Introduce an egregious hack to fix PR4828.

The problem this change addresses is that we treat __is_pod and
__is_empty as keywords in C++, because they are built-in type traits
in GCC >= 4.3. However, GNU libstdc++ 4.2 (and possibly earlier
versions) define implementation-detail struct templates named __is_pod
and __is_empty. 

This commit solves the problem by recognizing

  struct __is_pod

and

  struct __is_empty

as special token sequences. When one of these token sequences is
encountered, the keyword (__is_pod or __is_empty) is implicitly
downgraded to an identifier so that parsing can continue. This is an
egregious hack, but it has the virtue of "just working" whether
someone is using libstdc++ 4.2 or not, without the need for special
flags.

llvm-svn: 80988

clang/lib/Parse/ParseDeclCXX.cpp

@@ -536,6 +536,26 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
   if (Tok.is(tok::kw___declspec))
     Attr = ParseMicrosoftDeclSpec(Attr);
   
+  if (TagType == DeclSpec::TST_struct && Tok.is(tok::kw___is_pod)) {
+    // GNU libstdc++ 4.2 uses __is_pod as the name of a struct template, but
+    // __is_pod is a keyword in GCC >= 4.3. Therefore, when we see the
+    // token sequence "struct __is_pod", make __is_pod into a normal 
+    // identifier rather than a keyword, to allow libstdc++ 4.2 to work
+    // properly.
+    Tok.getIdentifierInfo()->setTokenID(tok::identifier);
+    Tok.setKind(tok::identifier);
+  }
+
+  if (TagType == DeclSpec::TST_struct && Tok.is(tok::kw___is_empty)) {
+    // GNU libstdc++ 4.2 uses __is_empty as the name of a struct template, but
+    // __is_empty is a keyword in GCC >= 4.3. Therefore, when we see the
+    // token sequence "struct __is_empty", make __is_empty into a normal 
+    // identifier rather than a keyword, to allow libstdc++ 4.2 to work
+    // properly.
+    Tok.getIdentifierInfo()->setTokenID(tok::identifier);
+    Tok.setKind(tok::identifier);
+  }
+  
   // Parse the (optional) nested-name-specifier.
   CXXScopeSpec SS;
   if (getLang().CPlusPlus && 

clang/test/SemaCXX/libstdcxx_is_pod_hack.cpp

@@ -0,0 +1,7 @@
+// RUN: clang-cc -fsyntax-only %s
+
+template<typename T>
+struct __is_pod {
+};
+
+__is_pod<int> ipi;