[Bug libstdc++/109150] std::fill should use __gnu_cxx::__is_scalar overloads for all scalars

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109150

--- Comment #8 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:b3743181899c5490a94c4dbde56a69ab77a40f11

commit r15-1549-gb3743181899c5490a94c4dbde56a69ab77a40f11
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 19 16:14:56 2024 +0100

    libstdc++: Fix std::fill and std::fill_n optimizations [PR109150]

    As noted in the PR, the optimization used for scalar types in std::fill
    and std::fill_n is non-conforming, because it doesn't consider that
    assigning a scalar type might have non-trivial side effects which are
    affected by the optimization.

    By changing the condition under which the optimization is done we ensure
    it's only performed when safe to do so, and we also enable it for
    additional types, which was the original subject of the PR.

    Instead of two overloads using __enable_if<__is_scalar<T>::__value, R>
    we can combine them into one and create a local variable which is either
    a local copy of __value or another reference to it, depending on whether
    the optimization is allowed.

    This removes a use of std::__is_scalar, which is a step towards fixing
    PR 115497 by removing std::__is_pointer from <bits/cpp_type_traits.h>

    libstdc++-v3/ChangeLog:

            PR libstdc++/109150
            * include/bits/stl_algobase.h (__fill_a1): Combine the
            !__is_scalar and __is_scalar overloads into one and rewrite the
            condition used to decide whether to perform the load outside the
            loop.
            * testsuite/25_algorithms/fill/109150.cc: New test.
            * testsuite/25_algorithms/fill_n/109150.cc: New test.