c++: deducing from class type of NTTP [PR105110]

Here when attempting to deduce T in the NTTP type A<T> from the argument
type 'const A<int>', we give up due to the const:

  types ‘A<T>’ and ‘const A<int>’ have incompatible cv-qualifiers

But since the type of an NTTP cannot be cv-qualified, it seems natural
to ignore cv-qualifiers on the argument type before attempting to unify
the two types.

	PR c++/105110

gcc/cp/ChangeLog:

	* pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Drop cv-quals from
	the argument type of an NTTP before deducing from it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class52.C: New test.

gcc/cp/pt.cc

@@ -24266,8 +24266,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
 	      && !(strict & UNIFY_ALLOW_INTEGER)
 	      && TEMPLATE_PARM_LEVEL (parm) <= TMPL_ARGS_DEPTH (targs))
 	    {
-	      /* Deduce it from the non-type argument.  */
-	      tree atype = TREE_TYPE (arg);
+	      /* Deduce it from the non-type argument.  As above, ignore
+		 top-level quals here too.  */
+	      tree atype = cv_unqualified (TREE_TYPE (arg));
 	      RECUR_AND_CHECK_FAILURE (tparms, targs,
 				       tparm, atype,
 				       UNIFY_ALLOW_NONE, explain_p);

gcc/testsuite/g++.dg/cpp2a/nontype-class52.C

@@ -0,0 +1,13 @@
+// PR c++/105110
+// { dg-do compile { target c++20 } }
+
+template<class> struct A { };
+
+template<auto> struct B { };
+
+template<class T, A<T> V> void f(B<V>);
+
+int main() {
+  constexpr A<int> a;
+  f(B<a>{});
+}