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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/108556] std::sort changes objects' member values
Date: Thu, 26 Jan 2023 10:20:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108556

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Compiling with -D_GLIBCXX_DEBUG shows the problem:

/home/jwakely/gcc/13/include/c++/13.0.1/bits/stl_algo.h:4892:
In function:
    void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter =3D=20
    gnu_debug::_Safe_iterator<gnu_cxx::normal_iterator<Item*, vector<Item,=
=20
    allocator<Item> > >, debug::vector<Item>, random_access_iterator_tag>;=
=20
    _Compare =3D main()::<lambda(const Item&, const Item&)>]

Error: comparison doesn't meet irreflexive requirements, assert(!(a < a)).

Objects involved in the operation:
    instance "functor" @ 0x7ffee875415f {
      type =3D main::{lambda(Item const&, Item const&)#1};
    }
    iterator::value_type "ordered type"  {
      type =3D Item;
    }
Aborted (core dumped)


Your comparison function fails to meet the requirements of a strict weak
ordering, so your program has undefined behaviour.

https://www.boost.org/sgi/stl/StrictWeakOrdering.html
https://en.cppreference.com/w/cpp/named_req/Compare=