feat: add override keyword

src/common.ts

@@ -36,49 +36,52 @@ export enum CommonFlags {
   GET = 1 << 11,
   /** Has a `set` modifier. */
   SET = 1 << 12,
+  /** Has a `override` modifier.  */
+  OVERRIDE = 1 << 13,
+
   /** Has a definite assignment assertion `!` as in `x!: i32;`. */
-  DEFINITELY_ASSIGNED = 1 << 13,
+  DEFINITELY_ASSIGNED = 1 << 14,
 
   // Extended modifiers usually derived from basic modifiers
 
   /** Is ambient, that is either declared or nested in a declared element. */
-  AMBIENT = 1 << 14,
+  AMBIENT = 1 << 15,
   /** Is generic. */
-  GENERIC = 1 << 15,
+  GENERIC = 1 << 16,
   /** Is part of a generic context. */
-  GENERIC_CONTEXT = 1 << 16,
+  GENERIC_CONTEXT = 1 << 17,
   /** Is an instance member. */
-  INSTANCE = 1 << 17,
+  INSTANCE = 1 << 18,
   /** Is a constructor. */
-  CONSTRUCTOR = 1 << 18,
+  CONSTRUCTOR = 1 << 19,
   /** Is a module export. */
-  MODULE_EXPORT = 1 << 19,
+  MODULE_EXPORT = 1 << 20,
   /** Is a module import. */
-  MODULE_IMPORT = 1 << 20,
+  MODULE_IMPORT = 1 << 21,
 
   // Compilation states
 
   /** Is resolved. */
-  RESOLVED = 1 << 21,
+  RESOLVED = 1 << 22,
   /** Is compiled. */
-  COMPILED = 1 << 22,
+  COMPILED = 1 << 23,
   /** Did error. */
-  ERRORED = 1 << 23,
+  ERRORED = 1 << 24,
   /** Has a constant value and is therefore inlined. */
-  INLINED = 1 << 24,
+  INLINED = 1 << 25,
   /** Is scoped. */
-  SCOPED = 1 << 25,
+  SCOPED = 1 << 26,
   /** Is a stub. */
-  STUB = 1 << 26,
+  STUB = 1 << 27,
   /** Is a virtual method. */
-  VIRTUAL = 1 << 27,
+  VIRTUAL = 1 << 28,
   /** Is (part of) a closure. */
-  CLOSURE = 1 << 28,
+  CLOSURE = 1 << 29,
 
   // Other
 
   /** Is quoted. */
-  QUOTED = 1 << 29
+  QUOTED = 1 << 30
 }
 
 /** Path delimiter inserted between file system levels. */

src/diagnosticMessages.json

@@ -184,6 +184,7 @@
   "Duplicate property '{0}'.": 2718,
   "Property '{0}' is missing in type '{1}' but required in type '{2}'.": 2741,
   "Type '{0}' has no call signatures.": 2757,
+  "This member cannot have an 'override' modifier because it is not declared in the base class '{0}'.": 4117,
 
   "File '{0}' not found.": 6054,
   "Numeric separators are not allowed here.": 6188,

src/parser.ts

@@ -1862,6 +1862,7 @@ export class Parser extends DiagnosticEmitter {
     //   'declare'?
     //   ('public' | 'private' | 'protected')?
     //   ('static' | 'abstract')?
+    //   'override'?
     //   'readonly'?
     //   ('get' | 'set')?
     //   Identifier ...
@@ -1991,6 +1992,22 @@ export class Parser extends DiagnosticEmitter {
       if (parent.flags & CommonFlags.GENERIC) flags |= CommonFlags.GENERIC_CONTEXT;
     }
 
+    var overrideStart = 0;
+    var overrideEnd = 0;
+    if (tn.skip(Token.OVERRIDE)) {
+      if (isInterface || parent.extendsType == null) {
+        this.error(
+          DiagnosticCode._0_modifier_cannot_be_used_here,
+          tn.range(), "override"
+        );
+      } else {
+        flags |= CommonFlags.OVERRIDE;
+        overrideStart = tn.tokenPos;
+        overrideEnd = tn.pos;
+      }
+      if (!startPos) startPos = tn.tokenPos;
+    }
+
     var readonlyStart = 0;
     var readonlyEnd = 0;
     if (tn.peek() == Token.READONLY) {
@@ -2104,6 +2121,12 @@ export class Parser extends DiagnosticEmitter {
             tn.range(staticStart, staticEnd), "static"
           ); // recoverable
         }
+        if (flags & CommonFlags.OVERRIDE) {
+          this.error(
+            DiagnosticCode._0_modifier_cannot_be_used_here,
+            tn.range(overrideStart, overrideEnd), "override"
+          );
+        }
         if (flags & CommonFlags.ABSTRACT) {
           this.error(
             DiagnosticCode._0_modifier_cannot_be_used_here,

src/program.ts

@@ -1524,6 +1524,12 @@ export class Program extends DiagnosticEmitter {
                 }
               }
             }
+            if (thisMember.is(CommonFlags.OVERRIDE) && !baseInstanceMembers.has(thisMember.name)) {
+              this.error(
+                DiagnosticCode.This_member_cannot_have_an_override_modifier_because_it_is_not_declared_in_the_base_class_0,
+                thisMember.identifierNode.range, basePrototype.name
+              );
+            }
           }
         }
         let nextPrototype = basePrototype.basePrototype;

src/tokenizer.ts

@@ -80,6 +80,7 @@ export enum Token {
   NEW,          // ES2017
   NULL,         // ES
   OF,
+  OVERRIDE,
   PACKAGE,      // ES2017 non-lexical
   PRIVATE,      // ES2017 non-lexical
   PROTECTED,    // ES2017 non-lexical
@@ -291,6 +292,7 @@ export function tokenFromKeyword(text: string): Token {
     }
     case CharCode.o: {
       if (text == "of") return Token.OF;
+      if (text == "override") return Token.OVERRIDE;
       break;
     }
     case CharCode.p: {

tests/compiler/class-overloading-cast.ts

@@ -4,7 +4,7 @@ class A<T> {
   }
 }
 class B<T, V> extends A<T> {
-  foo(a: T): string {
+  override foo(a: T): string {
     return "B";
   }
 }

tests/compiler/override-error.json

@@ -0,0 +1,6 @@
+{
+  "asc_flags": [],
+  "stderr": [
+    "TS4117: This member cannot have an 'override' modifier because it is not declared in the base class 'A'."
+  ]
+}

tests/compiler/override-error.ts

@@ -0,0 +1,14 @@
+class A {
+  method(): void {}
+}
+
+class B extends A {
+  override method(): void {}
+  // TS4117: This member cannot have an 'override' modifier because it is not declared in the base class 'A'.
+  override method_error(): void {}
+}
+
+export function test(): void {
+  new A();
+  new B();
+}

tests/parser/class.ts

@@ -37,4 +37,7 @@ export class Invalid<T> {
   // 1031: 'declare' modifier cannot appear on class elements of this kind.
   // 1183: An implementation cannot be declared in ambient contexts.
   declare declareMethod(): i32 {}
+
+  // ERROR 1042: "'override' modifier cannot be used here."
+  override overrideMethod(): void {}
 }

tests/parser/class.ts.fixture.ts

@@ -17,6 +17,7 @@ export class Invalid<T> {
   declare declareField: i32;
   declare declareInitializer: i32 = 0;
   declare declareMethod(): i32 {}
+  overrideMethod(): void {}
 }
 // ERROR 1092: "Type parameters cannot appear on a constructor declaration." in class.ts(15,14+3)
 // ERROR 1110: "Type expected." in class.ts(18,21+0)
@@ -31,3 +32,4 @@ export class Invalid<T> {
 // ERROR 1039: "Initializers are not allowed in ambient contexts." in class.ts(35,35+1)
 // ERROR 1031: "'declare' modifier cannot appear on class elements of this kind." in class.ts(39,3+7)
 // ERROR 1183: "An implementation cannot be declared in ambient contexts." in class.ts(39,32+1)
+// ERROR 1042: "'override' modifier cannot be used here." in class.ts(42,3+8)