c++, tree: walk TREE_VEC (and VECTOR_CST) in natural order [PR101886]

Unfortunately the extract_autos_r fix in r13-4799-ga7c8036b26082d is
derailed by the fact that walk_tree_1 currently walks the elements of a
TREE_VEC in reverse, which means for A<auto, auto> in the below testcase
extract_autos_r ends up adjusting the TEMPLATE_TYPE_IDX of the first
auto instead of the second one, and the first auto is the canonical auto
of level 2 (and index 0), so we rightfully trigger the sanity check
added in that commit.

It seems TREE_VEC and VECTOR_CST are the only trees we walk in reverse,
and this has been the case since r21884 whence the original version of
walk_tree_1 was introduced.  But that's arguably unnatural and not
consistent with how we walk all other compound trees such as CONSTRUCTORs
and EXPR_P trees in forward order.

So this patch makes walk_tree_1 walk TREE_VEC (and VECTOR_CST) in
forward order as well, which fixes the testcase.  Doing so revealed that
keep_template_parm grows the list of found template parameters from the
front, which previously compensated for the TREE_VEC behavior, so now we
should grow it from the back.

	PR c++/101886

gcc/cp/ChangeLog:

	* pt.cc (find_template_parameter_info::parm_list_tail):
	New data member.
	(keep_template_parm): Use parm_list_tail to append rather
	than prepend to parm_list.

gcc/ChangeLog:

	* tree.cc (walk_tree_1) <case TREE_VEC>: Walk the elements
	in forward instead of reverse order.
	<case VECTOR_CST>: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic12.C: Adjust expected order of
	template parameters within pretty printed parameter mapping.
	* g++.dg/concepts/auto6.C: New test.

0 files changed, 0 insertions, 0 deletions