diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -3388,6 +3388,12 @@
     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() ||
diff --git a/clang/test/SemaObjC/format-arg-attribute.m b/clang/test/SemaObjC/format-arg-attribute.m
--- a/clang/test/SemaObjC/format-arg-attribute.m
+++ b/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)));