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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/95322] std::list | take | transform, expression does
 not work cbegin() == end()
Date: Wed, 27 May 2020 21:08:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95322

--- Comment #9 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:6c2582c0406250c66e2eb3651f8e8638796b7f53

commit r11-673-g6c2582c0406250c66e2eb3651f8e8638796b7f53
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 27 22:08:15 2020 +0100

    libstdc++: Fix view adaptors for mixed-const sentinels and iterators (PR
95322)

    The bug report is that transform_view's sentinel<false> cannot be
    compared to its iterator<true>.  The comparison is supposed to use
    operator=3D=3D(iterator<Const>, sentinel<Const>) after converting
    sentinel<false> to sentinel<true>. However, the operator=3D=3D is a hid=
den
    friend so is not a candidate when comparing iterator<true> with
    sentinel<false>. The required conversion would only happen if we'd found
    the operator, but we can't find the operator until after the conversion
    happens.

    A new LWG issue has been reported, but not yet assigned a number.  The
    solution suggested by Casey Carter is to make the hidden friends of the
    sentinel types work with iterators of any const-ness, so that no
    conversions are required.

    Patrick Palka observed that join_view has a similar problem and a
    similar fix is used for its sentinel.

            PR libstdc++/95322
            * include/std/ranges (transform_view::_Sentinel): Allow hidden
            friends to work with _Iterator<true> and _Iterator<false>.
            (join_view::_Sentinel): Likewise.
            * testsuite/std/ranges/adaptors/95322.cc: New test.=