c++: Fix ICE with variadic concepts and aliases [PR93907]

This patch (naively) extends the PR93907 fix to also apply to variadic
concepts invoked with a type argument pack.  Without this, we ICE on
the below testcase (a variadic version of concepts-using2.C) in the same
manner as we used to on concepts-using2.C before r10-7133.

gcc/cp/ChangeLog:

	PR c++/93907
	* constraint.cc (tsubst_parameter_mapping): Also canonicalize
	the type arguments of a TYPE_ARGUMENT_PACk.

gcc/testsuite/ChangeLog:

	PR c++/93907
	* g++.dg/cpp2a/concepts-using3.C: New test, based off of
	concepts-using2.C.

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index b6f6f0d..c871a8a 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2252,6 +2252,16 @@ tsubst_parameter_mapping (tree map, tree args, subst_info info)
 	  new_arg = tsubst_template_arg (arg, args, complain, in_decl);
 	  if (TYPE_P (new_arg))
 	    new_arg = canonicalize_type_argument (new_arg, complain);
+	  if (TREE_CODE (new_arg) == TYPE_ARGUMENT_PACK)
+	    {
+	      tree pack_args = ARGUMENT_PACK_ARGS (new_arg);
+	      for (int i = 0; i < TREE_VEC_LENGTH (pack_args); i++)
+		{
+		  tree& pack_arg = TREE_VEC_ELT (pack_args, i);
+		  if (TYPE_P (pack_arg))
+		    pack_arg = canonicalize_type_argument (pack_arg, complain);
+		}
+	    }
 	}
       if (new_arg == error_mark_node)
 	return error_mark_node;