Index: lib/Basic/Targets.cpp
===================================================================
--- lib/Basic/Targets.cpp
+++ lib/Basic/Targets.cpp
@@ -571,8 +571,6 @@
       Builder.defineMacro("_MSC_VER", Twine(Opts.MSCVersion));
 
     if (Opts.MicrosoftExt) {
-      Builder.defineMacro("_MSC_EXTENSIONS");
-
       if (Opts.CPlusPlus11) {
         Builder.defineMacro("_RVALUE_REFERENCES_V2_SUPPORTED");
         Builder.defineMacro("_RVALUE_REFERENCES_SUPPORTED");
Index: lib/Frontend/InitPreprocessor.cpp
===================================================================
--- lib/Frontend/InitPreprocessor.cpp
+++ lib/Frontend/InitPreprocessor.cpp
@@ -508,6 +508,8 @@
   }
 
   if (LangOpts.MicrosoftExt) {
+    Builder.defineMacro("_MSC_EXTENSIONS");
+
     // Both __PRETTY_FUNCTION__ and __FUNCTION__ are GCC extensions, however
     // VC++ appears to only like __FUNCTION__.
     Builder.defineMacro("__PRETTY_FUNCTION__", "__FUNCTION__");
Index: test/Preprocessor/predefined-macros.c
===================================================================
--- test/Preprocessor/predefined-macros.c
+++ test/Preprocessor/predefined-macros.c
@@ -10,6 +10,9 @@
 // CHECK-MS: #define _WIN32 1
 // CHECK-MS-NOT: #define __GNUC__
 //
+// RUN: %clang_cc1 %s -E -dM -fms-extensions -o - | FileCheck %s --check-prefix=CHECK-MS-EXT
+// CHECK-MS-EXT: #define _MSC_EXTENSIONS 1
+//
 // RUN: %clang_cc1 %s -E -dM -ffast-math -o - \
 // RUN:   | FileCheck %s --check-prefix=CHECK-FAST-MATH
 // CHECK-FAST-MATH: #define __FAST_MATH__