Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1262,6 +1262,8 @@
 def err_mutable_const : Error<"'mutable' and 'const' cannot be mixed">;
 def err_mutable_nonmember : Error<
   "'mutable' can only be applied to member variables">;
+def err_virtual_in_union : Error<
+  "unions cannot have virtual functions">;
 def err_virtual_non_function : Error<
   "'virtual' can only appear on non-static member functions">;
 def err_virtual_out_of_class : Error<
Index: cfe/trunk/lib/Sema/SemaDecl.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDecl.cpp
+++ cfe/trunk/lib/Sema/SemaDecl.cpp
@@ -7202,6 +7202,11 @@
           dyn_cast<CXXRecordDecl>(NewFD->getDeclContext())) {
       if (Parent->isInterface() && cast<CXXMethodDecl>(NewFD)->isUserProvided())
         NewFD->setPure(true);
+
+      // C++ [class.union]p2
+      //   A union can have member functions, but not virtual functions.
+      if (isVirtual && Parent->isUnion())
+        Diag(D.getDeclSpec().getVirtualSpecLoc(), diag::err_virtual_in_union);
     }
 
     SetNestedNameSpecifier(NewFD, D);
Index: cfe/trunk/test/SemaCXX/virtual-function-in-union.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/virtual-function-in-union.cpp
+++ cfe/trunk/test/SemaCXX/virtual-function-in-union.cpp
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+union x {
+  virtual void f(); // expected-error {{unions cannot have virtual functions}}
+};