Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -3238,8 +3238,17 @@
     Diag(TheCall->getArg(1)->getLocStart(),
          diag::warn_second_arg_of_va_start_not_last_named_param);
   else if (IsCRegister || Type->isReferenceType() ||
-           Type->isPromotableIntegerType() ||
-           Type->isSpecificBuiltinType(BuiltinType::Float)) {
+           Type->isSpecificBuiltinType(BuiltinType::Float) || [=] {
+             // Promotable integers are UB, but enumerations need a bit of
+             // extra checking to see what their promotable type actually is.
+             if (!Type->isPromotableIntegerType())
+               return false;
+             if (!Type->isEnumeralType())
+               return true;
+             const EnumDecl *ED = Type->getAs<EnumType>()->getDecl();
+             return !(ED &&
+                      Context.typesAreCompatible(ED->getPromotionType(), Type));
+           }()) {
     unsigned Reason = 0;
     if (Type->isReferenceType())  Reason = 1;
     else if (IsCRegister)         Reason = 2;
Index: test/Sema/varargs.c
===================================================================
--- test/Sema/varargs.c
+++ test/Sema/varargs.c
@@ -1,5 +1,6 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify %s -triple i386-pc-unknown
 // RUN: %clang_cc1 -fsyntax-only -verify %s -triple x86_64-apple-darwin9
+// RUN: %clang_cc1 -fsyntax-only -fms-compatibility -DMS -verify %s
 
 void f1(int a)
 {
@@ -94,3 +95,20 @@
   __builtin_va_start(ap, i); // expected-warning {{passing a parameter declared with the 'register' keyword to 'va_start' has undefined behavior}}
   __builtin_va_end(ap);
 }
+
+enum __attribute__((packed)) E1 {
+  one1
+};
+
+void f13(enum E1 e, ...) {
+  __builtin_va_list va;
+  __builtin_va_start(va, e);
+#ifndef MS
+  // In Microsoft compatibility mode, all enum types are int, but in
+  // non-ms-compatibility mode, this enumeration type will undergo default
+  // argument promotions.
+  // expected-note@-7 {{parameter of type 'enum E1' is declared here}}
+  // expected-warning@-6 {{passing an object that undergoes default argument promotion to 'va_start' has undefined behavior}}
+#endif
+  __builtin_va_end(va);
+}