[Bug c++/85723] [C++17][DR 1496] __is_trivial intrinsic fails with no trivial non-deleted default c'tor

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85723

--- Comment #6 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:510ce5eed69ee1bea9c2c696fe3b2301e16d1486

commit r15-1533-g510ce5eed69ee1bea9c2c696fe3b2301e16d1486
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 18 13:27:02 2024 +0100

    libstdc++: Fix std::to_array for trivial-ish types [PR115522]

    Due to PR c++/85723 the std::is_trivial trait is true for types with a
    deleted default constructor, so the use of std::is_trivial in
    std::to_array is not sufficient to ensure the type can be trivially
    default constructed then filled using memcpy.

    I also forgot that a type with a deleted assignment operator can still
    be trivial, so we also need to check that it's assignable because the
    is_constant_evaluated() path can't use memcpy.

    Replace the uses of std::is_trivial with std::is_trivially_copyable
    (needed for memcpy), std::is_trivially_default_constructible (needed so
    that the default construction is valid and does no work) and
    std::is_copy_assignable (needed for the constant evaluation case).

    libstdc++-v3/ChangeLog:

            PR libstdc++/115522
            * include/std/array (to_array): Workaround the fact that
            std::is_trivial is not sufficient to check that a type is
            trivially default constructible and assignable.
            * testsuite/23_containers/array/creation/115522.cc: New test.