[Bug libstdc++/102480] std::regex fails to match ^ when match_prev_avail is used

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102480

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:e61c16fd9ba89e7bc12284e524dcd8379c2bebfc

commit r11-10120-ge61c16fd9ba89e7bc12284e524dcd8379c2bebfc
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 29 13:48:19 2021 +0100

    libstdc++: Implement std::regex_constants::multiline (LWG 2503)

    This implements LWG 2503, which allows ^ and $ to match line terminator
    characters, rather than only matching the beginning and end of the
    entire input. The multiline option is only valid for ECMAScript, but
    for other grammars we ignore it rather than throwing an exception.

    This is related to PR libstdc++/102480, which incorrectly said that
    ECMAscript should match the beginning of a line when match_prev_avail
    is used. I think that's only supposed to happen when multiline is used.

    The new regex_constants::multiline and basic_regex::multiline constants
    are not defined for strict -std=c++11 and -std=c++14 modes, but
    regex_constants::__multiline is always defined, so that the
    implementation can use it internally.

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

    libstdc++-v3/ChangeLog:

            * include/bits/regex.h (basic_regex::multiline): Define constant
            for C++17.
            * include/bits/regex_constants.h (regex_constants::multiline):
            Define constant for C++17.
            (regex_constants::__multiline): Define duplicate constant for
            internal use in C++11 and C++14.
            * include/bits/regex_executor.h (_Executor::_M_match_multiline()):
            New member function.
            (_Executor::_M_is_line_terminator(_CharT)): New member function.
            (_Executor::_M_at_begin(), _Executor::_M_at_end()): Use new
            member functions to support multiline matches.
            * testsuite/28_regex/algorithms/regex_match/multiline.cc: New test.

    (cherry picked from commit f38cd3bdb4cd429a5f7082ea91793a59b37d47b9)