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Subject: [Bug libstdc++/97828] std::ranges::search_n does not work with
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Date: Tue, 17 Nov 2020 16:28:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97828

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:04cb64dadb5c115b4ad093ff92c3f5a0a87ef15f

commit r10-9036-g04cb64dadb5c115b4ad093ff92c3f5a0a87ef15f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Nov 17 10:28:20 2020 -0500

    libstdc++: Fix ranges::search_n for random access iterators [PR97828]

    My ranges transcription of the std::search_n implementation for random
    access iterators missed a crucial part of the algorithm which the
    existing tests didn't exercise.  When __remainder is less than __count
    at the start of an iteration of the outer while loop, it means we're
    continuing a partial match of __count - __remainder elements from the
    previous iteration.  If at the end of the iteration we don't complete
    this partial match, we need to reset __remainder so that it's only
    offset by the size of the most recent partial match before starting the
    next iteration.

    This patch fixes this appropriately, mirroring how it's done in the
    corresponding std::search_n implementation.

    libstdc++-v3/ChangeLog:

            PR libstdc++/97828
            * include/bits/ranges_algo.h (__search_n_fn::operator()): Check
            random_access_iterator before using the backtracking
            implementation.  When the backwards scan fails prematurely,
            reset __remainder appropriately.
            * testsuite/25_algorithms/search_n/97828.cc: New test.

    (cherry picked from commit 8661f4faa875f361cd22a197774c1fa04cd0580b)=