[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466

--- Comment #12 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:f2af87b9705d5a7e37b65bf342146ff25f025e49

commit r12-10143-gf2af87b9705d5a7e37b65bf342146ff25f025e49
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jan 11 15:09:12 2024 +0000

    libstdc++: Fix non-portable results from 64-bit
std::subtract_with_carry_engine [PR107466]

    I implemented the resolution of LWG 3809 in r13-4364-ga64775a0edd469 but
    it was recently noted in the MSVC STL github repo that the change causes
    possible truncation for 64-bit seeds. Whether the truncation occurs (and
    to what value) depends on the width of uint_least32_t which is not
    portable, so the output of the PRNG for 64-bit seed values is no longer
    the same as in C++20, and no longer portable across platforms.

    That new issue was filed as LWG 4014. I proposed a new change which
    reduces the seed by the LCG's modulus before the conversion to
    uint_least32_t. This ensures that 64-bit seed values are consistently
    reduced by the modulus before any truncation. This removes the
    platform-dependent behaviour and restores the old behaviour for
    std::subtract_with_carry_engine specializations using a 64-bit result
    type (such as std::ranlux48_base).

    libstdc++-v3/ChangeLog:

            PR libstdc++/107466
            * include/bits/random.tcc (subtract_with_carry_engine::seed):
            Implement proposed resolution of LWG 4014.
            * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
            line number.
            *
testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc:
            Check for expected result of 64-bit engine with seed that
            doesn't fit in 32-bits.

    (cherry picked from commit c224dec0e7c88e7a95633023018cdcb6ee87c65f)