--noImplicitOverride

src/compiler/checker.ts

@@ -34854,7 +34854,8 @@ namespace ts {
             checkClassForDuplicateDeclarations(node);
 
             // Only check for reserved static identifiers on non-ambient context.
-            if (!(node.flags & NodeFlags.Ambient)) {
+            const nodeInAmbientContext = !!(node.flags & NodeFlags.Ambient);
+            if (!nodeInAmbientContext) {
                 checkClassForStaticPropertyNameConflicts(node);
             }
 
@@ -34910,6 +34911,8 @@ namespace ts {
                 }
             }
 
+            checkMembersForMissingOverrideModifier(node, type, typeWithThis);
+
             const implementedTypeNodes = getEffectiveImplementsTypeNodes(node);
             if (implementedTypeNodes) {
                 for (const typeRefNode of implementedTypeNodes) {
@@ -34944,6 +34947,48 @@ namespace ts {
             }
         }
 
+        function checkMembersForMissingOverrideModifier(node: ClassLikeDeclaration, type: InterfaceType, typeWithThis: Type) {
+            const nodeInAmbientContext = !!(node.flags & NodeFlags.Ambient);
+            const baseTypeNode = getEffectiveBaseTypeNode(node);
+            const baseTypes = baseTypeNode && getBaseTypes(type);
+            const baseWithThis = baseTypes?.length ? getTypeWithThisArgument(first(baseTypes), type.thisType) : undefined;
+
+            for (const member of node.members) {
+                if (isConstructorDeclaration(member)) {
+                    forEach(member.parameters, param => {
+                        if (isParameterPropertyDeclaration(param, member)) {
+                            checkClassMember(param, /*memberIsParameterProperty*/ true);
+                        }
+                    });
+                }
+                checkClassMember(member);
+            }
+            function checkClassMember(member: ClassElement | ParameterPropertyDeclaration, memberIsParameterProperty?: boolean) {
+                const hasOverride = hasOverrideModifier(member);
+                if (baseWithThis) {
+                    const declaredProp = member.name && getSymbolAtLocation(member.name) || getSymbolAtLocation(member);
+                    if (declaredProp) {
+                        const baseClassName = typeToString(baseWithThis);
+                        const prop = getPropertyOfType(typeWithThis, declaredProp.escapedName);
+                        const baseProp = getPropertyOfType(baseWithThis, declaredProp.escapedName);
+                        if (prop && !baseProp && hasOverride) {
+                            error(member, Diagnostics.This_member_cannot_have_an_override_modifier_because_it_is_not_declared_in_the_base_class_0, baseClassName);
+                        }
+                        else if (prop && baseProp && !hasOverride && compilerOptions.noImplicitOverride && !nodeInAmbientContext) {
+                            const diag = memberIsParameterProperty ?
+                                Diagnostics.This_parameter_must_convert_into_property_declaration_because_it_overrides_a_member_in_the_base_class_0 :
+                                Diagnostics.This_member_must_have_an_override_modifier_because_it_overrides_a_member_in_the_base_class_0;
+                            error(member, diag, baseClassName);
+                        }
+                    }
+                }
+                else if (hasOverride) {
+                    const className = typeToString(type);
+                    error(member, Diagnostics.This_member_cannot_have_an_override_modifier_because_its_containing_class_0_does_not_extend_another_class, className);
+                }
+            }
+        }
+
         function issueMemberSpecificError(node: ClassLikeDeclaration, typeWithThis: Type, baseWithThis: Type, broadDiag: DiagnosticMessage) {
             // iterate over all implemented properties and issue errors on each one which isn't compatible, rather than the class as a whole, if possible
             let issuedMemberError = false;
@@ -38276,7 +38321,7 @@ namespace ts {
                 return quickResult;
             }
 
-            let lastStatic: Node | undefined, lastDeclare: Node | undefined, lastAsync: Node | undefined, lastReadonly: Node | undefined;
+            let lastStatic: Node | undefined, lastDeclare: Node | undefined, lastAsync: Node | undefined, lastReadonly: Node | undefined, lastOverride: Node | undefined;
             let flags = ModifierFlags.None;
             for (const modifier of node.modifiers!) {
                 if (modifier.kind !== SyntaxKind.ReadonlyKeyword) {
@@ -38293,6 +38338,20 @@ namespace ts {
                             return grammarErrorOnNode(node, Diagnostics.A_class_member_cannot_have_the_0_keyword, tokenToString(SyntaxKind.ConstKeyword));
                         }
                         break;
+                    case SyntaxKind.OverrideKeyword:
+                        if (flags & ModifierFlags.Override) {
+                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "override");
+                        }
+                        else if (flags & ModifierFlags.Ambient) {
+                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "override", "declare");
+                        }
+                        else if (flags & ModifierFlags.Static) {
+                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "static", "override");
+                        }
+                        flags |= ModifierFlags.Override;
+                        lastOverride = modifier;
+                        break;
+
                     case SyntaxKind.PublicKeyword:
                     case SyntaxKind.ProtectedKeyword:
                     case SyntaxKind.PrivateKeyword:
@@ -38301,6 +38360,9 @@ namespace ts {
                         if (flags & ModifierFlags.AccessibilityModifier) {
                             return grammarErrorOnNode(modifier, Diagnostics.Accessibility_modifier_already_seen);
                         }
+                        else if (compilerOptions.noImplicitOverride && flags & ModifierFlags.Override) {
+                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "override");
+                        }
                         else if (flags & ModifierFlags.Static) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "static");
                         }
@@ -38334,6 +38396,9 @@ namespace ts {
                         else if (flags & ModifierFlags.Readonly) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "static", "readonly");
                         }
+                        else if (compilerOptions.noImplicitOverride && flags & ModifierFlags.Override) {
+                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "static", "override");
+                        }
                         else if (flags & ModifierFlags.Async) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "static", "async");
                         }
@@ -38401,6 +38466,9 @@ namespace ts {
                         else if (flags & ModifierFlags.Async) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                         }
+                        else if (compilerOptions.noImplicitOverride && flags & ModifierFlags.Override) {
+                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "override");
+                        }
                         else if (isClassLike(node.parent) && !isPropertyDeclaration(node)) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_class_element, "declare");
                         }
@@ -38474,6 +38542,9 @@ namespace ts {
                 if (flags & ModifierFlags.Abstract) {
                     return grammarErrorOnNode(lastStatic!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "abstract"); // TODO: GH#18217
                 }
+                if (compilerOptions.noImplicitOverride && flags & ModifierFlags.Override) {
+                    return grammarErrorOnNode(lastOverride!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "override"); // TODO: GH#18217
+                }
                 else if (flags & ModifierFlags.Async) {
                     return grammarErrorOnNode(lastAsync!, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "async");
                 }

src/compiler/commandLineParser.ts

@@ -632,6 +632,14 @@ namespace ts {
             category: Diagnostics.Additional_Checks,
             description: Diagnostics.Include_undefined_in_index_signature_results
         },
+        {
+            name: "noImplicitOverride",
+            type: "boolean",
+            affectsSemanticDiagnostics: true,
+            showInSimplifiedHelpView: false,
+            category: Diagnostics.Additional_Checks,
+            description: Diagnostics.Force_overriding_members_in_derived_classes_to_be_marked_with_an_override_modifier
+        },
 
         // Module Resolution
         {

src/compiler/diagnosticMessages.json

@@ -3408,6 +3408,22 @@
         "category": "Error",
         "code": 4110
     },
+    "This member cannot have an 'override' modifier because its containing class '{0}' does not extend another class.": {
+        "category": "Error",
+        "code": 4111
+    },
+    "This member cannot have an 'override' modifier because it is not declared in the base class '{0}'.": {
+        "category": "Error",
+        "code": 4112
+    },
+    "This member must have an 'override' modifier because it overrides a member in the base class '{0}'.": {
+        "category": "Error",
+        "code": 4113
+    },
+    "This parameter must convert into property declaration because it overrides a member in the base class '{0}'.": {
+        "category": "Error",
+        "code": 4114
+    },
 
     "The current host does not support the '{0}' option.": {
         "category": "Error",
@@ -4720,6 +4736,10 @@
         "category": "Error",
         "code": 6504
     },
+    "Force overriding members in derived classes to be marked with an 'override' modifier.": {
+        "category": "Error",
+        "code": 6801
+    },
 
     "Include 'undefined' in index signature results": {
         "category": "Message",
@@ -5923,6 +5943,30 @@
         "category": "Message",
         "code": 95142
     },
+    "Add 'override' modifier": {
+        "category": "Message",
+        "code": 95143
+    },
+    "Remove 'override' modifier": {
+        "category": "Message",
+        "code": 95144
+    },
+    "Add all 'override' modifier": {
+        "category": "Message",
+        "code": 95145
+    },
+    "Remove all 'override' modifier": {
+        "category": "Message",
+        "code": 95146
+    },
+    "Convert to property declaration and add 'override' modifier": {
+        "category": "Message",
+        "code": 95147
+    },
+    "Convert all to property declaration and add 'override' modifier": {
+        "category": "Message",
+        "code": 95148
+    },
 
     "No value exists in scope for the shorthand property '{0}'. Either declare one or provide an initializer.": {
         "category": "Error",

src/compiler/factory/nodeFactory.ts

@@ -1035,6 +1035,7 @@ namespace ts {
             if (flags & ModifierFlags.Static) { result.push(createModifier(SyntaxKind.StaticKeyword)); }
             if (flags & ModifierFlags.Readonly) { result.push(createModifier(SyntaxKind.ReadonlyKeyword)); }
             if (flags & ModifierFlags.Async) { result.push(createModifier(SyntaxKind.AsyncKeyword)); }
+            if (flags & ModifierFlags.Override) { result.push(createModifier(SyntaxKind.OverrideKeyword)); }
             return result;
         }
 

src/compiler/scanner.ts

@@ -125,6 +125,7 @@ namespace ts {
         private: SyntaxKind.PrivateKeyword,
         protected: SyntaxKind.ProtectedKeyword,
         public: SyntaxKind.PublicKeyword,
+        override: SyntaxKind.OverrideKeyword,
         readonly: SyntaxKind.ReadonlyKeyword,
         require: SyntaxKind.RequireKeyword,
         global: SyntaxKind.GlobalKeyword,

src/compiler/types.ts

@@ -186,6 +186,7 @@ namespace ts {
         FromKeyword,
         GlobalKeyword,
         BigIntKeyword,
+        OverrideKeyword,
         UppercaseKeyword,
         LowercaseKeyword,
         CapitalizeKeyword,
@@ -591,6 +592,7 @@ namespace ts {
         | SyntaxKind.ProtectedKeyword
         | SyntaxKind.PublicKeyword
         | SyntaxKind.ReadonlyKeyword
+        | SyntaxKind.OverrideKeyword
         | SyntaxKind.RequireKeyword
         | SyntaxKind.ReturnKeyword
         | SyntaxKind.SetKeyword
@@ -628,6 +630,7 @@ namespace ts {
         | SyntaxKind.ProtectedKeyword
         | SyntaxKind.PublicKeyword
         | SyntaxKind.ReadonlyKeyword
+        | SyntaxKind.OverrideKeyword
         | SyntaxKind.StaticKeyword
         ;
 
@@ -803,16 +806,17 @@ namespace ts {
         HasComputedJSDocModifiers = 1 << 12, // Indicates the computed modifier flags include modifiers from JSDoc.
 
         Deprecated =         1 << 13, // Deprecated tag.
+        Override =           1 << 14, // Override method.
         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,
         NonPublicAccessibilityModifier = Private | Protected,
 
-        TypeScriptModifier = Ambient | Public | Private | Protected | Readonly | Abstract | Const,
+        TypeScriptModifier = Ambient | Public | Private | Protected | Readonly | Abstract | Const | Override,
         ExportDefault = Export | Default,
-        All = Export | Ambient | Public | Private | Protected | Static | Readonly | Abstract | Async | Default | Const | Deprecated
+        All = Export | Ambient | Public | Private | Protected | Static | Readonly | Abstract | Async | Default | Const | Deprecated | Override
     }
 
     export const enum JsxFlags {
@@ -1032,6 +1036,7 @@ namespace ts {
     export type ProtectedKeyword = ModifierToken<SyntaxKind.ProtectedKeyword>;
     export type PublicKeyword = ModifierToken<SyntaxKind.PublicKeyword>;
     export type ReadonlyKeyword = ModifierToken<SyntaxKind.ReadonlyKeyword>;
+    export type OverrideKeyword = ModifierToken<SyntaxKind.OverrideKeyword>;
     export type StaticKeyword = ModifierToken<SyntaxKind.StaticKeyword>;
 
     /** @deprecated Use `ReadonlyKeyword` instead. */
@@ -1047,6 +1052,7 @@ namespace ts {
         | PrivateKeyword
         | ProtectedKeyword
         | PublicKeyword
+        | OverrideKeyword
         | ReadonlyKeyword
         | StaticKeyword
         ;
@@ -5792,6 +5798,7 @@ namespace ts {
         /*@internal*/ pathsBasePath?: string;
         /*@internal*/ plugins?: PluginImport[];
         preserveConstEnums?: boolean;
+        noImplicitOverride?: boolean;
         preserveSymlinks?: boolean;
         /* @internal */ preserveWatchOutput?: boolean;
         project?: string;

src/compiler/utilities.ts

@@ -4567,6 +4567,14 @@ namespace ts {
         return hasSyntacticModifier(node, ModifierFlags.Static);
     }
 
+    export function hasOverrideModifier(node: Node): boolean {
+        return hasSyntacticModifier(node, ModifierFlags.Override);
+    }
+
+    export function hasAmbientModifier(node: Node): boolean {
+        return hasSyntacticModifier(node, ModifierFlags.Ambient);
+    }
+
     export function hasEffectiveReadonlyModifier(node: Node): boolean {
         return hasEffectiveModifier(node, ModifierFlags.Readonly);
     }
@@ -4677,10 +4685,15 @@ 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;
     }
 
+    export function createModifiers(modifierFlags: ModifierFlags): ModifiersArray | undefined {
+        return modifierFlags ? factory.createNodeArray(factory.createModifiersFromModifierFlags(modifierFlags)) : undefined;
+    }
+
     export function isLogicalOperator(token: SyntaxKind): boolean {
         return token === SyntaxKind.BarBarToken
             || token === SyntaxKind.AmpersandAmpersandToken

src/compiler/utilitiesPublic.ts

@@ -1129,6 +1129,7 @@ namespace ts {
             case SyntaxKind.ProtectedKeyword:
             case SyntaxKind.ReadonlyKeyword:
             case SyntaxKind.StaticKeyword:
+            case SyntaxKind.OverrideKeyword:
                 return true;
         }
         return false;
@@ -1141,7 +1142,7 @@ namespace ts {
 
     /* @internal */
     export function isClassMemberModifier(idToken: SyntaxKind): boolean {
-        return isParameterPropertyModifier(idToken) || idToken === SyntaxKind.StaticKeyword;
+        return isParameterPropertyModifier(idToken) || idToken === SyntaxKind.StaticKeyword || idToken === SyntaxKind.OverrideKeyword;
     }
 
     export function isModifier(node: Node): node is Modifier {

src/harness/fourslashInterfaceImpl.ts

@@ -1138,6 +1138,7 @@ namespace FourSlashInterface {
                     case "symbol":
                     case "type":
                     case "unique":
+                    case "override":
                     case "unknown":
                     case "global":
                     case "bigint":
@@ -1149,7 +1150,7 @@ namespace FourSlashInterface {
         }
 
         export const classElementKeywords: readonly ExpectedCompletionEntryObject[] =
-            ["private", "protected", "public", "static", "abstract", "async", "constructor", "declare", "get", "readonly", "set"].map(keywordEntry);
+            ["private", "protected", "public", "static", "abstract", "async", "constructor", "declare", "get", "readonly", "set", "override"].map(keywordEntry);
 
         export const classElementInJsKeywords = getInJsKeywords(classElementKeywords);
 

src/services/codeFixProvider.ts

@@ -18,6 +18,10 @@ namespace ts.codefix {
         return createCodeFixActionWorker(fixName, diagnosticToString(description), changes, fixId, diagnosticToString(fixAllDescription), command);
     }
 
+    export function createCodeFixActionMaybeFixAll(fixName: string, changes: FileTextChanges[], description: DiagnosticAndArguments, fixId?: {}, fixAllDescription?: DiagnosticAndArguments, command?: CodeActionCommand) {
+        return createCodeFixActionWorker(fixName, diagnosticToString(description), changes, fixId, fixAllDescription && diagnosticToString(fixAllDescription), command);
+    }
+
     function createCodeFixActionWorker(fixName: string, description: string, changes: FileTextChanges[], fixId?: {}, fixAllDescription?: string, command?: CodeActionCommand): CodeFixAction {
         return { fixName, description, changes, fixId, fixAllDescription, commands: command ? [command] : undefined };
     }