[Bug c++/107853] [10/11/12 Regression] variadic template with a variadic template friend with a requires of fold expression

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107853

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:cb39089c5a0da1e9b16ad27db557a2c3f930e668

commit r12-9229-gcb39089c5a0da1e9b16ad27db557a2c3f930e668
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Dec 23 09:18:37 2022 -0500

    c++: template friend with variadic constraints [PR107853]

    When instantiating a constrained hidden template friend, we substitute
    into its template-head requirements in tsubst_friend_function.  For this
    substitution we use the template's full argument vector whose outer
    levels correspond to the instantiated class's arguments and innermost
    level corresponds to the template's own level-lowered generic arguments.

    But for A<int>::f here, for which the relevant argument vector is
    {{int}, {Us...}}, the substitution into (C<Ts, Us> && ...) triggers the
    assert in use_pack_expansion_extra_args_p since one argument is a pack
    expansion and the other isn't.

    And for A<int, int>::f, for which the relevant argument vector is
    {{int, int}, {Us...}}, the use_pack_expansion_extra_args_p assert would
    also trigger but we first get a bogus "mismatched argument pack lengths"
    error from tsubst_pack_expansion.

    Sidestepping the question of whether tsubst_pack_expansion should be
    able to handle such substitutions, it seems we can work around this by
    using only the instantiated class's arguments and not also the template
    friend's own generic arguments, which is consistent with how we normally
    substitute into the signature of a member template.

            PR c++/107853

    gcc/cp/ChangeLog:

            * constraint.cc (maybe_substitute_reqs_for): Substitute into
            the template-head requirements of a template friend using only
            its outer arguments via outer_template_args.
            * cp-tree.h (outer_template_args): Declare.
            * pt.cc (outer_template_args): Define, factored out and
            generalized from ...
            (ctor_deduction_guides_for): ... here.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-friend12.C: New test.
            * g++.dg/cpp2a/concepts-friend13.C: New test.

    (cherry picked from commit bd1fc4a219d8c0fad0ec41002e895b49e384c1c2)