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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/109536] Failure to compile constexpr std::vector with
 -D_GLIBCXX_DEBUG
Date: Thu, 18 Jan 2024 15:36:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109536

--- Comment #7 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:3d3145e9e1461e92bef02d55d36990261dd0444d

commit r14-8252-g3d3145e9e1461e92bef02d55d36990261dd0444d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jan 18 10:36:07 2024 -0500

    libstdc++/debug: Fix constexpr _Safe_iterator in C++20 mode [PR109536]

    Some _Safe_iterator member functions define a variable of non-literal
    type __gnu_cxx::__scoped_lock, which automatically disqualifies them fr=
om
    being constexpr in C++20 mode even if that code path is never constant
    evaluated.  This restriction was lifted by P2242R3 for C++23, but we
    need to work around it in C++20 mode.  To that end this patch defines
    a pair of macros that encapsulate the lambda-based workaround mentioned
    in that paper and uses it to make these functions valid C++20 constexpr
    functions.  The augmented std::vector test element_access/constexpr.cc
    now successfully compiles in C++20 mode with -D_GLIBCXX_DEBUG (and it
    should test all member functions modified by this patch).

            PR libstdc++/109536

    libstdc++-v3/ChangeLog:

            * include/debug/safe_base.h (_Safe_sequence_base::_M_swap):
            Remove _GLIBCXX20_CONSTEXPR from non-inline member function.
            * include/debug/safe_iterator.h
            (_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN): Define.
            (_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END): Define.
            (_Safe_iterator::operator=3D): Use them around the code path th=
at
            defines a variable of type __gnu_cxx::__scoped_lock.
            (_Safe_iterator::operator++): Likewise.
            (_Safe_iterator::operator--): Likewise.
            (_Safe_iterator::operator+=3D): Likewise.
            (_Safe_iterator::operator-=3D): Likewise.
            * testsuite/23_containers/vector/element_access/constexpr.cc
            (test_iterators): Test more iterator operations.
            * testsuite/23_containers/vector/bool/element_access/constexpr.=
cc
            (test_iterators): Likewise.
            * testsuite/std/ranges/adaptors/all.cc (test08) [_GLIBCXX_DEBUG=
]:
            Remove.

    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>=