Fix39458

src/compiler/checker.ts

@@ -5094,6 +5094,9 @@ namespace ts {
                 if (parameterDeclaration && isRequiredInitializedParameter(parameterDeclaration)) {
                     parameterType = getOptionalType(parameterType);
                 }
+                if ((context.flags & NodeBuilderFlags.NoUndefinedOptionalParameterType) && parameterDeclaration && !isJSDocParameterTag(parameterDeclaration) && isOptionalUninitializedParameter(parameterDeclaration)) {
+                    parameterType = getTypeWithFacts(parameterType, TypeFacts.NEUndefined);
+                }
                 const parameterTypeNode = serializeTypeForDeclaration(context, parameterType, parameterSymbol, context.enclosingDeclaration, privateSymbolVisitor, bundledImports);
 
                 const modifiers = !(context.flags & NodeBuilderFlags.OmitParameterModifiers) && preserveModifierFlags && parameterDeclaration && parameterDeclaration.modifiers ? parameterDeclaration.modifiers.map(factory.cloneNode) : undefined;
@@ -36958,6 +36961,12 @@ namespace ts {
                 hasSyntacticModifier(parameter, ModifierFlags.ParameterPropertyModifier);
         }
 
+        function isOptionalUninitializedParameter(parameter: ParameterDeclaration) {
+            return !!strictNullChecks &&
+                isOptionalParameter(parameter) &&
+                !parameter.initializer;
+        }
+
         function isExpandoFunctionDeclaration(node: Declaration): boolean {
             const declaration = getParseTreeNode(node, isFunctionDeclaration);
             if (!declaration) {

src/compiler/types.ts

@@ -4160,6 +4160,7 @@ namespace ts {
         UseAliasDefinedOutsideCurrentScope      = 1 << 14,  // Allow non-visible aliases
         UseSingleQuotesForStringLiteralType     = 1 << 28,  // Use single quotes for string literal type
         NoTypeReduction                         = 1 << 29,  // Don't call getReducedType
+        NoUndefinedOptionalParameterType        = 1 << 30,  // Do not add undefined to optional parameter type
 
         // Error handling
         AllowThisInObjectLiteral                = 1 << 15,

src/services/codefixes/helpers.ts

@@ -164,7 +164,7 @@ namespace ts.codefix {
         const program = context.program;
         const checker = program.getTypeChecker();
         const scriptTarget = getEmitScriptTarget(program.getCompilerOptions());
-        const flags = NodeBuilderFlags.NoTruncation | NodeBuilderFlags.SuppressAnyReturnType | (quotePreference === QuotePreference.Single ? NodeBuilderFlags.UseSingleQuotesForStringLiteralType : 0);
+        const flags = NodeBuilderFlags.NoTruncation | NodeBuilderFlags.NoUndefinedOptionalParameterType | NodeBuilderFlags.SuppressAnyReturnType | (quotePreference === QuotePreference.Single ? NodeBuilderFlags.UseSingleQuotesForStringLiteralType : 0);
         const signatureDeclaration = <MethodDeclaration>checker.signatureToSignatureDeclaration(signature, SyntaxKind.MethodDeclaration, enclosingDeclaration, flags, getNoopSymbolTrackerWithResolver(context));
         if (!signatureDeclaration) {
             return undefined;

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2232,6 +2232,7 @@ declare namespace ts {
         UseAliasDefinedOutsideCurrentScope = 16384,
         UseSingleQuotesForStringLiteralType = 268435456,
         NoTypeReduction = 536870912,
+        NoUndefinedOptionalParameterType = 1073741824,
         AllowThisInObjectLiteral = 32768,
         AllowQualifedNameInPlaceOfIdentifier = 65536,
         AllowAnonymousIdentifier = 131072,

tests/baselines/reference/api/typescript.d.ts

@@ -2232,6 +2232,7 @@ declare namespace ts {
         UseAliasDefinedOutsideCurrentScope = 16384,
         UseSingleQuotesForStringLiteralType = 268435456,
         NoTypeReduction = 536870912,
+        NoUndefinedOptionalParameterType = 1073741824,
         AllowThisInObjectLiteral = 32768,
         AllowQualifedNameInPlaceOfIdentifier = 65536,
         AllowAnonymousIdentifier = 131072,

tests/cases/fourslash/codeFixClassImplementInterface_noUndefinedOnOptionalParameter.ts

@@ -0,0 +1,23 @@
+/// <reference path="fourslash.ts" />
+
+////interface IFoo {
+////    bar(x?: number | string): void;
+////}
+////
+////class Foo implements IFoo {
+////}
+
+//https://github.com/microsoft/TypeScript/issues/39458
+verify.codeFix({
+    description: [ts.Diagnostics.Implement_interface_0.message, "IFoo"],
+    newFileContent:
+`interface IFoo {
+    bar(x?: number | string): void;
+}
+
+class Foo implements IFoo {
+    bar(x?: string | number): void {
+        throw new Error("Method not implemented.");
+    }
+}`,
+});