c++: trait as typename scope [PR116052]

The stdexec library currently wrongly ends up using __decay as the scope of
a typename, which leads to a crash.  Let's give an error instead.

	PR c++/116052

gcc/cp/ChangeLog:

	* mangle.cc (write_prefix): Handle TRAIT_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/decay1.C: New test.

gcc/cp/mangle.cc

@@ -1300,7 +1300,8 @@ write_prefix (const tree node)
 
   MANGLE_TRACE_TREE ("prefix", node);
 
-  if (TREE_CODE (node) == DECLTYPE_TYPE)
+  if (TREE_CODE (node) == DECLTYPE_TYPE
+      || TREE_CODE (node) == TRAIT_TYPE)
     {
       write_type (node);
       return;

gcc/testsuite/g++.dg/ext/decay1.C

@@ -0,0 +1,16 @@
+// PR c++/116052
+// { dg-do compile { target c++11 } }
+
+template <class T> using decay_t = __decay(T);
+template <class T> void g(typename decay_t<T>::foo); // { dg-error "built-in" }
+template <class T> void f()
+{
+  g<T>(0);
+}
+
+struct A { using foo = int; };
+
+int main()
+{
+  f<A>();
+}