fix(55765): Implement Interface Code Action is not available under certain circumstances

src/services/codefixes/helpers.ts

@@ -400,7 +400,7 @@ export function createSignatureDeclarationFromSignature(
 
     let typeParameters = isJs ? undefined : signatureDeclaration.typeParameters;
     let parameters = signatureDeclaration.parameters;
-    let type = isJs ? undefined : signatureDeclaration.type;
+    let type = isJs ? undefined : getSynthesizedDeepClone(signatureDeclaration.type);
     if (importAdder) {
         if (typeParameters) {
             const newTypeParameters = sameMap(typeParameters, typeParameterDecl => {

tests/cases/fourslash/codeFixClassImplementInterfaceTypeLiterals.ts

@@ -0,0 +1,27 @@
+/// <reference path='fourslash.ts' />
+
+////type Builtin = Date | Function | Uint8Array | string | number | boolean | undefined;
+////
+////export type DeepPartial<T> = T extends Builtin ? T :
+////    T extends Array<infer U> ? Array<DeepPartial<U>> :
+////        T extends ReadonlyArray<infer U> ? ReadonlyArray<DeepPartial<U>> :
+////            T extends {} ? { [K in keyof T]?: DeepPartial<T[K]> } : Partial<T>;
+////
+////export interface Nested {
+////    field: string;
+////}
+////
+////interface Foo {
+////    request(): DeepPartial<{ nested1: Nested; test2: Nested }>;
+////}
+////[|export class C implements Foo {}|]
+
+verify.codeFix({
+    description: "Implement interface 'Foo'",
+    newRangeContent:
+`export class C implements Foo {
+    request(): { nested1?: { field?: string; }; test2?: { field?: string; }; } {
+        throw new Error("Method not implemented.");
+    }
+}`,
+});