c++: ICE with -Wmismatched-tags and member template [PR106259]

-Wmismatched-tags warns about the (harmless) struct/class mismatch.
For, e.g.,

  template<typename T> struct A { };
  class A<int> a;

it works by adding A<T> to the class2loc hash table while parsing the
class-head and then, while parsing the elaborate type-specifier, we
add A<int>.  At the end of c_parse_file we go through the table and
warn about the class-key mismatches.  In this PR we crash though; we
have

  template<typename T> struct A {
    template<typename U> struct W { };
  };
  struct A<int>::W<int> w; // #1

where while parsing A and #1 we've stashed
   A<T>
   A<T>::W<U>
   A<int>::W<int>
into class2loc.  Then in class_decl_loc_t::diag_mismatched_tags TYPE
is A<int>::W<int>, and specialization_of gets us A<int>::W<U>, which
is not in class2loc, so we crash on gcc_assert (cdlguide).  But it's
OK not to have found A<int>::W<U>, we should just look one "level" up,
that is, A<T>::W<U>.

It's important to handle class specializations, so e.g.

  template<>
  struct A<char> {
    template<typename U>
    class W { };
  };

where W's class-key is different than in the primary template above,
so we should warn depending on whether we're looking into A<char>
or into a different instantiation.

	PR c++/106259

gcc/cp/ChangeLog:

	* parser.cc (class_decl_loc_t::diag_mismatched_tags): If the first
	lookup of SPEC didn't find anything, try to look for
	most_general_template.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wmismatched-tags-11.C: New test.

gcc/cp/parser.cc

@@ -34473,14 +34473,32 @@ class_decl_loc_t::diag_mismatched_tags (tree type_decl)
 	 be (and inevitably is) at index zero.  */
       tree spec = specialization_of (type);
       cdlguide = class2loc.get (spec);
+      /* It's possible that we didn't find SPEC.  Consider:
+
+	   template<typename T> struct A {
+	     template<typename U> struct W { };
+	   };
+	   struct A<int>::W<int> w; // #1
+
+	 where while parsing A and #1 we've stashed
+	   A<T>
+	   A<T>::W<U>
+	   A<int>::W<int>
+	 into CLASS2LOC.  If TYPE is A<int>::W<int>, specialization_of
+	 will yield A<int>::W<U> which may be in CLASS2LOC if we had
+	 an A<int> class specialization, but otherwise won't be in it.
+	 So try to look up A<T>::W<U>.  */
+      if (!cdlguide)
+	{
+	  spec = DECL_TEMPLATE_RESULT (most_general_template (spec));
+	  cdlguide = class2loc.get (spec);
+	}
+      /* Now we really should have found something.  */
       gcc_assert (cdlguide != NULL);
     }
-  else
-    {
-      /* Skip declarations that consistently use the same class-key.  */
-      if (def_class_key != none_type)
-	return;
-    }
+  /* Skip declarations that consistently use the same class-key.  */
+  else if (def_class_key != none_type)
+    return;
 
   /* Set if a definition for the class has been seen.  */
   const bool def_p = cdlguide->def_p ();

gcc/testsuite/g++.dg/warn/Wmismatched-tags-11.C

@@ -0,0 +1,23 @@
+// PR c++/106259
+// { dg-do compile }
+// { dg-options "-Wmismatched-tags" }
+
+template<typename T> struct A {
+  template<typename U>
+  struct W { };
+};
+
+template<>
+struct A<char> {
+  template<typename U>
+  class W { };
+};
+
+void
+g ()
+{
+  struct A<char>::W<int> w1; // { dg-warning "mismatched" }
+  struct A<int>::W<int> w2;
+  class A<char>::W<int> w3;
+  class A<int>::W<int> w4; // { dg-warning "mismatched" }
+}