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Subject: [Bug c++/89074] valid pointer equality constexpr comparison rejected
Date: Thu, 18 Nov 2021 16:14:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D89074

--- Comment #6 from CVS 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:ca243ada71656651a8753e88164a1f0f019be1c3

commit r12-5376-gca243ada71656651a8753e88164a1f0f019be1c3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 18 12:39:20 2021 +0000

    libstdc++: Fix std::char_traits<C>::move for constexpr

    The constexpr branch in __gnu_cxx::char_traits::move compares the string
    arguments to see if they overlap, but relational comparisons between
    unrelated pointers are not core constant expressions.

    I want to replace the comparisons with a loop using pointer equality to
    determine whether the end of the source string is in the destination
    string. However, that doesn't work with GCC, due to PR c++/89074 so
    allocate a temporary buffer instead and copy out into that first, so
    that overlapping source and destination don't matter. The allocation
    isn't supported by the current Intel icc so use the loop as a fallback.

    libstdc++-v3/ChangeLog:

            * include/bits/char_traits.h (__gnu_cxx::char_traits::move):
            Do not compare unrelated pointers during constant evaluation.
            *
testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc:
            Improve tests for char_traits::move.=