C++ front-end: permit GCC attributes in using declarations

LLVM's standard library uses these in type_traits.

Author: tautschnig

regression/cpp/type_traits_essentials1/main.cpp

@@ -26,6 +26,10 @@ struct G<T(A......)>
 using i=int;
 template<typename T> using x=V<T>;
 
+#ifdef __GNUC__
+using type __attribute__((__nodebug__)) = int;
+#endif
+
 template<typename T>
 class C
 {

src/cpp/parse.cpp

@@ -583,7 +583,23 @@ bool Parser::rDefinition(cpp_itemt &item)
   else if(t==TOK_INLINE && lex.LookAhead(1)==TOK_NAMESPACE)
     return rNamespaceSpec(item.make_namespace_spec());
   else if(t==TOK_USING)
-    return rUsingOrTypedef(item);
+  {
+    cpp_token_buffert::post pos = lex.Save();
+
+    cpp_tokent tk;
+    lex.get_token(tk);
+    typet discard;
+    if(
+      is_identifier(lex.get_token(tk)) && optAttribute(discard) &&
+      lex.LookAhead(0) == '=')
+    {
+      lex.Restore(pos);
+      return rTypedefUsing(item.make_declaration());
+    }
+
+    lex.Restore(pos);
+    return rUsing(item.make_using());
+  }
   else if(t==TOK_STATIC_ASSERT)
     return rStaticAssert(item.make_static_assert());
   else