Index: cfe/trunk/include/clang/Sema/Sema.h
===================================================================
--- cfe/trunk/include/clang/Sema/Sema.h
+++ cfe/trunk/include/clang/Sema/Sema.h
@@ -7737,6 +7737,7 @@
     VAK_Valid,
     VAK_ValidInCXX11,
     VAK_Undefined,
+    VAK_MSVCUndefined,
     VAK_Invalid
   };
 
Index: cfe/trunk/lib/Sema/SemaChecking.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaChecking.cpp
+++ cfe/trunk/lib/Sema/SemaChecking.cpp
@@ -3569,6 +3569,7 @@
       break;
 
     case Sema::VAK_Undefined:
+    case Sema::VAK_MSVCUndefined:
       EmitFormatDiagnostic(
         S.PDiag(diag::warn_non_pod_vararg_with_format_string)
           << S.getLangOpts().CPlusPlus11
Index: cfe/trunk/lib/Sema/SemaExpr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaExpr.cpp
+++ cfe/trunk/lib/Sema/SemaExpr.cpp
@@ -800,6 +800,9 @@
   if (Ty->isObjCObjectType())
     return VAK_Invalid;
 
+  if (getLangOpts().MSVCCompat)
+    return VAK_MSVCUndefined;
+
   // FIXME: In C++11, these cases are conditionally-supported, meaning we're
   // permitted to reject them. We should consider doing so.
   return VAK_Undefined;
@@ -829,6 +832,7 @@
     break;
 
   case VAK_Undefined:
+  case VAK_MSVCUndefined:
     DiagRuntimeBehavior(
         E->getLocStart(), nullptr,
         PDiag(diag::warn_cannot_pass_non_pod_arg_to_vararg)
Index: cfe/trunk/test/CodeGenCXX/vararg-non-pod-ms-compat.cpp
===================================================================
--- cfe/trunk/test/CodeGenCXX/vararg-non-pod-ms-compat.cpp
+++ cfe/trunk/test/CodeGenCXX/vararg-non-pod-ms-compat.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -Wno-error=non-pod-varargs -triple i686-pc-win32 -fms-compatibility -emit-llvm -o - %s | FileCheck %s -check-prefix=X86 -check-prefix=CHECK
+// RUN: %clang_cc1 -Wno-error=non-pod-varargs -triple x86_64-pc-win32 -fms-compatibility -emit-llvm -o - %s | FileCheck %s -check-prefix=X64 -check-prefix=CHECK
+
+struct X {
+  X();
+  ~X();
+  int data;
+};
+
+void vararg(...);
+
+void test(X x) {
+  // CHECK-LABEL: define void @"\01?test@@YAXUX@@@Z"
+
+  // X86: %[[argmem:[^ ]*]] = alloca inalloca <{ %struct.X }>
+  // X86: call void (<{ %struct.X }>*, ...)* bitcast (void (...)* @"\01?vararg@@YAXZZ" to void (<{ %struct.X }>*, ...)*)(<{ %struct.X }>* inalloca %[[argmem]])
+
+  // X64: %[[valptr:[^ ]*]] = getelementptr %struct.X* %{{[^ ]*}}, i32 0, i32 0
+  // X64: %[[val:[^ ]*]] = load i32* %[[valptr]]
+  // X64: call void (...)* @"\01?vararg@@YAXZZ"(i32 %[[val]])
+
+  // CHECK-NOT: llvm.trap
+  vararg(x);
+  // CHECK: ret void
+}
+
Index: cfe/trunk/test/SemaCXX/vararg-non-pod.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/vararg-non-pod.cpp
+++ cfe/trunk/test/SemaCXX/vararg-non-pod.cpp
@@ -1,5 +1,8 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -fblocks %s -Wno-error=non-pod-varargs
 
+// Check that the warning is still there under -fms-compatibility.
+// RUN: %clang_cc1 -fsyntax-only -verify -fblocks %s -Wno-error=non-pod-varargs -fms-compatibility
+
 extern char version[];
 
 class C {