[Bug libstdc++/108118] std::weak_ptr lacks a self-usurp check on move

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108118

--- Comment #8 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:9578570e305e77b8c105583506781158c137f743

commit r11-10768-g9578570e305e77b8c105583506781158c137f743
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Dec 15 09:52:48 2022 +0000

    libstdc++: Fix self-move for std::weak_ptr [PR108118]

    I think an alternative fix would be something like:

      _M_ptr = std::exchange(rhs._M_ptr, nullptr);
      _M_refcount = std::move(rhs._M_refcount);

    The standard's move-and-swap implementation generates smaller code at
    all levels except -O0 and -Og, so it seems simplest to just do what the
    standard says.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108118
            * include/bits/shared_ptr_base.h (weak_ptr::operator=):
            Implement as move-and-swap exactly as specified in the standard.
            * testsuite/20_util/weak_ptr/cons/self_move.cc: New test.

    (cherry picked from commit 92eb0adc14a5f84acce7e5bc780b81b1544b24aa)