C++ front-end: support attributes with tag-less structs

Seen in GCC's STL.

Author: tautschnig

regression/cpp/type_traits_essentials1/main.cpp

@@ -28,6 +28,10 @@ template<typename T> using x=V<T>;
 
 #ifdef __GNUC__
 using type __attribute__((__nodebug__)) = int;
+template <std::size_t _Align = __alignof__(int)>
+struct __attribute__((__aligned__((_Align))))
+{
+} __align;
 #endif
 
 struct alignas(0x1) t

src/cpp/parse.cpp

@@ -4470,6 +4470,12 @@ bool Parser::rClassSpec(typet &spec)
   std::cout << std::string(__indent, ' ') << "Parser::rClassSpec 3\n";
 #endif
 
+  if(!optAlignas(spec))
+    return false;
+
+  if(!optAttribute(spec))
+    return false;
+
   if(lex.LookAhead(0)=='{')
   {
     // no tag
@@ -4479,12 +4485,6 @@ bool Parser::rClassSpec(typet &spec)
   }
   else
   {
-    if(!optAlignas(spec))
-      return false;
-
-    if(!optAttribute(spec))
-      return false;
-
     irept name;
 
     if(!rName(name))