Add override keyword

src/compiler/binder.ts

@@ -3104,6 +3104,7 @@ namespace ts {
             case SyntaxKind.TypeAssertionExpression:
             case SyntaxKind.AsExpression:
             case SyntaxKind.NonNullExpression:
+            case SyntaxKind.OverrideKeyword:
             case SyntaxKind.ReadonlyKeyword:
                 // These nodes are TypeScript syntax.
                 transformFlags |= TransformFlags.AssertTypeScript;

src/compiler/commandLineParser.ts

@@ -144,6 +144,11 @@ namespace ts {
             type: "boolean",
             description: Diagnostics.Raise_error_on_expressions_and_declarations_with_an_implied_any_type,
         },
+        {
+            name: "noImplicitOverride",
+            type: "boolean",
+            description: Diagnostics.Raise_an_error_when_inherited_Slashoverridden_class_members_are_not_explicitly_marked_override,
+        },
         {
             name: "noImplicitThis",
             type: "boolean",

src/compiler/declarationEmitter.ts

@@ -727,6 +727,9 @@ namespace ts {
             if (flags & ModifierFlags.Static) {
                 write("static ");
             }
+            if (flags & ModifierFlags.Override) {
+                write("override ");
+            }
             if (flags & ModifierFlags.Readonly) {
                 write("readonly ");
             }

src/compiler/diagnosticMessages.json

@@ -767,7 +767,7 @@
         "category": "Error",
         "code": 1241
     },
-    "'abstract' modifier can only appear on a class, method, or property declaration.": {
+    "'{0}' modifier can only appear on a class, method, or property declaration.": {
         "category": "Error",
         "code": 1242
     },
@@ -1683,6 +1683,10 @@
         "category": "Error",
         "code": 2518
     },
+    "Overload signatures must all be override or non-override.": {
+        "category": "Error",
+        "code": 2519
+    },
     "Duplicate identifier '{0}'. Compiler uses declaration '{1}' to support async functions.": {
         "category": "Error",
         "code": 2520
@@ -2015,6 +2019,10 @@
         "category": "Error",
         "code": 2698
     },
+    "Accessors must both be override or non-override.": {
+        "category": "Error",
+        "code": 2699
+    },
     "Rest types may only be created from object types.": {
         "category": "Error",
         "code": 2700
@@ -3279,6 +3287,30 @@
         "category": "Message",
         "code": 90015
     },
+    "Raise an error when inherited/overridden class members are not explicitly marked 'override'": {
+        "category": "Error",
+        "code": 90030
+    },
+    "Class member '{0}' was marked 'override', but no matching declaration was found in any supertype of '{1}'": {
+        "category": "Error",
+        "code": 90032
+    },
+    "Class member '{0}' must be marked 'override' when noImplicitOverride is enabled (inherited from {1})": {
+        "category": "Error",
+        "code": 90033
+    },
+    "Class member '{0}' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.{1})": {
+        "category": "Error",
+        "code": 90034
+    },
+    "override modifier cannot be used with an optional property declaration": {
+        "category": "Error",
+        "code": 90035
+    },
+    "All declarations of an override method must be consecutive.": {
+        "category": "Error",
+        "code": 90036
+    },
     "Octal literal types must use ES2015 syntax. Use the syntax '{0}'.": {
         "category": "Error",
         "code": 8017

src/compiler/emitter.ts

@@ -340,6 +340,7 @@ namespace ts {
                 case SyntaxKind.ModuleKeyword:
                 case SyntaxKind.NamespaceKeyword:
                 case SyntaxKind.NeverKeyword:
+                case SyntaxKind.OverrideKeyword:
                 case SyntaxKind.ReadonlyKeyword:
                 case SyntaxKind.RequireKeyword:
                 case SyntaxKind.NumberKeyword:

src/compiler/parser.ts

@@ -4692,6 +4692,7 @@ namespace ts {
                     case SyntaxKind.PrivateKeyword:
                     case SyntaxKind.ProtectedKeyword:
                     case SyntaxKind.PublicKeyword:
+                    case SyntaxKind.OverrideKeyword:
                     case SyntaxKind.ReadonlyKeyword:
                         nextToken();
                         // ASI takes effect for this modifier.
@@ -4776,6 +4777,7 @@ namespace ts {
                 case SyntaxKind.PublicKeyword:
                 case SyntaxKind.PrivateKeyword:
                 case SyntaxKind.ProtectedKeyword:
+                case SyntaxKind.OverrideKeyword:
                 case SyntaxKind.StaticKeyword:
                 case SyntaxKind.ReadonlyKeyword:
                     // When these don't start a declaration, they may be the start of a class member if an identifier
@@ -4857,6 +4859,7 @@ namespace ts {
                 case SyntaxKind.PrivateKeyword:
                 case SyntaxKind.ProtectedKeyword:
                 case SyntaxKind.PublicKeyword:
+                case SyntaxKind.OverrideKeyword:
                 case SyntaxKind.AbstractKeyword:
                 case SyntaxKind.StaticKeyword:
                 case SyntaxKind.ReadonlyKeyword:
@@ -5154,6 +5157,7 @@ namespace ts {
                 case SyntaxKind.PublicKeyword:
                 case SyntaxKind.PrivateKeyword:
                 case SyntaxKind.ProtectedKeyword:
+                case SyntaxKind.OverrideKeyword:
                 case SyntaxKind.StaticKeyword:
                 case SyntaxKind.ReadonlyKeyword:
                     return true;

src/compiler/program.ts

@@ -1061,6 +1061,7 @@ namespace ts {
                             case SyntaxKind.PublicKeyword:
                             case SyntaxKind.PrivateKeyword:
                             case SyntaxKind.ProtectedKeyword:
+                            case SyntaxKind.OverrideKeyword:
                             case SyntaxKind.ReadonlyKeyword:
                             case SyntaxKind.DeclareKeyword:
                             case SyntaxKind.AbstractKeyword:

src/compiler/scanner.ts

@@ -99,6 +99,7 @@ namespace ts {
         "null": SyntaxKind.NullKeyword,
         "number": SyntaxKind.NumberKeyword,
         "object": SyntaxKind.ObjectKeyword,
+        "override": SyntaxKind.OverrideKeyword,
         "package": SyntaxKind.PackageKeyword,
         "private": SyntaxKind.PrivateKeyword,
         "protected": SyntaxKind.ProtectedKeyword,

src/compiler/transformers/ts.ts

@@ -306,6 +306,7 @@ namespace ts {
                 case SyntaxKind.ConstKeyword:
                 case SyntaxKind.DeclareKeyword:
                 case SyntaxKind.ReadonlyKeyword:
+                case SyntaxKind.OverrideKeyword:
                     // TypeScript accessibility and readonly modifiers are elided.
 
                 case SyntaxKind.ArrayType:

src/compiler/types.ts

@@ -183,6 +183,7 @@ namespace ts {
         UndefinedKeyword,
         FromKeyword,
         GlobalKeyword,
+        OverrideKeyword,
         OfKeyword, // LastKeyword and LastToken
 
         // Parse tree nodes
@@ -453,6 +454,7 @@ namespace ts {
         Abstract =           1 << 7,  // Class/Method/ConstructSignature
         Async =              1 << 8,  // Property/Method/Function
         Default =            1 << 9,  // Function/Class (export default declaration)
+        Override =           1 << 10, // Property/Method
         Const =              1 << 11, // Variable declaration
         HasComputedFlags =   1 << 29, // Modifier flags have been computed
 
@@ -461,7 +463,7 @@ namespace ts {
         ParameterPropertyModifier = AccessibilityModifier | Readonly,
         NonPublicAccessibilityModifier = Private | Protected,
 
-        TypeScriptModifier = Ambient | Public | Private | Protected | Readonly | Abstract | Const,
+        TypeScriptModifier = Ambient | Public | Private | Protected | Override | Readonly | Abstract | Const,
         ExportDefault = Export | Default,
     }
 
@@ -532,6 +534,7 @@ namespace ts {
         | Token<SyntaxKind.PublicKeyword>
         | Token<SyntaxKind.PrivateKeyword>
         | Token<SyntaxKind.ProtectedKeyword>
+        | Token<SyntaxKind.OverrideKeyword>
         | Token<SyntaxKind.ReadonlyKeyword>
         | Token<SyntaxKind.StaticKeyword>
         ;
@@ -3192,6 +3195,7 @@ namespace ts {
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
         noImplicitAny?: boolean;
+        noImplicitOverride?: boolean;
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;
         noUnusedLocals?: boolean;

src/compiler/utilities.ts

@@ -1964,6 +1964,7 @@ namespace ts {
             case SyntaxKind.PublicKeyword:
             case SyntaxKind.PrivateKeyword:
             case SyntaxKind.ProtectedKeyword:
+            case SyntaxKind.OverrideKeyword:
             case SyntaxKind.ReadonlyKeyword:
             case SyntaxKind.StaticKeyword:
                 return true;
@@ -3123,6 +3124,7 @@ namespace ts {
             case SyntaxKind.DefaultKeyword: return ModifierFlags.Default;
             case SyntaxKind.AsyncKeyword: return ModifierFlags.Async;
             case SyntaxKind.ReadonlyKeyword: return ModifierFlags.Readonly;
+            case SyntaxKind.OverrideKeyword: return ModifierFlags.Override;
         }
         return ModifierFlags.None;
     }

src/harness/unittests/transpile.ts

@@ -341,6 +341,10 @@ var x = 0;`, {
             options: { compilerOptions: { noImplicitAny: true }, fileName: "input.js", reportDiagnostics: true }
         });
 
+        transpilesCorrectly("Supports setting 'noImplicitOverride'", "x;", {
+            options: { compilerOptions: { noImplicitOverride: true }, fileName: "input.js", reportDiagnostics: true }
+        });
+
         transpilesCorrectly("Supports setting 'noImplicitReturns'", "x;", {
             options: { compilerOptions: { noImplicitReturns: true }, fileName: "input.js", reportDiagnostics: true }
         });

src/server/protocol.ts

@@ -417,7 +417,7 @@ namespace ts.server.protocol {
         startOffset: number;
 
         /**
-         * Position (can be specified instead of line/offset pair) 
+         * Position (can be specified instead of line/offset pair)
          */
         /* @internal */
         startPosition?: number;
@@ -433,7 +433,7 @@ namespace ts.server.protocol {
         endOffset: number;
 
         /**
-         * Position (can be specified instead of line/offset pair) 
+         * Position (can be specified instead of line/offset pair)
          */
         /* @internal */
         endPosition?: number;
@@ -445,7 +445,7 @@ namespace ts.server.protocol {
     }
 
     /**
-     * Response for GetCodeFixes request. 
+     * Response for GetCodeFixes request.
      */
     export interface GetCodeFixesResponse extends Response {
         body?: CodeAction[];
@@ -2237,6 +2237,7 @@ namespace ts.server.protocol {
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
         noImplicitAny?: boolean;
+        noImplicitOverride?: boolean;
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;
         noUnusedLocals?: boolean;

src/services/jsDoc.ts

@@ -30,6 +30,7 @@ namespace ts.JsDoc {
         "private",
         "property",
         "public",
+        "override",
         "requires",
         "returns",
         "see",

tests/baselines/reference/noImplicitOverride.errors.txt

@@ -0,0 +1,86 @@
+tests/cases/compiler/noImplicitOverride.ts(11,5): error TS90034: Class member 'toString' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Object)
+tests/cases/compiler/noImplicitOverride.ts(12,5): error TS90034: Class member 'hasOwnProperty' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Object)
+tests/cases/compiler/noImplicitOverride.ts(18,5): error TS90034: Class member 'toString' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.toString)
+tests/cases/compiler/noImplicitOverride.ts(19,5): error TS90034: Class member 'hasOwnProperty' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.hasOwnProperty)
+tests/cases/compiler/noImplicitOverride.ts(25,5): error TS2322: Type '(prop: string) => number' is not assignable to type '(v: string) => boolean'.
+  Class member 'hasOwnProperty' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.hasOwnProperty)
+tests/cases/compiler/noImplicitOverride.ts(31,12): error TS90034: Class member 'userId' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Base)
+tests/cases/compiler/noImplicitOverride.ts(35,9): error TS90034: Class member 'name' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Base)
+tests/cases/compiler/noImplicitOverride.ts(43,5): error TS1070: 'override' modifier cannot appear on a type member.
+tests/cases/compiler/noImplicitOverride.ts(52,29): error TS90034: Class member 'userId' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Base)
+
+
+==== tests/cases/compiler/noImplicitOverride.ts (9 errors) ====
+
+    class Base {
+        get name(): string {
+            return 'Base';
+        }
+        getMeaningOfLife(): number { return 42; }
+        public userId: number = 1;
+    }
+
+    class RejectWhenOverrideMissingOnInheritedMethod extends Object {
+        toString(): string { return 'foo'; };
+        ~~~~~~~~
+!!! error TS90034: Class member 'toString' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Object)
+        hasOwnProperty(prop: string): boolean {
+        ~~~~~~~~~~~~~~
+!!! error TS90034: Class member 'hasOwnProperty' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Object)
+            return super.hasOwnProperty(prop);
+        }
+    }
+
+    class RejectWhenOverrideMissingOnAugmentedProperty {
+        toString(): string { return 'foo'; };
+        ~~~~~~~~
+!!! error TS90034: Class member 'toString' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.toString)
+        hasOwnProperty(prop: string): boolean {
+        ~~~~~~~~~~~~~~
+!!! error TS90034: Class member 'hasOwnProperty' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.hasOwnProperty)
+            return false;
+        }
+    }
+
+    class RejectWhenOverrideTypeMismatchOnMethodThatMasksObjectTypeMember {
+        hasOwnProperty(prop: string): number {
+        ~~~~~~~~~~~~~~
+!!! error TS2322: Type '(prop: string) => number' is not assignable to type '(v: string) => boolean'.
+!!! error TS2322:   Class member 'hasOwnProperty' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.hasOwnProperty)
+            return -1;
+        }
+    }
+
+    class RejectWhenOverrideMissingOnInheritedProperty extends Base {
+        public userId = 2;
+               ~~~~~~
+!!! error TS90034: Class member 'userId' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Base)
+    }
+
+    class RejectWhenOverrideMissingOnInheritedAccessor extends Base {
+        get name(): string { return 'foo'; };
+            ~~~~
+!!! error TS90034: Class member 'name' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Base)
+    }
+
+    interface Shape {
+        getWidth(): number;
+    }
+
+    interface RejectWhenOverrideOnAbstractDeclaration_Line extends Shape {
+        override getWidth(): number;
+        ~~~~~~~~
+!!! error TS1070: 'override' modifier cannot appear on a type member.
+    }
+
+    interface AcceptWhenOverrideNotOnAbstractDeclaration_Line extends Shape {
+        // abstract members don't need to be declared override
+        getWidth(): number;
+    }
+
+    class FIXME_AcceptWhenOverrideSpecifiedByJSDocAnnotation extends Base {
+        /** @override */ public userId: number = 2;
+                                ~~~~~~
+!!! error TS90034: Class member 'userId' must be marked 'override' when noImplicitOverride is enabled (augmented from Object.Base)
+    }
+