[Bug c++/99287] std::source_location::function_name breaks constexpr

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99287

--- Comment #7 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:d1bba463bd0d5692b7fa58ee37a61a55b2517456

commit r11-7546-gd1bba463bd0d5692b7fa58ee37a61a55b2517456
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Mar 6 17:09:07 2021 -0500

    c++: Fix constexpr evaluation of pre-increment when !lval [PR99287]

    Here, during cxx_eval_increment_expression (with lval=false) of
    ++__first where __first is &"mystr"[0], we correctly update __first
    to &"mystr"[1] but we end up returning &"mystr"[0] + 1 instead of
    &"mystr"[1].  This unreduced return value inhibits other pointer
    arithmetic folding during later constexpr evaluation, which ultimately
    causes the constexpr evaluation to fail.

    It turns out the simplification of &"mystr"[0] + 1 to &"mystr"[1]
    is performed by cxx_fold_pointer_plus_expression, not by fold_build2.
    So we perform this simplification during constexpr evaluation of
    the temporary MODIFY_EXPR (during which we assign to __first the
    simplified value), but then we return 'mod' which has only been folded
    via fold_build2 and hasn't gone through cxx_fold_pointer_plus_expression.

    This patch fixes this by updating 'mod' with the result of the
    MODIFY_EXPR evaluation appropriately, so that it captures any additional
    folding of the expression when !lval.  We now need to be wary of this
    evaluation failing and returning e.g. the MODIFY_EXPR or NULL_TREE; it
    seems checking *non_constant_p should cover our bases here and is
    generally prudent.

    gcc/cp/ChangeLog:

            PR c++/99287
            * constexpr.c (cxx_eval_increment_expression): Pass lval when
            evaluating the MODIFY_EXPR, and update 'mod' with the result of
            this evaluation.  Check *non_constant_p afterwards.  For prefix
            ops, just return 'mod'.

    gcc/testsuite/ChangeLog:

            PR c++/99287
            * g++.dg/cpp2a/constexpr-99287.C: New test.

    Co-authored-by: Jakub Jelinek <jakub@redhat.com>