[Bug c++/101764] ICE for constexpr if within fold expression within lambda expression within a template

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101764

--- Comment #2 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:c8b2b89358481d36755dbc99e585a251780453b0

commit r12-3488-gc8b2b89358481d36755dbc99e585a251780453b0
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Sep 13 10:29:32 2021 -0400

    c++: parameter pack inside constexpr if [PR101764]

    Here when partially instantiating the first pack expansion, substitution
    into the condition of the constexpr if yields a still-dependent tree, so
    tsubst_expr returns an IF_STMT with an unsubstituted IF_COND and with
    IF_STMT_EXTRA_ARGS added to.  Hence after partial instantiation the pack
    expansion pattern still refers to the unlowered parameter pack 'ts' of
    level 2, and it's thusly recorded in the new
PACK_EXPANSION_PARAMETER_PACKS.
    During the subsequent final instantiation of the regenerated lambda we
    crash in tsubst_pack_expansion because it can't find an argument pack
    for this unlowered 'ts', due to the level mismatch.  (Likewise when the
    constexpr if is replaced by a requires-expr, which also uses the extra
    args mechanism for avoiding partial instantiation.)

    So essentially, a pack expansion pattern that contains an "extra args"
    tree doesn't play well with partial instantiation.  This patch fixes
    this by forcing such pack expansions to use the extra args mechanism as
    well.

            PR c++/101764

    gcc/cp/ChangeLog:

            * cp-tree.h (PACK_EXPANSION_FORCE_EXTRA_ARGS_P): New accessor
            macro.
            * pt.c (has_extra_args_mechanism_p): New function.
            (find_parameter_pack_data::found_extra_args_tree_p): New data
            member.
            (find_parameter_packs_r): Set ppd->found_extra_args_tree_p
            appropriately.
            (make_pack_expansion): Set PACK_EXPANSION_FORCE_EXTRA_ARGS_P if
            ppd.found_extra_args_tree_p.
            (use_pack_expansion_extra_args_p): Return true if there were
            unsubstituted packs and PACK_EXPANSION_FORCE_EXTRA_ARGS_P.
            (tsubst_pack_expansion): Pass the pack expansion to
            use_pack_expansion_extra_args_p.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/constexpr-if35.C: New test.