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.