Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td	(revision 182601)
+++ include/clang/Basic/DiagnosticSemaKinds.td	(working copy)
@@ -1179,6 +1179,9 @@
 def warn_cxx98_compat_static_data_member_in_union : Warning<
   "static data member %0 in union is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
+def ext_union_member_of_reference_type : ExtWarn<
+  "union member %0 has reference type %1, which is a Microsoft extension">,
+  InGroup<Microsoft>;
 def err_union_member_of_reference_type : Error<
   "union member %0 has reference type %1">;
 def ext_anonymous_struct_union_qualified : Extension<
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp	(revision 182601)
+++ lib/Sema/SemaDecl.cpp	(working copy)
@@ -10680,10 +10680,14 @@
       }
 
       // C++ [class.union]p1: If a union contains a member of reference type,
-      // the program is ill-formed.
+      // the program is ill-formed.  Except when compiling with MSVC extensions
+      // enabled.
       if (EltTy->isReferenceType()) {
-        Diag(NewFD->getLocation(), diag::err_union_member_of_reference_type)
+        Diag(NewFD->getLocation(), getLangOpts().MicrosoftExt ? 
+                                    diag::ext_union_member_of_reference_type :
+                                    diag::err_union_member_of_reference_type)
           << NewFD->getDeclName() << EltTy;
+        if (!getLangOpts().MicrosoftExt)
         NewFD->setInvalidDecl();
       }
     }
Index: test/SemaCXX/MicrosoftExtensions.cpp
===================================================================
--- test/SemaCXX/MicrosoftExtensions.cpp	(revision 182601)
+++ test/SemaCXX/MicrosoftExtensions.cpp	(working copy)
@@ -333,3 +333,8 @@
   c3.g(); // Overloaded incdec op operand
   c3.h(); // Overloaded unary op operand
 }
+
+union u {
+  int *i1;
+  int &i2;  // expected-warning {{union member 'i2' has reference type 'int &', which is a Microsoft extension"}}
+};