[Bug c++/102786] [c++20] virtual pmf sometimes rejected as not a constant

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102786

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:f45610a45236e97616726ca042898d6ac46a082e

commit r12-4491-gf45610a45236e97616726ca042898d6ac46a082e
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Oct 19 09:24:57 2021 +0200

    c++: Don't reject calls through PMF during constant evaluation [PR102786]

    The following testcase incorrectly rejects the c initializer,
    while in the s.*a case cxx_eval_* sees .__pfn reads etc.,
    in the s.*&S::foo case get_member_function_from_ptrfunc creates
    expressions which use INTEGER_CSTs with type of pointer to METHOD_TYPE.
    And cxx_eval_constant_expression rejects any INTEGER_CSTs with pointer
    type if they aren't 0.
    Either we'd need to make sure we defer such folding till cp_fold but the
    function and pfn_from_ptrmemfunc is used from lots of places, or
    the following patch just tries to reject only non-zero INTEGER_CSTs
    with pointer types if they don't point to METHOD_TYPE in the hope that
    all such INTEGER_CSTs with POINTER_TYPE to METHOD_TYPE are result of
    folding valid pointer-to-member function expressions.
    I don't immediately see how one could create such INTEGER_CSTs otherwise,
    cast of integers to PMF is rejected and would have the PMF RECORD_TYPE
    anyway, etc.

    2021-10-19  Jakub Jelinek  <jakub@redhat.com>

            PR c++/102786
            * constexpr.c (cxx_eval_constant_expression): Don't reject
            INTEGER_CSTs with type POINTER_TYPE to METHOD_TYPE.

            * g++.dg/cpp2a/constexpr-virtual19.C: New test.