[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:7d00a59229ee17af009a3c6c6208b0611740ed49

commit r14-6553-g7d00a59229ee17af009a3c6c6208b0611740ed49
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Dec 6 13:39:52 2023 +0000

    libstdc++: Make __gnu_debug::vector usable in constant expressions
[PR109536]

    This makes constexpr std::vector (mostly) work in Debug Mode. All safe
    iterator instrumentation and checking is disabled during constant
    evaluation, because it requires mutex locks and calls to non-inline
    functions defined in libstdc++.so. It should be OK to disable the safety
    checks, because most UB should be detected during constant evaluation
    anyway.

    We could try to enable the full checking in constexpr, but it would mean
    wrapping all the non-inline functions like _M_attach with an inline
    _M_constexpr_attach that does the iterator housekeeping inline without
    mutex locks when called for constant evaluation, and calls the
    non-inline function at runtime. That could be done in future if we find
    that we've lost safety or useful checking by disabling the safe
    iterators.

    There are a few test failures in C++20 mode, which I'm unable to
    explain. The _Safe_iterator::operator++() member gives errors for using
    non-constexpr functions during constant evaluation, even though those
    functions are guarded by std::is_constant_evaluated() checks. The same
    code works fine for C++23 and up.

    libstdc++-v3/ChangeLog:

            PR libstdc++/109536
            * include/bits/c++config (__glibcxx_constexpr_assert): Remove
            macro.
            * include/bits/stl_algobase.h (__niter_base, __copy_move_a)
            (__copy_move_backward_a, __fill_a, __fill_n_a, __equal_aux)
            (__lexicographical_compare_aux): Add constexpr to overloads for
            debug mode iterators.
            * include/debug/helper_functions.h (__unsafe): Add constexpr.
            * include/debug/macros.h (_GLIBCXX_DEBUG_VERIFY_COND_AT): Remove
            macro, folding it into ...
            (_GLIBCXX_DEBUG_VERIFY_AT_F): ... here. Do not use
            __glibcxx_constexpr_assert.
            * include/debug/safe_base.h (_Safe_iterator_base): Add constexpr
            to some member functions. Omit attaching, detaching and checking
            operations during constant evaluation.
            * include/debug/safe_container.h (_Safe_container): Likewise.
            * include/debug/safe_iterator.h (_Safe_iterator): Likewise.
            * include/debug/safe_iterator.tcc (__niter_base, __copy_move_a)
            (__copy_move_backward_a, __fill_a, __fill_n_a, __equal_aux)
            (__lexicographical_compare_aux): Add constexpr.
            * include/debug/vector (_Safe_vector, vector): Add constexpr.
            Omit safe iterator operations during constant evaluation.
            * testsuite/23_containers/vector/bool/capacity/constexpr.cc:
            Remove dg-xfail-if for debug mode.
            * testsuite/23_containers/vector/bool/cmp_c++20.cc: Likewise.
            * testsuite/23_containers/vector/bool/cons/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/element_access/1.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/element_access/constexpr.cc:
            Likewise.
            *
testsuite/23_containers/vector/bool/modifiers/assign/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/modifiers/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/capacity/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/cmp_c++20.cc: Likewise.
            * testsuite/23_containers/vector/cons/constexpr.cc: Likewise.
            * testsuite/23_containers/vector/data_access/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/element_access/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/modifiers/assign/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/modifiers/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/modifiers/swap/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/cons/destructible_debug_neg.cc:
            Adjust dg-error line number.