Add only 'undefined' to optional parameter types

src/compiler/checker.ts

@@ -2698,7 +2698,7 @@ namespace ts {
             // Use type from type annotation if one is present
             if (declaration.type) {
                 const type = getTypeFromTypeNode(declaration.type);
-                return declaration.questionToken ? getNullableType(type) : type;
+                return strictNullChecks && declaration.questionToken ? addNullableKind(type, TypeFlags.Undefined) : type;
             }
 
             if (declaration.kind === SyntaxKind.Parameter) {
@@ -2713,7 +2713,7 @@ namespace ts {
                 // Use contextual parameter type if one is available
                 const type = getContextuallyTypedParameterType(<ParameterDeclaration>declaration);
                 if (type) {
-                    return declaration.questionToken ? getNullableType(type) : type;
+                    return strictNullChecks && declaration.questionToken ? addNullableKind(type, TypeFlags.Undefined) : type;
                 }
             }