format_arg attribute should allow instancetype in NSString definition

- [[format_arg(N)]] tells Clang that a method returns a format string with
  specifiers equivalent to those passed in the string at argument #N. It
  obviously requires the argument and the return type to be strings both.
- `instancetype` is a special return type available in Objective-C class
  definitions that Clang expands to the most-derived statically known type on
  use.
- In Objective-C mode, NSString is allowed in lieu of a C string, both as input
  and output. However, _in the definition of NSString_, Clang rejects format_arg
  on methods that return NSString. This PR addresses this issue by substituting
  `instancetype` with the enclosing definition's type during the validation of
  `format_arg`.

Reviewed By: ahatanak

Differential Revision: https://reviews.llvm.org/D112670

Radar-Id: rdar://84729746

Author: apple-fcloutier

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3397,6 +3397,12 @@ static void handleFormatArgAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     return;
   }
   Ty = getFunctionOrMethodResultType(D);
+  // replace instancetype with the class type
+  if (Ty.getTypePtr() == S.Context.getObjCInstanceTypeDecl()->getTypeForDecl())
+    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() ||

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

@@ -1,6 +1,14 @@
 // RUN: %clang_cc1 -verify -fsyntax-only %s
 
-@class NSString;
+@interface NSString
++(instancetype)stringWithCString:(const char *)cstr __attribute__((format_arg(1)));
++(instancetype)stringWithString:(NSString *)cstr __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;
 
 extern NSString *fa2 (const NSString *) __attribute__((format_arg(1)));