c++: use TREE_VEC for trailing args of variadic built-in traits

This patch makes us use TREE_VEC instead of TREE_LIST to represent the
trailing arguments of a variadic built-in trait.  These built-ins are
typically passed a simple pack expansion as the second argument, e.g.

   __is_constructible(T, Ts...)

and the main benefit of this representation change is that substituting
into this argument list is now basically free since tsubst_template_args
makes sure we reuse the TREE_VEC of the corresponding ARGUMENT_PACK when
expanding such a pack expansion.  In the previous TREE_LIST representation
we would need need to convert the expanded pack expansion into a TREE_LIST
(via tsubst_tree_list).

Note that an empty set of trailing arguments is now represented as an
empty TREE_VEC instead of NULL_TREE, so now TRAIT_TYPE/EXPR_TYPE2 will
be empty only for unary traits.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Convert a TREE_VEC
	of arguments into a TREE_LIST for sake of pretty printing.
	* cxx-pretty-print.cc (pp_cxx_trait): Handle TREE_VEC
	instead of TREE_LIST of trailing variadic trait arguments.
	* method.cc (constructible_expr): Likewise.
	(is_xible_helper): Likewise.
	* parser.cc (cp_parser_trait): Represent trailing variadic trait
	arguments as a TREE_VEC instead of TREE_LIST.
	* pt.cc (value_dependent_expression_p): Handle TREE_VEC
	instead of TREE_LIST of trailing variadic trait arguments.
	* semantics.cc (finish_type_pack_element): Likewise.
	(check_trait_type): Likewise.

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 273d15a..675299a 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3675,6 +3675,16 @@ diagnose_trait_expr (tree expr, tree args)
 
   tree t1 = TRAIT_EXPR_TYPE1 (expr);
   tree t2 = TRAIT_EXPR_TYPE2 (expr);
+  if (t2 && TREE_CODE (t2) == TREE_VEC)
+    {
+      /* Convert the TREE_VEC of arguments into a TREE_LIST, since we can't
+	 directly print a TREE_VEC but we can a TREE_LIST via the E format
+	 specifier.  */
+      tree list = NULL_TREE;
+      for (tree t : tree_vec_range (t2))
+	list = tree_cons (NULL_TREE, t, list);
+      t2 = nreverse (list);
+    }
   switch (TRAIT_EXPR_KIND (expr))
     {
     case CPTK_HAS_NOTHROW_ASSIGN: