends V, V> extends C3<T> {
         [x: string]: T;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D6<T extends U, U extends V, V> extends C3<U> {
         [x: string]: U;
         foo: T;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D7<T extends U, U extends V, V> extends C3<V> {
         [x: string]: V;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     // test if U is a subtype of T, U, V
@@ -115,16 +158,22 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
 !!! error TS2416:       Type 'V' is not assignable to type 'T'.
 !!! error TS2416:         'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D9<T extends U, U extends V, V> extends C3<U> {
         [x: string]: U;
         foo: U; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D10<T extends U, U extends V, V> extends C3<V> {
         [x: string]: V;
         foo: U; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     // test if V is a subtype of T, U, V
@@ -138,6 +187,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D11<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'T'.
 !!! error TS2416:     'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D12<T extends U, U extends V, V> extends C3<U> {
@@ -149,11 +200,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D12<T, U, V>' is not assignable to the same property in base type 'C3<U>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'U'.
 !!! error TS2416:     'V' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '{}'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D13<T extends U, U extends V, V> extends C3<V> {
         [x: string]: V;
         foo: V; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     // Date > V > U > T
@@ -162,21 +217,29 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D14<T extends U, U extends V, V extends Date> extends C3<Date> {
         [x: string]: Date;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D15<T extends U, U extends V, V extends Date> extends C3<T> {
         [x: string]: T;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D16<T extends U, U extends V, V extends Date> extends C3<U> {
         [x: string]: U;
         foo: T;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D17<T extends U, U extends V, V extends Date> extends C3<V> {
         [x: string]: V;
         foo: T;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     // test if U is a subtype of T, U, V, Date
@@ -184,6 +247,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D18<T extends U, U extends V, V extends Date> extends C3<Date> {
         [x: string]: Date;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D19<T extends U, U extends V, V extends Date> extends C3<T> {
@@ -199,16 +264,22 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:         'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
 !!! error TS2416:           Type 'Date' is not assignable to type 'T'.
 !!! error TS2416:             'Date' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D20<T extends U, U extends V, V extends Date> extends C3<U> {
         [x: string]: U;
         foo: U; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D21<T extends U, U extends V, V extends Date> extends C3<V> {
         [x: string]: V;
         foo: U;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     // test if V is a subtype of T, U, V, Date
@@ -216,6 +287,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D22<T extends U, U extends V, V extends Date> extends C3<Date> {
         [x: string]: Date;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D23<T extends U, U extends V, V extends Date> extends C3<T> {
@@ -229,6 +302,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     'V' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
 !!! error TS2416:       Type 'Date' is not assignable to type 'T'.
 !!! error TS2416:         'Date' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D24<T extends U, U extends V, V extends Date> extends C3<U> {
@@ -242,11 +317,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     'V' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Date'.
 !!! error TS2416:       Type 'Date' is not assignable to type 'U'.
 !!! error TS2416:         'Date' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Date'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D25<T extends U, U extends V, V extends Date> extends C3<V> {
         [x: string]: V;
         foo: V; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     // test if Date is a subtype of T, U, V, Date
@@ -254,6 +333,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D26<T extends U, U extends V, V extends Date> extends C3<Date> {
         [x: string]: Date;
         foo: Date; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<Date>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D27<T extends U, U extends V, V extends Date> extends C3<T> {
@@ -265,6 +346,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D27<T, U, V>' is not assignable to the same property in base type 'C3<T>'.
 !!! error TS2416:   Type 'Date' is not assignable to type 'T'.
 !!! error TS2416:     'Date' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Date'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D28<T extends U, U extends V, V extends Date> extends C3<U> {
@@ -276,6 +359,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D28<T, U, V>' is not assignable to the same property in base type 'C3<U>'.
 !!! error TS2416:   Type 'Date' is not assignable to type 'U'.
 !!! error TS2416:     'Date' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Date'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D29<T extends U, U extends V, V extends Date> extends C3<V> {
@@ -287,4 +372,6 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D29<T, U, V>' is not assignable to the same property in base type 'C3<V>'.
 !!! error TS2416:   Type 'Date' is not assignable to type 'V'.
 !!! error TS2416:     'Date' is assignable to the constraint of type 'V', but 'V' could be instantiated with a different subtype of constraint 'Date'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'C3<V>'. Add a 'declare' modifier or an initializer to avoid this.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/subtypesOfTypeParameterWithConstraints4.errors.txt b/tests/baselines/reference/subtypesOfTypeParameterWithConstraints4.errors.txt
index 03656c08c4ef2..d951b47e4cff9 100644
--- a/tests/baselines/reference/subtypesOfTypeParameterWithConstraints4.errors.txt
+++ b/tests/baselines/reference/subtypesOfTypeParameterWithConstraints4.errors.txt
@@ -1,27 +1,36 @@
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(37,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<Foo>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(42,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<Foo>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(47,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'Foo'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(47,5): error TS2416: Property 'foo' in type 'D3<T, U, V>' is not assignable to the same property in base type 'B1<Foo>'.
   Type 'V' is not assignable to type 'Foo'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(47,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<Foo>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(52,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(57,5): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(57,5): error TS2416: Property 'foo' in type 'D5<T, U, V>' is not assignable to the same property in base type 'B1<T>'.
   Type 'U' is not assignable to type 'T'.
     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Foo'.
       Type 'Foo' is not assignable to type 'T'.
         'Foo' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Foo'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(57,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(62,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(62,5): error TS2416: Property 'foo' in type 'D6<T, U, V>' is not assignable to the same property in base type 'B1<T>'.
   Type 'V' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(62,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(67,5): error TS2411: Property 'foo' of type 'T' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(67,5): error TS2416: Property 'foo' in type 'D7<T, U, V>' is not assignable to the same property in base type 'B1<U>'.
   Type 'T' is not assignable to type 'U'.
     'T' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Foo'.
       Type 'Foo' is not assignable to type 'U'.
         'Foo' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Foo'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(67,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(72,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(77,5): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(77,5): error TS2416: Property 'foo' in type 'D9<T, U, V>' is not assignable to the same property in base type 'B1<U>'.
   Type 'V' is not assignable to type 'U'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts(77,5): error TS2610: Property 'foo' will overwrite the base property in 'B1<U>'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts (10 errors) ====
+==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithConstraints4.ts (19 errors) ====
     // checking whether other types are subtypes of type parameters with constraints
     
     class Foo { foo: number; }
@@ -59,11 +68,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
     class D1<T extends Foo, U extends Foo, V> extends B1<Foo> {
         [x: string]: Foo;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<Foo>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D2<T extends Foo, U extends Foo, V> extends B1<Foo> {
         [x: string]: Foo;
         foo: U; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<Foo>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D3<T extends Foo, U extends Foo, V> extends B1<Foo> {
@@ -74,11 +87,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
         ~~~
 !!! error TS2416: Property 'foo' in type 'D3<T, U, V>' is not assignable to the same property in base type 'B1<Foo>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'Foo'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<Foo>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D4<T extends Foo, U extends Foo, V> extends B1<T> {
         [x: string]: T;
         foo: T; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D5<T extends Foo, U extends Foo, V> extends B1<T> {
@@ -92,6 +109,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Foo'.
 !!! error TS2416:       Type 'Foo' is not assignable to type 'T'.
 !!! error TS2416:         'Foo' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Foo'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D6<T extends Foo, U extends Foo, V> extends B1<T> {
@@ -102,6 +121,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
         ~~~
 !!! error TS2416: Property 'foo' in type 'D6<T, U, V>' is not assignable to the same property in base type 'B1<T>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'T'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<T>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D7<T extends Foo, U extends Foo, V> extends B1<U> {
@@ -115,11 +136,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     'T' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Foo'.
 !!! error TS2416:       Type 'Foo' is not assignable to type 'U'.
 !!! error TS2416:         'Foo' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint 'Foo'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D8<T extends Foo, U extends Foo, V> extends B1<U> {
         [x: string]: U;
         foo: U; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class D9<T extends Foo, U extends Foo, V> extends B1<U> {
@@ -130,4 +155,6 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
         ~~~
 !!! error TS2416: Property 'foo' in type 'D9<T, U, V>' is not assignable to the same property in base type 'B1<U>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'U'.
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'B1<U>'. Add a 'declare' modifier or an initializer to avoid this.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/subtypesOfTypeParameterWithRecursiveConstraints.errors.txt b/tests/baselines/reference/subtypesOfTypeParameterWithRecursiveConstraints.errors.txt
index 700076db4d190..f1c67351d3370 100644
--- a/tests/baselines/reference/subtypesOfTypeParameterWithRecursiveConstraints.errors.txt
+++ b/tests/baselines/reference/subtypesOfTypeParameterWithRecursiveConstraints.errors.txt
@@ -1,66 +1,84 @@
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(63,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(68,9): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(68,9): error TS2416: Property 'foo' in type 'D2<T, U, V>' is not assignable to the same property in base type 'Base<T>'.
   Type 'U' is not assignable to type 'T'.
     Type 'Foo<T>' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(68,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(73,9): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(73,9): error TS2416: Property 'foo' in type 'D3<T, U, V>' is not assignable to the same property in base type 'Base<T>'.
   Type 'V' is not assignable to type 'T'.
     Type 'Foo<V>' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(73,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(78,9): error TS2411: Property 'foo' of type 'T' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(78,9): error TS2416: Property 'foo' in type 'D4<T, U, V>' is not assignable to the same property in base type 'Base<U>'.
   Type 'T' is not assignable to type 'U'.
     Type 'Foo<U>' is not assignable to type 'U'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(78,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<U>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(83,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(88,9): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(88,9): error TS2416: Property 'foo' in type 'D6<T, U, V>' is not assignable to the same property in base type 'Base<U>'.
   Type 'V' is not assignable to type 'U'.
     Type 'Foo<V>' is not assignable to type 'U'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(88,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(93,9): error TS2411: Property 'foo' of type 'T' is not assignable to string index type 'V'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(93,9): error TS2416: Property 'foo' in type 'D7<T, U, V>' is not assignable to the same property in base type 'Base<V>'.
   Type 'T' is not assignable to type 'V'.
     Type 'Foo<U>' is not assignable to type 'V'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(93,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<V>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(98,9): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'V'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(98,9): error TS2416: Property 'foo' in type 'D8<T, U, V>' is not assignable to the same property in base type 'Base<V>'.
   Type 'U' is not assignable to type 'V'.
     Type 'Foo<T>' is not assignable to type 'V'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(98,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<V>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(103,9): error TS2610: Property 'foo' will overwrite the base property in 'Base<V>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(115,9): error TS2416: Property 'foo' in type 'D1<T, U, V>' is not assignable to the same property in base type 'Base2<T>'.
   Type 'T' is not assignable to type 'Foo<T>'.
     Type 'Foo<U>' is not assignable to type 'Foo<T>'.
       Type 'U' is not assignable to type 'T'.
         Type 'Foo<T>' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(115,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(120,9): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(120,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(125,9): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'T'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(125,9): error TS2416: Property 'foo' in type 'D3<T, U, V>' is not assignable to the same property in base type 'Base2<T>'.
   Type 'V' is not assignable to type 'Foo<T>'.
     Type 'Foo<V>' is not assignable to type 'Foo<T>'.
       Type 'V' is not assignable to type 'T'.
         Type 'Foo<V>' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(125,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<T>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(130,9): error TS2411: Property 'foo' of type 'T' is not assignable to string index type 'U'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(130,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(135,9): error TS2416: Property 'foo' in type 'D5<T, U, V>' is not assignable to the same property in base type 'Base2<U>'.
   Type 'U' is not assignable to type 'Foo<U>'.
     Type 'Foo<T>' is not assignable to type 'Foo<U>'.
       Type 'T' is not assignable to type 'U'.
         Type 'Foo<U>' is not assignable to type 'U'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(135,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(140,9): error TS2411: Property 'foo' of type 'V' is not assignable to string index type 'U'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(140,9): error TS2416: Property 'foo' in type 'D6<T, U, V>' is not assignable to the same property in base type 'Base2<U>'.
   Type 'V' is not assignable to type 'Foo<U>'.
     Type 'Foo<V>' is not assignable to type 'Foo<U>'.
       Type 'V' is not assignable to type 'U'.
         Type 'Foo<V>' is not assignable to type 'U'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(140,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<U>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(145,9): error TS2411: Property 'foo' of type 'T' is not assignable to string index type 'V'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(145,9): error TS2416: Property 'foo' in type 'D7<T, U, V>' is not assignable to the same property in base type 'Base2<V>'.
   Type 'T' is not assignable to type 'Foo<V>'.
     Type 'Foo<U>' is not assignable to type 'Foo<V>'.
       Type 'U' is not assignable to type 'V'.
         Type 'Foo<T>' is not assignable to type 'V'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(145,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<V>'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(150,9): error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'V'.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(150,9): error TS2416: Property 'foo' in type 'D8<T, U, V>' is not assignable to the same property in base type 'Base2<V>'.
   Type 'U' is not assignable to type 'Foo<V>'.
     Type 'Foo<T>' is not assignable to type 'Foo<V>'.
       Type 'T' is not assignable to type 'V'.
         Type 'Foo<U>' is not assignable to type 'V'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(150,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<V>'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts(155,9): error TS2610: Property 'foo' will overwrite the base property in 'Base2<V>'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts (24 errors) ====
+==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOfTypeParameterWithRecursiveConstraints.ts (42 errors) ====
     // checking whether other types are subtypes of type parameters with constraints
     
     class Foo<T> { foo: T; }
@@ -124,6 +142,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
         class D1<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<T> {
             [x: string]: T;
             foo: T
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<T>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D2<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<T> {
@@ -135,6 +155,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D2<T, U, V>' is not assignable to the same property in base type 'Base<T>'.
 !!! error TS2416:   Type 'U' is not assignable to type 'T'.
 !!! error TS2416:     Type 'Foo<T>' is not assignable to type 'T'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<T>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D3<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<T> {
@@ -146,6 +168,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D3<T, U, V>' is not assignable to the same property in base type 'Base<T>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'T'.
 !!! error TS2416:     Type 'Foo<V>' is not assignable to type 'T'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<T>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D4<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<U> {
@@ -157,11 +181,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D4<T, U, V>' is not assignable to the same property in base type 'Base<U>'.
 !!! error TS2416:   Type 'T' is not assignable to type 'U'.
 !!! error TS2416:     Type 'Foo<U>' is not assignable to type 'U'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<U>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D5<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<U> {
             [x: string]: U;
             foo: U
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<U>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D6<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<U> {
@@ -173,6 +201,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D6<T, U, V>' is not assignable to the same property in base type 'Base<U>'.
 !!! error TS2416:   Type 'V' is not assignable to type 'U'.
 !!! error TS2416:     Type 'Foo<V>' is not assignable to type 'U'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<U>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D7<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<V> {
@@ -184,6 +214,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D7<T, U, V>' is not assignable to the same property in base type 'Base<V>'.
 !!! error TS2416:   Type 'T' is not assignable to type 'V'.
 !!! error TS2416:     Type 'Foo<U>' is not assignable to type 'V'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<V>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D8<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<V> {
@@ -195,11 +227,15 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416: Property 'foo' in type 'D8<T, U, V>' is not assignable to the same property in base type 'Base<V>'.
 !!! error TS2416:   Type 'U' is not assignable to type 'V'.
 !!! error TS2416:     Type 'Foo<T>' is not assignable to type 'V'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<V>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D9<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base<V> {
             [x: string]: V;
             foo: V
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base<V>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     }
     
@@ -218,6 +254,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     Type 'Foo<U>' is not assignable to type 'Foo<T>'.
 !!! error TS2416:       Type 'U' is not assignable to type 'T'.
 !!! error TS2416:         Type 'Foo<T>' is not assignable to type 'T'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<T>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D2<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<T> {
@@ -225,6 +263,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
             foo: U
             ~~~
 !!! error TS2411: Property 'foo' of type 'U' is not assignable to string index type 'T'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<T>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D3<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<T> {
@@ -238,6 +278,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     Type 'Foo<V>' is not assignable to type 'Foo<T>'.
 !!! error TS2416:       Type 'V' is not assignable to type 'T'.
 !!! error TS2416:         Type 'Foo<V>' is not assignable to type 'T'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<T>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D4<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<U> {
@@ -245,6 +287,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
             foo: T
             ~~~
 !!! error TS2411: Property 'foo' of type 'T' is not assignable to string index type 'U'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<U>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D5<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<U> {
@@ -256,6 +300,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     Type 'Foo<T>' is not assignable to type 'Foo<U>'.
 !!! error TS2416:       Type 'T' is not assignable to type 'U'.
 !!! error TS2416:         Type 'Foo<U>' is not assignable to type 'U'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<U>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D6<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<U> {
@@ -269,6 +315,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     Type 'Foo<V>' is not assignable to type 'Foo<U>'.
 !!! error TS2416:       Type 'V' is not assignable to type 'U'.
 !!! error TS2416:         Type 'Foo<V>' is not assignable to type 'U'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<U>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D7<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<V> {
@@ -282,6 +330,8 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     Type 'Foo<U>' is not assignable to type 'Foo<V>'.
 !!! error TS2416:       Type 'U' is not assignable to type 'V'.
 !!! error TS2416:         Type 'Foo<T>' is not assignable to type 'V'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<V>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D8<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<V> {
@@ -295,10 +345,14 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypesOf
 !!! error TS2416:     Type 'Foo<T>' is not assignable to type 'Foo<V>'.
 !!! error TS2416:       Type 'T' is not assignable to type 'V'.
 !!! error TS2416:         Type 'Foo<U>' is not assignable to type 'V'.
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<V>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class D9<T extends Foo<U>, U extends Foo<T>, V extends Foo<V>> extends Base2<V> {
             [x: string]: V;
             foo: V
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base2<V>'. Add a 'declare' modifier or an initializer to avoid this.
         }
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/subtypingTransitivity.errors.txt b/tests/baselines/reference/subtypingTransitivity.errors.txt
new file mode 100644
index 0000000000000..aed2b0a7220b7
--- /dev/null
+++ b/tests/baselines/reference/subtypingTransitivity.errors.txt
@@ -0,0 +1,28 @@
+tests/cases/compiler/subtypingTransitivity.ts(6,12): error TS2610: Property 'x' will overwrite the base property in 'B'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/compiler/subtypingTransitivity.ts(9,12): error TS2610: Property 'x' will overwrite the base property in 'B'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/compiler/subtypingTransitivity.ts (2 errors) ====
+    class B {
+        x: Object;
+    }
+    
+    class D extends B {
+        public x: string;
+               ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'B'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    class D2 extends B {
+        public x: number;
+               ~
+!!! error TS2610: Property 'x' will overwrite the base property in 'B'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    var b: B;
+    var d: D;
+    var d2: D2;
+    
+    d.x = '';
+    b = d;
+    b.x = 1; // assigned number to string
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/subtypingWithObjectMembers.errors.txt b/tests/baselines/reference/subtypingWithObjectMembers.errors.txt
index 1e2f7bc9114f1..140fef4236192 100644
--- a/tests/baselines/reference/subtypingWithObjectMembers.errors.txt
+++ b/tests/baselines/reference/subtypingWithObjectMembers.errors.txt
@@ -1,18 +1,30 @@
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(13,5): error TS2610: Property 'foo' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(14,5): error TS2416: Property 'bar' in type 'B' is not assignable to the same property in base type 'A'.
   Type 'string' is not assignable to type 'Base'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(14,5): error TS2610: Property 'bar' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(23,5): error TS2610: Property '1' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(24,5): error TS2416: Property '2' in type 'B2' is not assignable to the same property in base type 'A2'.
   Type 'string' is not assignable to type 'Base'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(24,5): error TS2610: Property '2.0' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(33,5): error TS2610: Property ''1'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(34,5): error TS2416: Property ''2.0'' in type 'B3' is not assignable to the same property in base type 'A3'.
   Type 'string' is not assignable to type 'Base'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(34,5): error TS2610: Property ''2.0'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(44,9): error TS2610: Property 'foo' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(45,9): error TS2416: Property 'bar' in type 'B' is not assignable to the same property in base type 'A'.
   Type 'string' is not assignable to type 'Base'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(45,9): error TS2610: Property 'bar' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(54,9): error TS2610: Property '1' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(55,9): error TS2416: Property '2' in type 'B2' is not assignable to the same property in base type 'A2'.
   Type 'string' is not assignable to type 'Base'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(55,9): error TS2610: Property '2.0' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(64,9): error TS2610: Property ''1'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
 tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(65,9): error TS2416: Property ''2.0'' in type 'B3' is not assignable to the same property in base type 'A3'.
   Type 'string' is not assignable to type 'Base'.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts(65,9): error TS2610: Property ''2.0'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
 
 
-==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts (6 errors) ====
+==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingWithObjectMembers.ts (18 errors) ====
     class Base { foo: string; }
     class Derived extends Base { bar: string; }
     class Derived2 extends Derived { baz: string; }
@@ -26,10 +38,14 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingW
     
     class B extends A {
         foo: Derived; // ok
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
         bar: string; // error
         ~~~
 !!! error TS2416: Property 'bar' in type 'B' is not assignable to the same property in base type 'A'.
 !!! error TS2416:   Type 'string' is not assignable to type 'Base'.
+        ~~~
+!!! error TS2610: Property 'bar' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class A2 {
@@ -39,10 +55,14 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingW
     
     class B2 extends A2 {
         1: Derived; // ok
+        ~
+!!! error TS2610: Property '1' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
         2: string; // error
         ~
 !!! error TS2416: Property '2' in type 'B2' is not assignable to the same property in base type 'A2'.
 !!! error TS2416:   Type 'string' is not assignable to type 'Base'.
+        ~
+!!! error TS2610: Property '2.0' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     class A3 {
@@ -52,10 +72,14 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingW
     
     class B3 extends A3 {
         '1': Derived; // ok
+        ~~~
+!!! error TS2610: Property ''1'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
         '2.0': string; // error
         ~~~~~
 !!! error TS2416: Property ''2.0'' in type 'B3' is not assignable to the same property in base type 'A3'.
 !!! error TS2416:   Type 'string' is not assignable to type 'Base'.
+        ~~~~~
+!!! error TS2610: Property ''2.0'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
     }
     
     module TwoLevels {
@@ -66,10 +90,14 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingW
     
         class B extends A {
             foo: Derived2; // ok
+            ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
             bar: string; // error
             ~~~
 !!! error TS2416: Property 'bar' in type 'B' is not assignable to the same property in base type 'A'.
 !!! error TS2416:   Type 'string' is not assignable to type 'Base'.
+            ~~~
+!!! error TS2610: Property 'bar' will overwrite the base property in 'A'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class A2 {
@@ -79,10 +107,14 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingW
     
         class B2 extends A2 {
             1: Derived2; // ok
+            ~
+!!! error TS2610: Property '1' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
             2: string; // error
             ~
 !!! error TS2416: Property '2' in type 'B2' is not assignable to the same property in base type 'A2'.
 !!! error TS2416:   Type 'string' is not assignable to type 'Base'.
+            ~
+!!! error TS2610: Property '2.0' will overwrite the base property in 'A2'. Add a 'declare' modifier or an initializer to avoid this.
         }
     
         class A3 {
@@ -92,9 +124,13 @@ tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/subtypingW
     
         class B3 extends A3 {
             '1': Derived2; // ok
+            ~~~
+!!! error TS2610: Property ''1'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
             '2.0': string; // error
             ~~~~~
 !!! error TS2416: Property ''2.0'' in type 'B3' is not assignable to the same property in base type 'A3'.
 !!! error TS2416:   Type 'string' is not assignable to type 'Base'.
+            ~~~~~
+!!! error TS2610: Property ''2.0'' will overwrite the base property in 'A3'. Add a 'declare' modifier or an initializer to avoid this.
         }
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/tsxGenericAttributesType5.errors.txt b/tests/baselines/reference/tsxGenericAttributesType5.errors.txt
new file mode 100644
index 0000000000000..602526de4dfcc
--- /dev/null
+++ b/tests/baselines/reference/tsxGenericAttributesType5.errors.txt
@@ -0,0 +1,19 @@
+tests/cases/conformance/jsx/file.tsx(9,5): error TS2610: Property 'props' will overwrite the base property in 'Component<U, {}>'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/jsx/file.tsx (1 errors) ====
+    import React = require('react');
+    
+    class B1<T extends { x: string }> extends React.Component<T, {}> {
+        render() {
+            return <div>hi</div>; 
+        }
+    }
+    class B<U> extends React.Component<U, {}> {
+        props: U;
+        ~~~~~
+!!! error TS2610: Property 'props' will overwrite the base property in 'Component<U, {}>'. Add a 'declare' modifier or an initializer to avoid this.
+        render() {
+            return <B1 {...this.props} x="hi" />;
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/tsxGenericAttributesType6.errors.txt b/tests/baselines/reference/tsxGenericAttributesType6.errors.txt
new file mode 100644
index 0000000000000..a75ae10b22a2d
--- /dev/null
+++ b/tests/baselines/reference/tsxGenericAttributesType6.errors.txt
@@ -0,0 +1,19 @@
+tests/cases/conformance/jsx/file.tsx(9,5): error TS2610: Property 'props' will overwrite the base property in 'Component<U, {}>'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/jsx/file.tsx (1 errors) ====
+    import React = require('react');
+    
+    class B1<T extends { x: string } = { x:string } > extends React.Component<T, {}> {
+        render() {
+            return <div>hi</div>; 
+        }
+    }
+    class B<U> extends React.Component<U, {}> {
+        props: U;
+        ~~~~~
+!!! error TS2610: Property 'props' will overwrite the base property in 'Component<U, {}>'. Add a 'declare' modifier or an initializer to avoid this.
+        render() {
+            return <B1 {...this.props} x="hi" />;
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/undefinedIsSubtypeOfEverything.errors.txt b/tests/baselines/reference/undefinedIsSubtypeOfEverything.errors.txt
new file mode 100644
index 0000000000000..021d240b8a533
--- /dev/null
+++ b/tests/baselines/reference/undefinedIsSubtypeOfEverything.errors.txt
@@ -0,0 +1,190 @@
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(8,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(12,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(16,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(20,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(25,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(29,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(34,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(38,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(43,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(47,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(52,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(56,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(61,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(68,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(73,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(78,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(86,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(95,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(100,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(105,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(114,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts(119,5): error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+
+
+==== tests/cases/conformance/types/typeRelationships/subtypesAndSuperTypes/undefinedIsSubtypeOfEverything.ts (22 errors) ====
+    // undefined is a subtype of every other types, no errors expected below
+    
+    class Base {
+        foo: typeof undefined;
+    } 
+    
+    class D0 extends Base {
+        foo: any;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class DA extends Base {
+        foo: typeof undefined; 
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class D1 extends Base {
+        foo: string;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class D1A extends Base {
+        foo: String;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D2 extends Base {
+        foo: number;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class D2A extends Base {
+        foo: Number;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D3 extends Base {
+        foo: boolean;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class D3A extends Base {
+        foo: Boolean;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D4 extends Base {
+        foo: RegExp;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class D5 extends Base {
+        foo: Date;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D6 extends Base {
+        foo: number[];
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    class D7 extends Base {
+        foo: { bar: number };
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D8 extends Base {
+        foo: D7;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    interface I1 {
+        bar: string;
+    }
+    class D9 extends Base {
+        foo: I1;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D10 extends Base {
+        foo: () => number;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    enum E { A }
+    class D11 extends Base {
+        foo: E;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    function f() { }
+    module f {
+        export var bar = 1;
+    }
+    class D12 extends Base {
+        foo: typeof f;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class c { baz: string }
+    module c {
+        export var bar = 1;
+    }
+    class D13 extends Base {
+        foo: typeof c;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D14<T> extends Base {
+        foo: T;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D15<T, U> extends Base {
+        foo: U;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    //class D15<T, U extends T> extends Base {
+    //    foo: U;
+    //}
+    
+    
+    class D16 extends Base {
+        foo: Object;
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
+    
+    class D17 extends Base {
+        foo: {};
+        ~~~
+!!! error TS2610: Property 'foo' will overwrite the base property in 'Base'. Add a 'declare' modifier or an initializer to avoid this.
+    }
+    
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts b/tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts
new file mode 100644
index 0000000000000..e2d7494dd6891
--- /dev/null
+++ b/tests/cases/conformance/classes/propertyMemberDeclarations/derivedUninitializedPropertyDeclaration.ts
@@ -0,0 +1,67 @@
+// @strict: true
+class A {
+    property = 'x';
+    m() { return 1 }
+}
+class B extends A {
+    property: any; // error
+}
+class BD extends A {
+    declare property: any; // ok because it's implicitly initialised
+}
+class BDBang extends A {
+    declare property!: any; // ! is not allowed, this is an ambient declaration
+}
+class BOther extends A {
+    declare m() { return 2 } // not allowed on methods
+    declare nonce: any; // ok, even though it's not in the base
+    declare property = 'y' // initialiser not allowed with declare
+}
+class U {
+    declare nonce: any; // ok, even though there's no base
+}
+
+class C {
+    p: string;
+}
+class D extends C {
+    p: 'hi'; // error
+}
+class DD extends C {
+    declare p: 'bye'; // ok
+}
+
+
+declare class E {
+    p1: string
+    p2: string
+}
+class F extends E {
+    p1!: 'z'
+    declare p2: 'alpha'
+}
+
+class G extends E {
+    p1: 'z'
+    constructor() {
+        super()
+        this.p1 = 'z'
+    }
+}
+
+abstract class H extends E {
+    abstract p1: 'a' | 'b' | 'c'
+    declare abstract p2: 'a' | 'b' | 'c'
+}
+
+interface I {
+    q: number
+}
+interface J extends I { }
+class J {
+    r = 5
+}
+class K extends J {
+    q!: 1 | 2 | 3 // ok, extends a property from an interface
+    r!: 4 | 5 // error, from class
+}
diff --git a/tests/cases/fourslash/codeFixAddMissingDeclareProperty.ts b/tests/cases/fourslash/codeFixAddMissingDeclareProperty.ts
new file mode 100644
index 0000000000000..90dcbdd86ba8d
--- /dev/null
+++ b/tests/cases/fourslash/codeFixAddMissingDeclareProperty.ts
@@ -0,0 +1,18 @@
+/// <reference path='fourslash.ts' />
+
+////class B {
+////    p = 1
+////}
+////class C extends B {
+////    p: number
+////}
+
+verify.codeFix({
+    description: "Prefix with 'declare'",
+    newFileContent: `class B {
+    p = 1
+}
+class C extends B {
+    declare p: number
+}`
+});
diff --git a/tests/cases/fourslash/codeFixAddMissingDeclareProperty2.ts b/tests/cases/fourslash/codeFixAddMissingDeclareProperty2.ts
new file mode 100644
index 0000000000000..32dc21d9a2753
--- /dev/null
+++ b/tests/cases/fourslash/codeFixAddMissingDeclareProperty2.ts
@@ -0,0 +1,25 @@
+/// <reference path='fourslash.ts' />
+
+////class B {
+////    p = 1
+////}
+////class C extends B {
+////    p: number
+////}
+////class D extends B {
+////    p: 1 | 2 | 3
+////}
+
+verify.codeFixAll({
+    fixId: "addMissingDeclareProperty",
+    fixAllDescription: "Prefix all incorrect property declarations with 'declare'",
+    newFileContent: `class B {
+    p = 1
+}
+class C extends B {
+    declare p: number
+}
+class D extends B {
+    declare p: 1 | 2 | 3
+}`
+});
diff --git a/tests/cases/fourslash/derivedTypeIndexerWithGenericConstraints.ts b/tests/cases/fourslash/derivedTypeIndexerWithGenericConstraints.ts
index 21489b5d8aa9b..82dfe9429e1b2 100644
--- a/tests/cases/fourslash/derivedTypeIndexerWithGenericConstraints.ts
+++ b/tests/cases/fourslash/derivedTypeIndexerWithGenericConstraints.ts
@@ -13,7 +13,7 @@
 ////}
 
 ////class DbSet<TEntity extends Entity> extends BaseCollection<TEntity> { // error
-////    _itemsByKey: { [key: string]: TEntity; };
+////    _itemsByKey: { [key: string]: TEntity; } = {};
 ////}
 
 ////var a: BaseCollection<CollectionItem>;
diff --git a/tests/cases/fourslash/incompatibleOverride.ts b/tests/cases/fourslash/incompatibleOverride.ts
index 0b3dd3230154c..1ce0b07def189 100644
--- a/tests/cases/fourslash/incompatibleOverride.ts
+++ b/tests/cases/fourslash/incompatibleOverride.ts
@@ -3,8 +3,8 @@
 // Squiggle for implementing a derived class with an incompatible override is too large
 
 //// class Foo { xyz: string; }
-//// class Bar extends Foo { /*1*/xyz/*2*/: number; }
-//// class Baz extends Foo { public /*3*/xyz/*4*/: number; }
+//// class Bar extends Foo { /*1*/xyz/*2*/: number = 1; }
+//// class Baz extends Foo { public /*3*/xyz/*4*/: number = 2; }
 //// class /*5*/Baf/*6*/ extends Foo {
 ////    constructor(public xyz: number) {
 ////       super();
@@ -14,4 +14,4 @@
 verify.errorExistsBetweenMarkers('1', '2');
 verify.errorExistsBetweenMarkers('3', '4');
 verify.errorExistsBetweenMarkers('5', '6');
-verify.numberOfErrorsInCurrentFile(3);
\ No newline at end of file
+verify.numberOfErrorsInCurrentFile(3);
diff --git a/tests/cases/fourslash/squiggleIllegalSubclassOverride.ts b/tests/cases/fourslash/squiggleIllegalSubclassOverride.ts
index 955d461b9f13a..c97c6c75fcf5b 100644
--- a/tests/cases/fourslash/squiggleIllegalSubclassOverride.ts
+++ b/tests/cases/fourslash/squiggleIllegalSubclassOverride.ts
@@ -5,8 +5,8 @@
 ////}
 ////
 ////class Bar extends Foo {
-////    public /*1*/x/*2*/: string;
+////    public /*1*/x/*2*/: string = 'hi';
 ////}
 
 verify.errorExistsBetweenMarkers("1", "2");
-verify.numberOfErrorsInCurrentFile(1);
\ No newline at end of file
+verify.numberOfErrorsInCurrentFile(1);