Index: include/clang/Lex/Preprocessor.h
===================================================================
--- include/clang/Lex/Preprocessor.h
+++ include/clang/Lex/Preprocessor.h
@@ -125,6 +125,7 @@
   IdentifierInfo *Ident__has_include;              // __has_include
   IdentifierInfo *Ident__has_include_next;         // __has_include_next
   IdentifierInfo *Ident__has_warning;              // __has_warning
+  IdentifierInfo *Ident__is_identifier;            // __is_identifier
   IdentifierInfo *Ident__building_module;          // __building_module
   IdentifierInfo *Ident__MODULE__;                 // __MODULE__
 
Index: lib/Lex/PPMacroExpansion.cpp
===================================================================
--- lib/Lex/PPMacroExpansion.cpp
+++ lib/Lex/PPMacroExpansion.cpp
@@ -114,6 +114,7 @@
   Ident__has_include      = RegisterBuiltinMacro(*this, "__has_include");
   Ident__has_include_next = RegisterBuiltinMacro(*this, "__has_include_next");
   Ident__has_warning      = RegisterBuiltinMacro(*this, "__has_warning");
+  Ident__is_identifier    = RegisterBuiltinMacro(*this, "__is_identifier");
 
   // Modules.
   if (LangOpts.Modules) {
@@ -1372,6 +1373,7 @@
   } else if (II == Ident__has_feature   ||
              II == Ident__has_extension ||
              II == Ident__has_builtin   ||
+             II == Ident__is_identifier ||
              II == Ident__has_attribute) {
     // The argument to these builtins should be a parenthesized identifier.
     SourceLocation StartLoc = Tok.getLocation();
@@ -1395,6 +1397,8 @@
     bool Value = false;
     if (!IsValid)
       Diag(StartLoc, diag::err_feature_check_malformed);
+    else if (II == Ident__is_identifier)
+      Value = FeatureII->getTokenID() == tok::identifier;
     else if (II == Ident__has_builtin) {
       // Check for a builtin is trivial.
       Value = FeatureII->getBuiltinID() != 0;
Index: test/Lexer/keywords_test.c
===================================================================
--- test/Lexer/keywords_test.c
+++ test/Lexer/keywords_test.c
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -std=c99 -E %s -o - | FileCheck --check-prefix=CHECK-NONE %s
+
+// RUN: %clang_cc1 -std=gnu89 -E %s -o - \
+// RUN:     | FileCheck --check-prefix=CHECK-GNU-KEYWORDS %s
+// RUN: %clang_cc1 -std=c99 -fgnu-keywords -E %s -o - \
+// RUN:     | FileCheck --check-prefix=CHECK-GNU-KEYWORDS %s
+// 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:     | FileCheck --check-prefix=CHECK-MS-KEYWORDS %s
+
+void f() {
+// CHECK-NONE: int asm
+// CHECK-GNU-KEYWORDS: asm ("ret" : :)
+#if __is_identifier(asm)
+  int asm;
+#else
+  asm ("ret" : :);
+#endif
+}
+
+// CHECK-NONE: no_ms_wchar
+// CHECK-MS-KEYWORDS: has_ms_wchar
+#if __is_identifier(__wchar_t)
+void no_ms_wchar();
+#else
+void has_ms_wchar();
+#endif