[Bug c++/110025] [C++23] ICE with default-argument for template-param with decltype and auto.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110025

--- Comment #8 from GCC 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:a6a1920b592b58c38137c5c891b3bbb02b084f38

commit r14-9260-ga6a1920b592b58c38137c5c891b3bbb02b084f38
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Mar 1 12:50:18 2024 -0500

    c++: auto(x) partial substitution [PR110025, PR114138]

    In r12-6773-g09845ad7569bac we gave CTAD placeholders a level of 0 and
    ensured we never replaced them via tsubst.  It turns out that autos
    representing an explicit cast need the same treatment and for the same
    reason: such autos appear in an expression context and so their level
    gets easily messed up after partial substitution, leading to premature
    replacement via an incidental tsubst instead of via do_auto_deduction.

    This patch fixes this by extending the r12-6773 approach to auto(x).

            PR c++/110025
            PR c++/114138

    gcc/cp/ChangeLog:

            * cp-tree.h (make_cast_auto): Declare.
            * parser.cc (cp_parser_functional_cast): If the type is an auto,
            replace it with a level-less one via make_cast_auto.
            * pt.cc (find_parameter_packs_r): Don't treat level-less auto
            as a type parameter pack.
            (tsubst) <case TEMPLATE_TYPE_PARM>: Generalize CTAD placeholder
            auto handling to all level-less autos.
            (make_cast_auto): Define.
            (do_auto_deduction): Handle replacement of a level-less auto.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp23/auto-fncast16.C: New test.
            * g++.dg/cpp23/auto-fncast17.C: New test.
            * g++.dg/cpp23/auto-fncast18.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>