Add override keyword

src/compiler/binder.ts

@@ -3332,6 +3332,7 @@ 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

@@ -263,6 +263,13 @@ namespace ts {
             category: Diagnostics.Strict_Type_Checking_Options,
             description: Diagnostics.Raise_error_on_expressions_and_declarations_with_an_implied_any_type,
         },
+        {
+            name: "noImplicitOverride",
+            type: "boolean",
+            showInSimplifiedHelpView: true,
+            category: Diagnostics.Strict_Type_Checking_Options,
+            description: Diagnostics.Raise_error_when_inherited_Slashoverridden_class_members_are_not_explicitly_marked_override,
+        },
         {
             name: "strictNullChecks",
             type: "boolean",

src/compiler/declarationEmitter.ts

@@ -768,6 +768,9 @@ namespace ts {
             if (flags & ModifierFlags.Static) {
                 write("static ");
             }
+            if (flags & ModifierFlags.Override) {
+                write("override ");
+            }
             if (flags & ModifierFlags.Readonly) {
                 write("readonly ");
             }

src/compiler/diagnosticMessages.json

@@ -779,7 +779,7 @@
         "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
     },
@@ -1964,6 +1964,34 @@
         "category": "Error",
         "code": 2565
     },
+    "Overload signatures must all be override or non-override.": {
+        "category": "Error",
+        "code": 2566
+    },
+    "Accessors must both be override or non-override.": {
+        "category": "Error",
+        "code": 2567
+    },
+    "Raise error when inherited/overridden class members are not explicitly marked 'override'": {
+        "category": "Error",
+        "code": 2568
+    },
+    "Class member '{0}' was marked 'override', but no matching declaration was found in any supertype of '{1}'": {
+        "category": "Error",
+        "code": 2569
+    },
+    "Class member '{0}' must be marked 'override' when noImplicitOverride is enabled (inherited from {1})": {
+        "category": "Error",
+        "code": 2570
+    },
+    "Override modifier cannot be used with an optional property declaration": {
+        "category": "Error",
+        "code": 2572
+    },
+    "All declarations of an override method must be consecutive.": {
+        "category": "Error",
+        "code": 2573
+    },
     "JSX element attributes type '{0}' may not be a union type.": {
         "category": "Error",
         "code": 2600
@@ -3786,7 +3814,6 @@
         "category": "Message",
         "code": 90017
     },
-
     "Disable checking for this file.": {
         "category": "Message",
         "code": 90018

src/compiler/parser.ts

@@ -4934,6 +4934,7 @@ namespace ts {
                     case SyntaxKind.PrivateKeyword:
                     case SyntaxKind.ProtectedKeyword:
                     case SyntaxKind.PublicKeyword:
+                    case SyntaxKind.OverrideKeyword:
                     case SyntaxKind.ReadonlyKeyword:
                         nextToken();
                         // ASI takes effect for this modifier.
@@ -5020,6 +5021,7 @@ 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
@@ -5101,6 +5103,7 @@ namespace ts {
                 case SyntaxKind.PrivateKeyword:
                 case SyntaxKind.ProtectedKeyword:
                 case SyntaxKind.PublicKeyword:
+                case SyntaxKind.OverrideKeyword:
                 case SyntaxKind.AbstractKeyword:
                 case SyntaxKind.StaticKeyword:
                 case SyntaxKind.ReadonlyKeyword:
@@ -5403,6 +5406,7 @@ 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

@@ -1468,6 +1468,7 @@ 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

@@ -104,6 +104,7 @@ 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

@@ -365,6 +365,7 @@ 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

@@ -215,6 +215,7 @@ namespace ts {
         UniqueKeyword,
         FromKeyword,
         GlobalKeyword,
+        OverrideKeyword,
         OfKeyword, // LastKeyword and LastToken and LastContextualKeyword
 
         // Parse tree nodes
@@ -497,15 +498,16 @@ 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
 
         AccessibilityModifier = Public | Private | Protected,
         // Accessibility modifiers and 'readonly' can be attached to a parameter in a constructor to make it a property.
-        ParameterPropertyModifier = AccessibilityModifier | Readonly,
+        ParameterPropertyModifier = AccessibilityModifier | Readonly | Override,
         NonPublicAccessibilityModifier = Private | Protected,
 
-        TypeScriptModifier = Ambient | Public | Private | Protected | Readonly | Abstract | Const,
+        TypeScriptModifier = Ambient | Public | Private | Protected | Override | Readonly | Abstract | Const,
         ExportDefault = Export | Default,
     }
 
@@ -618,6 +620,7 @@ namespace ts {
         | Token<SyntaxKind.PublicKeyword>
         | Token<SyntaxKind.PrivateKeyword>
         | Token<SyntaxKind.ProtectedKeyword>
+        | Token<SyntaxKind.OverrideKeyword>
         | Token<SyntaxKind.ReadonlyKeyword>
         | Token<SyntaxKind.StaticKeyword>
         ;
@@ -3836,6 +3839,7 @@ namespace ts {
         noEmitOnError?: boolean;
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
+        noImplicitOverride?: boolean; // Always combine with strict property (?)
         noImplicitAny?: boolean;  // Always combine with strict property
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;  // Always combine with strict property

src/compiler/utilities.ts

@@ -666,7 +666,7 @@ namespace ts {
             // construct.
             return getSpanOfTokenAtPosition(sourceFile, node.pos);
         }
-
+        
         const pos = nodeIsMissing(errorNode)
             ? errorNode.pos
             : skipTrivia(sourceFile.text, errorNode.pos);
@@ -3145,6 +3145,7 @@ 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;
     }
@@ -5113,6 +5114,7 @@ namespace ts {
             case SyntaxKind.PrivateKeyword:
             case SyntaxKind.ProtectedKeyword:
             case SyntaxKind.ReadonlyKeyword:
+            case SyntaxKind.OverrideKeyword:
             case SyntaxKind.StaticKeyword:
                 return true;
         }

src/harness/unittests/transpile.ts

@@ -338,6 +338,10 @@ 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

@@ -2518,6 +2518,7 @@ namespace ts.server.protocol {
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
         noImplicitAny?: boolean;
+        noImplicitOverride?: boolean;
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;
         noUnusedLocals?: boolean;