[Bug c++/101886] [11/12 Regression][concepts TS] ICE with auto as template argument since r11-7011-g6e0a231a4aa2407b

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101886

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:8aa045c97485d75e0b6eb1068464c0ddaa6beacb

commit r13-4810-g8aa045c97485d75e0b6eb1068464c0ddaa6beacb
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Dec 20 17:02:37 2022 -0500

    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.