[Bug libstdc++/96851] operator< on std::array<T, N> does not work in constexpr, for sizeof(T) == 1, and N > 1

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96851

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:33c34c4c2466fd4fd050ed8e2d5996c35ebdeef6

commit r10-8702-g33c34c4c2466fd4fd050ed8e2d5996c35ebdeef6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 2 15:17:24 2020 +0100

    libstdc++: Fix three-way comparison for std::array [PR 96851]

    The spaceship operator for std::array uses memcmp when the
    __is_byte<value_type> trait is true, but memcmp isn't usable in
    constexpr contexts. Also, memcmp should only be used for unsigned byte
    types, because it gives the wrong answer for signed chars with negative
    values.

    We can simply check std::is_constant_evaluated() so that we don't use
    memcmp during constant evaluation.

    To fix the problem of using memcmp for inappropriate types, this patch
    adds new __is_memcmp_ordered and __is_memcmp_ordered_with traits. These
    say whether using memcmp will give the right answer for ordering
    operations such as lexicographical_compare and three-way comparisons.
    The new traits can be used in several places.

    Unlike the trunk commit this was backported from, this commit for the
    branch doesn't extend the memcmp optimisations to all unsigned integers
    on big endian targets. Only narrow character types and std::byte will
    use memcmp.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96851
            * include/bits/cpp_type_traits.h (__is_memcmp_ordered):
            New trait that says if memcmp can be used for ordering.
            (__is_memcmp_ordered_with): Likewise, for two types.
            * include/bits/ranges_algo.h (__lexicographical_compare_fn):
            Use new traits instead of __is_byte and __numeric_traits.
            * include/bits/stl_algobase.h (__lexicographical_compare_aux1)
            (__is_byte_iter): Likewise.
            * include/std/array (operator<=>): Likewise. Only use memcmp
            when std::is_constant_evaluated() is false.
            * testsuite/23_containers/array/comparison_operators/96851.cc:
            New test.
            * testsuite/23_containers/array/tuple_interface/get_neg.cc:
            Adjust dg-error line numbers.

    (cherry picked from commit 2f983fa69005b603ea1758a013b4134d5b0f24a8)