c++: Fix ICE with -Wmismatched-tags [PR105725]

Here we ICE with -Wmismatched-tags on something like

  template <class T>
  bool B<T, enable_if_t<is_class_v<class T::foo>>>;

Specifically, the "class T::foo" bit.  There, class_decl_loc_t::add gets
a TYPENAME_TYPE as TYPE, rather than a class/union type, so checking
TYPE_BEING_DEFINED will crash.  I think it's OK to allow a TYPENAME_TYPE to
slip into that function; we just shouldn't consider the 'class' tag redundant
(which works as a 'typename').  In fact, every other compiler *requires* it.

	PR c++/105725

gcc/cp/ChangeLog:

	* parser.cc (class_decl_loc_t::add): Check CLASS_TYPE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wmismatched-tags-10.C: New test.

(cherry picked from commit d822f4bbd7)

gcc/cp/parser.cc

@@ -33508,7 +33508,8 @@ class_decl_loc_t::add (cp_parser *parser, location_t key_loc,
   bool key_redundant = (!def_p && !decl_p
 			&& (decl == type_decl
 			    || TREE_CODE (decl) == TEMPLATE_DECL
-			    || TYPE_BEING_DEFINED (type)));
+			    || (CLASS_TYPE_P (type)
+				&& TYPE_BEING_DEFINED (type))));
 
   if (key_redundant
       && class_key != class_type
@@ -33546,7 +33547,7 @@ class_decl_loc_t::add (cp_parser *parser, location_t key_loc,
 	}
       else
 	{
-	  /* TYPE was previously defined in some unknown precompiled hdeader.
+	  /* TYPE was previously defined in some unknown precompiled header.
 	     Simply add a record of its definition at an unknown location and
 	     proceed below to add a reference to it at the current location.
 	     (Declarations in precompiled headers that are not definitions

gcc/testsuite/g++.dg/warn/Wmismatched-tags-10.C

@@ -0,0 +1,10 @@
+// PR c++/105725
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wall -Wmismatched-tags" }
+
+template <bool> struct enable_if;
+template <bool Cond> using enable_if_t = typename enable_if<Cond>::type;
+template <typename> bool is_class_v;
+template <class, class> bool B;
+template <class T>
+bool B<T, enable_if_t<is_class_v<class T::foo>>>;