Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td
+++ include/clang/Basic/Attr.td
@@ -1543,6 +1543,7 @@
   let Spellings = [GCC<"transparent_union">];
 //  let Subjects = SubjectList<[Record, TypedefName]>;
   let Documentation = [Undocumented];
+  let LangOpts = [COnly];
 }
 
 def Unavailable : InheritableAttr {
Index: test/SemaCXX/attr-gnu.cpp
===================================================================
--- test/SemaCXX/attr-gnu.cpp
+++ test/SemaCXX/attr-gnu.cpp
@@ -27,3 +27,19 @@
   void test3() __attribute__((cf_unknown_transfer)) override {} // Ok, not known to GCC.
 };
 }
+
+template<typename T>
+union Tu { T b; } __attribute__((transparent_union)); // expected-warning {{'transparent_union' attribute ignored}}
+
+template<typename T>
+union Tu2 { int x; T b; } __attribute__((transparent_union)); // expected-warning {{'transparent_union' attribute ignored}}
+
+union Tu3 { int x; } __attribute((transparent_union)); // expected-warning {{'transparent_union' attribute ignored}}
+
+void tuTest1(Tu<int> u); // expected-note {{candidate function not viable: no known conversion from 'int' to 'Tu<int>' for 1st argument}}
+void tuTest2(Tu3 u); // expected-note {{candidate function not viable: no known conversion from 'int' to 'Tu3' for 1st argument}}
+void tu() {
+  int x = 2;
+  tuTest1(x); // expected-error {{no matching function for call to 'tuTest1'}}
+  tuTest2(x); // expected-error {{no matching function for call to 'tuTest2'}}
+}