Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -14743,8 +14743,7 @@
     // If this is an undefined enum, warn.
     if (TUK != TUK_Definition && !Invalid) {
       TagDecl *Def;
-      if (IsFixed && (getLangOpts().CPlusPlus11 || getLangOpts().ObjC) &&
-          cast<EnumDecl>(New)->isFixed()) {
+      if (IsFixed && cast<EnumDecl>(New)->isFixed()) {
         // C++0x: 7.2p2: opaque-enum-declaration.
         // Conflicts are diagnosed above. Do nothing.
       }
Index: clang/test/Sema/fixed-enum.c
===================================================================
--- clang/test/Sema/fixed-enum.c
+++ clang/test/Sema/fixed-enum.c
@@ -10,9 +10,29 @@
 #elif defined(CXX03)
 // expected-warning@-4{{enumeration types with a fixed underlying type are a C++11 extension}}
 #elif defined(OBJC)
-// expected-no-diagnostics
+// No diagnostic
 #elif defined(C11)
 // expected-warning@-8{{enumeration types with a fixed underlying type are a Clang extension}}
 #elif defined(MS)
 // expected-warning@-10{{enumeration types with a fixed underlying type are a Microsoft extension}}
 #endif
+
+// Don't warn about the forward declaration in any language mode.
+enum Fwd : int;
+enum Fwd : int { e2 };
+#ifndef OBJC
+// expected-warning@-3 {{enumeration types with a fixed underlying type}}
+// expected-warning@-3 {{enumeration types with a fixed underlying type}}
+#endif
+
+// Always error on the incompatible redeclaration.
+enum BadFwd : int;
+#ifndef OBJC
+// expected-warning@-2 {{enumeration types with a fixed underlying type}}
+#endif
+// expected-note@-4 {{previous declaration is here}}
+enum BadFwd : char { e3 };
+#ifndef OBJC
+// expected-warning@-2 {{enumeration types with a fixed underlying type}}
+#endif
+// expected-error@-4 {{enumeration redeclared with different underlying type 'char' (was 'int')}}