[Bug libstdc++/91488] [9/10/11/12 Regression] char_traits::length causes "inlining failed in call to always_inline" error with -fgnu-tm -O2 -std=c++17

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91488

--- Comment #8 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:b376b1ef38971b84975ad1540bf5d2ae0b924e76

commit r12-1603-gb376b1ef38971b84975ad1540bf5d2ae0b924e76
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 17 14:11:22 2021 +0100

    libstdc++: Simplify constexpr checks in std::char_traits [PR 91488]

    This removes the helper functions added by r8-1294 to detect whether the
    char_traits member functions can be evaluated at compile time. Instead,
    we can just use __builtin_constant_evaluated directly, which is well
    supported by non-GCC compilers by now.

    As a result, there is a chance that those members will no longer be
    usable in constant expressions when using old versions of non-GCC
    compilers. Make the relevant feature test macros depend on the
    availability of __builtin_constant_evaluated, so they are defined only
    when the feature is actualyl available.

    The new testcase from the PR is added to the libitm testsuite, because
    that's where we can be sure it's OK to use the -fgnu-tm option.

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

            PR libstdc++/91488

    libstdc++-v3/ChangeLog:

            * include/bits/basic_string.h (__cpp_lib_constexpr_string): Only
            define when is_constant_evaluated is available.
            * include/bits/char_traits.h (__cpp_lib_constexpr_char_traits):
            Likewise.
            (__constant_string_p, __constant_array_p): Remove.
            (char_traits): Use is_constant_evaluated directly.
            * include/std/version (__cpp_lib_constexpr_char_traits)
            (__cpp_lib_constexpr_string): Only define when
            is_constant_evaluated is available.

    libitm/ChangeLog:

            * testsuite/libitm.c++/libstdc++-pr91488.C: New test.