diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3108,7 +3108,7 @@
   "strftime format attribute requires 3rd parameter to be 0">;
 def err_format_attribute_requires_variadic : Error<
   "format attribute requires variadic function">;
-def err_format_attribute_not : Error<"format argument not %0">;
+def err_format_attribute_not : Error<"format argument not a string type">;
 def err_format_attribute_result_not : Error<"function does not return %0">;
 def err_format_attribute_implicit_this_format_string : Error<
   "format attribute cannot specify the implicit this argument as the format "
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
@@ -3661,8 +3661,7 @@
       (!Ty->isPointerType() ||
        !Ty->castAs<PointerType>()->getPointeeType()->isCharType())) {
     S.Diag(AL.getLoc(), diag::err_format_attribute_not)
-        << "a string type" << IdxExpr->getSourceRange()
-        << getFunctionOrMethodParamRange(D, 0);
+        << IdxExpr->getSourceRange() << getFunctionOrMethodParamRange(D, 0);
     return;
   }
   Ty = getFunctionOrMethodResultType(D);
@@ -3862,27 +3861,12 @@
   // make sure the format string is really a string
   QualType Ty = getFunctionOrMethodParamType(D, ArgIdx);
 
-  if (Kind == CFStringFormat) {
-    if (!isCFStringType(Ty, S.Context)) {
-      S.Diag(AL.getLoc(), diag::err_format_attribute_not)
-        << "a CFString" << IdxExpr->getSourceRange()
-        << getFunctionOrMethodParamRange(D, ArgIdx);
-      return;
-    }
-  } else if (Kind == NSStringFormat) {
-    // FIXME: do we need to check if the type is NSString*?  What are the
-    // semantics?
-    if (!isNSStringType(Ty, S.Context, /*AllowNSAttributedString=*/true)) {
-      S.Diag(AL.getLoc(), diag::err_format_attribute_not)
-        << "an NSString" << IdxExpr->getSourceRange()
-        << getFunctionOrMethodParamRange(D, ArgIdx);
-      return;
-    }
-  } else if (!Ty->isPointerType() ||
-             !Ty->castAs<PointerType>()->getPointeeType()->isCharType()) {
+  if (!isNSStringType(Ty, S.Context, true) &&
+      !isCFStringType(Ty, S.Context) &&
+      (!Ty->isPointerType() ||
+       !Ty->castAs<PointerType>()->getPointeeType()->isCharType())) {
     S.Diag(AL.getLoc(), diag::err_format_attribute_not)
-      << "a string type" << IdxExpr->getSourceRange()
-      << getFunctionOrMethodParamRange(D, ArgIdx);
+      << IdxExpr->getSourceRange() << getFunctionOrMethodParamRange(D, ArgIdx);
     return;
   }
 
diff --git a/clang/test/SemaObjC/format-strings-objc.m b/clang/test/SemaObjC/format-strings-objc.m
--- a/clang/test/SemaObjC/format-strings-objc.m
+++ b/clang/test/SemaObjC/format-strings-objc.m
@@ -60,8 +60,23 @@
 }
 
 // Check type validation
-extern void NSLog2(int format, ...) __attribute__((format(__NSString__, 1, 2))); // expected-error {{format argument not an NSString}}
-extern void CFStringCreateWithFormat2(int *format, ...) __attribute__((format(CFString, 1, 2))); // expected-error {{format argument not a CFString}}
+extern void NSLog2(int format, ...) __attribute__((format(__NSString__, 1, 2))); // expected-error {{format argument not a string type}}
+extern void CFStringCreateWithFormat2(int *format, ...) __attribute__((format(CFString, 1, 2))); // expected-error {{format argument not a string type}}
+
+// Check interoperability of strings
+extern void NSLog3(const char *, ...) __attribute__((format(__NSString__, 1, 2)));
+extern void CFStringCreateWithFormat3(CFStringRef, ...) __attribute__((format(__NSString__, 1, 2)));
+extern void printf2(NSString *format, ...) __attribute__((format(printf, 1, 2)));
+
+extern NSString *CStringToNSString(const char *) __attribute__((format_arg(1)));
+
+void NSLog3(const char *fmt, ...) {
+  NSString *const nsFmt = CStringToNSString(fmt);
+  va_list ap;
+  va_start(ap, fmt);
+  NSLogv(nsFmt, ap);
+  va_end(ap);
+}
 
 // <rdar://problem/7068334> - Catch use of long long with int arguments.
 void rdar_7068334(void) {