Index: include/clang/Basic/TokenKinds.def
===================================================================
--- include/clang/Basic/TokenKinds.def
+++ include/clang/Basic/TokenKinds.def
@@ -243,6 +243,7 @@
 //                are enabled.
 //   KEYGNU   - This is a keyword if GNU extensions are enabled
 //   KEYMS    - This is a keyword if Microsoft extensions are enabled
+//   KEYMSCOMPAT - This is a keyword if Microsoft compatibility mode is enabled
 //   KEYNOMS18 - This is a keyword that must never be enabled under
 //               MSVC <= v18.
 //   KEYOPENCLC   - This is a keyword in OpenCL C
@@ -363,7 +364,7 @@
 CXX11_KEYWORD(decltype              , 0)
 CXX11_KEYWORD(noexcept              , 0)
 CXX11_KEYWORD(nullptr               , 0)
-CXX11_KEYWORD(static_assert         , 0)
+CXX11_KEYWORD(static_assert         , KEYMSCOMPAT)
 CXX11_KEYWORD(thread_local          , 0)
 
 // C++2a / concepts TS keywords
Index: lib/Basic/IdentifierTable.cpp
===================================================================
--- lib/Basic/IdentifierTable.cpp
+++ lib/Basic/IdentifierTable.cpp
@@ -99,6 +99,7 @@
     KEYMODULES    = 0x100000,
     KEYCXX2A      = 0x200000,
     KEYOPENCLCXX  = 0x400000,
+    KEYMSCOMPAT   = 0x800000,
     KEYALLCXX = KEYCXX | KEYCXX11 | KEYCXX2A,
     KEYALL = (0xffffff & ~KEYNOMS18 &
               ~KEYNOOPENCL) // KEYNOMS18 and KEYNOOPENCL are used to exclude.
@@ -125,6 +126,7 @@
   if (LangOpts.C99 && (Flags & KEYC99)) return KS_Enabled;
   if (LangOpts.GNUKeywords && (Flags & KEYGNU)) return KS_Extension;
   if (LangOpts.MicrosoftExt && (Flags & KEYMS)) return KS_Extension;
+  if (LangOpts.MSVCCompat && (Flags & KEYMSCOMPAT)) return KS_Enabled;
   if (LangOpts.Borland && (Flags & KEYBORLAND)) return KS_Extension;
   if (LangOpts.Bool && (Flags & BOOLSUPPORT)) return KS_Enabled;
   if (LangOpts.Half && (Flags & HALFSUPPORT)) return KS_Enabled;
Index: test/Lexer/keywords_test.c
===================================================================
--- test/Lexer/keywords_test.c
+++ test/Lexer/keywords_test.c
@@ -7,7 +7,7 @@
 // RUN: %clang_cc1 -std=gnu89 -fno-gnu-keywords -E %s -o - \
 // RUN:     | FileCheck --check-prefix=CHECK-NONE %s
 
-// RUN: %clang_cc1 -std=c99 -fms-extensions -E %s -o - \
+// RUN: %clang_cc1 -std=c99 -fms-extensions -fms-compatibility -E %s -o - \
 // RUN:     | FileCheck --check-prefix=CHECK-MS-KEYWORDS %s
 // RUN: %clang_cc1 -std=c99 -fdeclspec -E %s -o - \
 // RUN:     | FileCheck --check-prefix=CHECK-DECLSPEC-KEYWORD %s
@@ -42,3 +42,13 @@
 #else
 void has_declspec();
 #endif
+
+// CHECK-NONE: no_static_assert
+// CHECK-GNU-KEYWORDS: no_static_assert
+// CHECK-MS-KEYWORDS: has_static_assert
+// CHECK-MS-KEYWORDS-WITHOUT-DECLSPEC: no_static_assert
+#if __is_identifier(static_assert)
+void no_static_assert();
+#else
+void has_static_assert();
+#endif
Index: test/Lexer/keywords_test.cpp
===================================================================
--- test/Lexer/keywords_test.cpp
+++ test/Lexer/keywords_test.cpp
@@ -11,9 +11,9 @@
 // RUN: %clang_cc1 -std=c++03 -fdeclspec -fno-declspec -fsyntax-only %s
 // RUN: %clang_cc1 -std=c++03 -fms-extensions -fno-declspec -fdeclspec -DDECLSPEC -fsyntax-only %s
 // RUN: %clang_cc1 -std=c++03 -fms-extensions -fdeclspec -fno-declspec -fsyntax-only %s
-// RUN: %clang -std=c++03 -target i686-windows-msvc -DDECLSPEC -fsyntax-only %s
+// RUN: %clang -std=c++03 -target i686-windows-msvc -DMS -DDECLSPEC -fsyntax-only %s
 // RUN: %clang -std=c++03 -target x86_64-scei-ps4 -DDECLSPEC -fsyntax-only %s
-// RUN: %clang -std=c++03 -target i686-windows-msvc -fno-declspec -fsyntax-only %s
+// RUN: %clang -std=c++03 -target i686-windows-msvc -DMS -fno-declspec -fsyntax-only %s
 // RUN: %clang -std=c++03 -target x86_64-scei-ps4 -fno-declspec -fsyntax-only %s
 
 #define IS_KEYWORD(NAME) _Static_assert(!__is_identifier(NAME), #NAME)
@@ -51,7 +51,12 @@
 CXX11_TYPE(char32_t);
 CXX11_KEYWORD(constexpr);
 CXX11_KEYWORD(noexcept);
+#ifndef MS
 CXX11_KEYWORD(static_assert);
+#else
+// MS compiler recognizes static_assert in all modes. So should we.
+IS_KEYWORD(static_assert);
+#endif
 CXX11_KEYWORD(thread_local);
 
 // Concepts TS keywords