Index: cfe/trunk/include/clang/Basic/IdentifierTable.h
===================================================================
--- cfe/trunk/include/clang/Basic/IdentifierTable.h
+++ cfe/trunk/include/clang/Basic/IdentifierTable.h
@@ -62,7 +62,7 @@
                                    // partially) from an AST file.
   bool ChangedAfterLoad       : 1; // True if identifier has changed from the
                                    // definition loaded from an AST file.
-  bool RevertedTokenID        : 1; // True if RevertTokenIDToIdentifier was
+  bool RevertedTokenID        : 1; // True if revertTokenIDToIdentifier was
                                    // called.
   bool OutOfDate              : 1; // True if there may be additional
                                    // information about this identifier
@@ -152,7 +152,7 @@
   /// tokens.
   tok::TokenKind getTokenID() const { return (tok::TokenKind)TokenID; }
 
-  /// \brief True if RevertTokenIDToIdentifier() was called.
+  /// \brief True if revertTokenIDToIdentifier() was called.
   bool hasRevertedTokenIDToIdentifier() const { return RevertedTokenID; }
 
   /// \brief Revert TokenID to tok::identifier; used for GNU libstdc++ 4.2
@@ -166,6 +166,11 @@
     TokenID = tok::identifier;
     RevertedTokenID = true;
   }
+  void revertIdentifierToTokenID(tok::TokenKind TK) {
+    assert(TokenID == tok::identifier && "Should be at tok::identifier");
+    TokenID = TK;
+    RevertedTokenID = false;
+  }
 
   /// \brief Return the preprocessor keyword ID for this identifier.
   ///
Index: cfe/trunk/include/clang/Basic/TokenKinds.def
===================================================================
--- cfe/trunk/include/clang/Basic/TokenKinds.def
+++ cfe/trunk/include/clang/Basic/TokenKinds.def
@@ -280,7 +280,7 @@
 KEYWORD(while                       , KEYALL)
 KEYWORD(_Alignas                    , KEYALL)
 KEYWORD(_Alignof                    , KEYALL)
-KEYWORD(_Atomic                     , KEYALL|KEYNOMS18|KEYNOOPENCL)
+KEYWORD(_Atomic                     , KEYALL|KEYNOOPENCL)
 KEYWORD(_Bool                       , KEYNOCXX)
 KEYWORD(_Complex                    , KEYALL)
 KEYWORD(_Generic                    , KEYALL)
Index: cfe/trunk/lib/Parse/ParseDecl.cpp
===================================================================
--- cfe/trunk/lib/Parse/ParseDecl.cpp
+++ cfe/trunk/lib/Parse/ParseDecl.cpp
@@ -2647,6 +2647,18 @@
     const char *PrevSpec = nullptr;
     unsigned DiagID = 0;
 
+    // HACK: MSVC doesn't consider _Atomic to be a keyword and its STL
+    // implementation for VS2013 uses _Atomic as an identifier for one of the
+    // classes in <atomic>.
+    //
+    // A typedef declaration containing _Atomic<...> is among the places where
+    // the class is used.  If we are currently parsing such a declaration, treat
+    // the token as an identifier.
+    if (getLangOpts().MSVCCompat && Tok.is(tok::kw__Atomic) &&
+        DS.getStorageClassSpec() == clang::DeclSpec::SCS_typedef &&
+        !DS.hasTypeSpecifier() && GetLookAheadToken(1).is(tok::less))
+      Tok.setKind(tok::identifier);
+
     SourceLocation Loc = Tok.getLocation();
 
     switch (Tok.getKind()) {
Index: cfe/trunk/lib/Parse/ParseDeclCXX.cpp
===================================================================
--- cfe/trunk/lib/Parse/ParseDeclCXX.cpp
+++ cfe/trunk/lib/Parse/ParseDeclCXX.cpp
@@ -1308,6 +1308,35 @@
     // allow libstdc++ 4.2 and libc++ to work properly.
     TryKeywordIdentFallback(true);
 
+  struct PreserveAtomicIdentifierInfoRAII {
+    PreserveAtomicIdentifierInfoRAII(Token &Tok, bool Enabled)
+        : AtomicII(nullptr) {
+      if (!Enabled)
+        return;
+      assert(Tok.is(tok::kw__Atomic));
+      AtomicII = Tok.getIdentifierInfo();
+      AtomicII->revertTokenIDToIdentifier();
+      Tok.setKind(tok::identifier);
+    }
+    ~PreserveAtomicIdentifierInfoRAII() {
+      if (!AtomicII)
+        return;
+      AtomicII->revertIdentifierToTokenID(tok::kw__Atomic);
+    }
+    IdentifierInfo *AtomicII;
+  };
+
+  // HACK: MSVC doesn't consider _Atomic to be a keyword and its STL
+  // implementation for VS2013 uses _Atomic as an identifier for one of the
+  // classes in <atomic>.  When we are parsing 'struct _Atomic', don't consider
+  // '_Atomic' to be a keyword.  We are careful to undo this so that clang can
+  // use '_Atomic' in its own header files.
+  bool ShouldChangeAtomicToIdentifier = getLangOpts().MSVCCompat &&
+                                        Tok.is(tok::kw__Atomic) &&
+                                        TagType == DeclSpec::TST_struct;
+  PreserveAtomicIdentifierInfoRAII AtomicTokenGuard(
+      Tok, ShouldChangeAtomicToIdentifier);
+
   // Parse the (optional) nested-name-specifier.
   CXXScopeSpec &SS = DS.getTypeSpecScope();
   if (getLangOpts().CPlusPlus) {
@@ -1859,6 +1888,15 @@
   CheckMisplacedCXX11Attribute(Attributes, StartLoc);
 
   // Parse the class-name.
+
+  // HACK: MSVC doesn't consider _Atomic to be a keyword and its STL
+  // implementation for VS2013 uses _Atomic as an identifier for one of the
+  // classes in <atomic>.  Treat '_Atomic' to be an identifier when we are
+  // parsing the class-name for a base specifier.
+  if (getLangOpts().MSVCCompat && Tok.is(tok::kw__Atomic) &&
+      NextToken().is(tok::less))
+    Tok.setKind(tok::identifier);
+
   SourceLocation EndLocation;
   SourceLocation BaseLoc;
   TypeResult BaseType = ParseBaseTypeSpecifier(BaseLoc, EndLocation);
Index: cfe/trunk/test/SemaCXX/MicrosoftCompatibility.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/MicrosoftCompatibility.cpp
+++ cfe/trunk/test/SemaCXX/MicrosoftCompatibility.cpp
@@ -9,11 +9,18 @@
 typedef unsigned int char32_t;
 #endif
 
-#if _MSC_VER >= 1900
 _Atomic(int) z;
-#else
-struct _Atomic {};
-#endif
+template <typename T>
+struct _Atomic {
+  _Atomic() {}
+  ~_Atomic() {}
+};
+template <typename T>
+struct atomic : _Atomic<T> {
+  typedef _Atomic<T> TheBase;
+  TheBase field;
+};
+_Atomic(int) alpha;
 
 typename decltype(3) a; // expected-warning {{expected a qualified name after 'typename'}}