format_arg attribute does not support nullable instancetype return type

[fcloutier]

Summary

Following up with D112670, it appears that the original fix was insufficient: it's not possible to use __attribute__((format_arg)) on methods that return instancetype in NSString when instancetype is sugared, as this breaks pointer equality. Although this is only possible in limited scenarios because instancetype is a contextual keyword, it can (crucially) be combined with nullability qualifiers.

rdar://85278860

[ahatanak]

LGTM

clang/lib/Sema/SemaDeclAttr.cpp

if (NotNSStringTy &&
        !Ty->castAs<PointerType>()->getPointeeType()->isCharType())) {
     S.Diag(AL.getLoc(), diag::err_format_attribute_not)
         << "a string type" << IdxExpr->getSourceRange()
         << getFunctionOrMethodParamRange(D, 0);
     return;
   }
   Ty = getFunctionOrMethodResultType(D);
   // replace instancetype with the class type
-  if (Ty.getTypePtr() == S.Context.getObjCInstanceTypeDecl()->getTypeForDecl())
+  auto Instancetype = S.Context.getObjCInstanceTypeDecl()->getTypeForDecl();
+  if (Ty->getAs<TypedefType>() == Instancetype)
     if (auto *OMD = dyn_cast<ObjCMethodDecl>(D))
       if (auto *Interface = OMD->getClassInterface())
         Ty = S.Context.getObjCObjectPointerType(
             QualType(Interface->getTypeForDecl(), 0));
   if (!isNSStringType(Ty, S.Context, /*AllowNSAttributedString=*/true) &&
       !isCFStringType(Ty, S.Context) &&
       (!Ty->isPointerType() ||
        !Ty->castAs<PointerType>()->getPointeeType()->isCharType())) {

clang/test/SemaObjC/format-arg-attribute.m

 // RUN: %clang_cc1 -verify -fsyntax-only %s
 
 @interface NSString
 +(instancetype)stringWithCString:(const char *)cstr __attribute__((format_arg(1)));
-+(instancetype)stringWithString:(NSString *)cstr __attribute__((format_arg(1)));
+-(instancetype)initWithString:(NSString *)str __attribute__((format_arg(1)));
+
++(instancetype _Nonnull)nonNullableString:(NSString *)str __attribute__((format_arg(1)));
++(instancetype _Nullable)nullableString:(NSString *)str __attribute__((format_arg(1)));
 @end
 
 @protocol MaybeString
 -(instancetype)maybeString:(const char *)cstr __attribute__((format_arg(1))); // expected-error {{function does not return string type}}
 @end
 
 @class NSAttributedString;