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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99402] [10 Regression] std::copy creates
 _GLIBCXX_DEBUG false positive for attempt to subscript a dereferenceable
 (start-of-sequence) iterator
Date: Tue, 13 Apr 2021 20:51:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99402

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[10/11 Regression]          |[10 Regression] std::copy
                   |std::copy creates           |creates _GLIBCXX_DEBUG
                   |_GLIBCXX_DEBUG false        |false positive for attempt
                   |positive for attempt to     |to subscript a
                   |subscript a dereferenceable |dereferenceable
                   |(start-of-sequence)         |(start-of-sequence)
                   |iterator                    |iterator

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This was fixed on trunk by r11-8100:

libstdc++: [_GLIBCXX_DEBUG] Fix management of __dp_sign_max_size [PR 99402]

__dp_sign precision indicates that we found out what iterator comes first or
last in the range. __dp_sign_max_size is the same plus it gives the informa=
tion
of the max size of the range that is to say the max_size value such that
distance(lhs, rhs) < max_size.
Thanks to this additional information we are able to tell when a copy of n
elements
to that range will fail even if we do not know exactly how large it is.

This patch makes sure that we are properly using this information.

libstdc++-v3/ChangeLog:

        PR libstdc++/99402
        * include/debug/helper_functions.h (__can_advance(_InputIterator,
        const std::pair<_Diff, _Distance_precision>&, int)): New.
        (__can_advance(const _Safe_iterator<>&,
        const std::pair<_Diff, _Distance_precision>&, int)): New.
        * include/debug/macros.h (__glibcxx_check_can_increment_dist): New,
        use latter.
        (__glibcxx_check_can_increment_range): Adapt to use latter.
        (__glibcxx_check_can_decrement_range): Likewise.
        * include/debug/safe_iterator.h
        (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff,
_Distance_precision>&,
        int)): New.
        (__can_advance(const _Safe_iterator<>&,
        const std::pair<_Diff, _Distance_precision>&, int)): New.
        * include/debug/safe_iterator.tcc
        (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff,
_Distance_precision>&,
        int)): New.
        (_Safe_iterator<>::_M_valid_range(const _Safe_iterator<>&,
        std::pair<difference_type, _Distance_precision>&, bool)): Adapt for
        __dp_sign_max_size.
        (__copy_move_a): Adapt to use __glibcxx_check_can_increment_dist.
        (__copy_move_backward_a): Likewise.
        (__equal_aux): Likewise.
        * include/debug/stl_iterator.h (__can_advance(const
std::reverse_iterator<>&,
        const std::pair<_Diff, _Distance_precision>&, int)): New.
        (__can_advance(const std::move_iterator<>&,
        const std::pair<_Diff, _Distance_precision>&, int)): New.
        * testsuite/25_algorithms/copy/debug/99402.cc: New test.=