Check for class expressions as well as class declarations

src/compiler/checker.ts

@@ -12625,6 +12625,10 @@ namespace ts {
             return s.flags & SymbolFlags.Instantiated ? getSymbolLinks(s).target : s;
         }
 
+        function getClassLikeDeclarationOfSymbol(symbol: Symbol): Declaration {
+            return forEach(symbol.declarations, d => isClassLike(d) ? d : undefined);
+        }
+
         function checkKindsOfPropertyMemberOverrides(type: InterfaceType, baseType: ObjectType): void {
 
             // TypeScript 1.0 spec (April 2014): 8.2.3
@@ -12662,14 +12666,20 @@ namespace ts {
                     if (derived === base) {
                         // derived class inherits base without override/redeclaration
 
-                        let derivedClassDecl = getDeclarationOfKind(type.symbol, SyntaxKind.ClassDeclaration);
+                        let derivedClassDecl = getClassLikeDeclarationOfSymbol(type.symbol);
 
                         // It is an error to inherit an abstract member without implementing it or being declared abstract.
                         // If there is no declaration for the derived class (as in the case of class expressions),
                         // then the class cannot be declared abstract.
-                        if ( baseDeclarationFlags & NodeFlags.Abstract && (!derivedClassDecl || !(derivedClassDecl.flags & NodeFlags.Abstract))) {
-                            error(derivedClassDecl, Diagnostics.Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2,
-                                typeToString(type), symbolToString(baseProperty), typeToString(baseType));
+                        if (baseDeclarationFlags & NodeFlags.Abstract && (!derivedClassDecl || !(derivedClassDecl.flags & NodeFlags.Abstract))) {
+                            if (derivedClassDecl.kind === SyntaxKind.ClassExpression) {
+                                error(derivedClassDecl, Diagnostics.Non_abstract_class_expression_does_not_implement_inherited_abstract_member_0_from_class_1,
+                                    symbolToString(baseProperty), typeToString(baseType));
+                            }
+                            else {
+                                error(derivedClassDecl, Diagnostics.Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2,
+                                    typeToString(type), symbolToString(baseProperty), typeToString(baseType));
+                            }
                         }
                     }
                     else {

src/compiler/diagnosticInformationMap.generated.ts

@@ -427,6 +427,7 @@ namespace ts {
         Cannot_emit_namespaced_JSX_elements_in_React: { code: 2650, category: DiagnosticCategory.Error, key: "Cannot emit namespaced JSX elements in React" },
         A_member_initializer_in_a_enum_declaration_cannot_reference_members_declared_after_it_including_members_defined_in_other_enums: { code: 2651, category: DiagnosticCategory.Error, key: "A member initializer in a enum declaration cannot reference members declared after it, including members defined in other enums." },
         Merged_declaration_0_cannot_include_a_default_export_declaration_Consider_adding_a_separate_export_default_0_declaration_instead: { code: 2652, category: DiagnosticCategory.Error, key: "Merged declaration '{0}' cannot include a default export declaration. Consider adding a separate 'export default {0}' declaration instead." },
+        Non_abstract_class_expression_does_not_implement_inherited_abstract_member_0_from_class_1: { code: 2653, category: DiagnosticCategory.Error, key: "Non-abstract class expression does not implement inherited abstract member '{0}' from class '{1}'." },
         Import_declaration_0_is_using_private_name_1: { code: 4000, category: DiagnosticCategory.Error, key: "Import declaration '{0}' is using private name '{1}'." },
         Type_parameter_0_of_exported_class_has_or_is_using_private_name_1: { code: 4002, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported class has or is using private name '{1}'." },
         Type_parameter_0_of_exported_interface_has_or_is_using_private_name_1: { code: 4004, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported interface has or is using private name '{1}'." },

src/compiler/diagnosticMessages.json

@@ -1692,11 +1692,16 @@
     "A member initializer in a enum declaration cannot reference members declared after it, including members defined in other enums.": {
         "category": "Error",
         "code": 2651
-    },    
+    },
     "Merged declaration '{0}' cannot include a default export declaration. Consider adding a separate 'export default {0}' declaration instead.": {
         "category": "Error",
         "code": 2652
     },
+    "Non-abstract class expression does not implement inherited abstract member '{0}' from class '{1}'.": {
+        "category": "Error",
+        "code": 2653
+    },
+
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000

tests/baselines/reference/classExpressionExtendingAbstractClass.errors.txt

@@ -0,0 +1,14 @@
+tests/cases/compiler/classExpressionExtendingAbstractClass.ts(5,9): error TS2653: Non-abstract class expression does not implement inherited abstract member 'foo' from class 'A'.
+
+
+==== tests/cases/compiler/classExpressionExtendingAbstractClass.ts (1 errors) ====
+    abstract class A {
+        abstract foo(): void;
+    }
+    
+    var C = class extends A {     // no error reported!
+            ~~~~~
+!!! error TS2653: Non-abstract class expression does not implement inherited abstract member 'foo' from class 'A'.
+    };
+    
+    

tests/baselines/reference/classExpressionExtendingAbstractClass.js

@@ -0,0 +1,28 @@
+//// [classExpressionExtendingAbstractClass.ts]
+abstract class A {
+    abstract foo(): void;
+}
+
+var C = class extends A {     // no error reported!
+};
+
+
+
+//// [classExpressionExtendingAbstractClass.js]
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var A = (function () {
+    function A() {
+    }
+    return A;
+})();
+var C = (function (_super) {
+    __extends(class_1, _super);
+    function class_1() {
+        _super.apply(this, arguments);
+    }
+    return class_1;
+})(A);

tests/cases/compiler/classExpressionExtendingAbstractClass.ts

@@ -0,0 +1,7 @@
+abstract class A {
+    abstract foo(): void;
+}
+
+var C = class extends A {     // no error reported!
+};
+