c++: variadic class template placeholder deduction [PR97134]

do_class_deduction handles specially the case where we're deducing one
placeholder from another equivalent one, but here the initializer passed
to do_class_deduction is wrapped in an EXPR_PACK_EXPANSION (we're being
called from unify during get_partial_spec_bindings).  This patch makes
do_class_deduction look through EXPR_PACK_EXPANSIONs so that we detect
this case as well.

gcc/cp/ChangeLog:

	PR c++/97134
	* pt.c (do_class_deduction): Look through EXPR_PACK_EXPANSION
	when checking if the initializer is an equivalent class
	placeholder template parameter.

gcc/testsuite/ChangeLog:

	PR c++/97134
	* g++.dg/cpp2a/nontype-class43.C: New test.

gcc/cp/pt.c

@@ -29286,7 +29286,11 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
     return ptype;
   /* Initializing one placeholder from another.  */
-  if (init && TREE_CODE (init) == TEMPLATE_PARM_INDEX
+  if (init
+      && (TREE_CODE (init) == TEMPLATE_PARM_INDEX
+	  || (TREE_CODE (init) == EXPR_PACK_EXPANSION
+	      && (TREE_CODE (PACK_EXPANSION_PATTERN (init))
+		  == TEMPLATE_PARM_INDEX)))
       && is_auto (TREE_TYPE (init))
       && CLASS_PLACEHOLDER_TEMPLATE (TREE_TYPE (init)) == tmpl)
     return cp_build_qualified_type (TREE_TYPE (init), cp_type_quals (ptype));

gcc/testsuite/g++.dg/cpp2a/nontype-class43.C

+// PR c++/97134
+// { dg-do compile { target c++20 } }
+
+template<typename T>
+struct templ {};
+
+template<templ... Vs>
+struct wrapper {};
+
+template<templ... Vs> requires true
+struct wrapper<Vs...> {};