The C++ LWG recently confirmed that self-move assignment should not have
undefined behaviour for standard containers (see the proposed resolution
of LWG 2839). The result should be a valid but unspecified value, just
like other times when a container is moved from.

Our std::list, std::__cxx11::basic_string and unordered containers all
have bugs which result in undefined behaviour.

For std::list the problem is that we clear the previous contents using
_M_clear() instead of clear(). This means the _M_next, _M_prev and
_M_size members are not zeroed, and so after we "update" them (with
their existing values), we are left with dangling pointers and a
non-zero size, but no elements.

For the unordered containers the problem is similar. _Hashtable first
deallocates the existing contents, then takes ownership of the pointers
from the RHS object (which has just had its contents deallocated so the
pointers are dangling).

For std::basic_string it's a little more subtle. When the string is
local (i.e. fits in the SSO buffer) we use char_traits::copy to copy the
contents from this->data() to __rhs.data(). When &__rhs == this that
copy violates the precondition that the ranges don't overlap. We only
need to check for self-move for this case where it's local, because the
only other case that can be true for self-move is that it's non-local
but the allocators compare equal. In that case the data pointer is
neither deallocated nor leaked, so the result is well-defined.

This patch also makes a small optimization for std::deque move
assignment, to use the efficient move when is_always_equal is false, but
the allocators compare equal at runtime.

Finally, we need to remove all the Debug Mode checks which abort the
program when a self-move is detected, because it's not undefined to do
that.

Before PR 85828 can be closed we should also look into fixing
std::shuffle so it doesn't do any redundant self-swaps.

libstdc++-v3/ChangeLog:

	PR libstdc++/85828
	* include/bits/basic_string.h (operator=(basic_string&&)): Check
	for self-move before copying with char_traits::copy.
	* include/bits/hashtable.h (operator=(_Hashtable&&)): Check for
	self-move.
	* include/bits/stl_deque.h (_M_move_assign1(deque&&, false_type)):
	Check for equal allocators.
	* include/bits/stl_list.h (_M_move_assign(list&&, true_type)):
	Call clear() instead of _M_clear().
	* include/debug/formatter.h (__msg_self_move_assign): Change
	comment.
	* include/debug/macros.h (__glibcxx_check_self_move_assign):
	(_GLIBCXX_DEBUG_VERIFY): Remove.
	* include/debug/safe_container.h (operator=(_Safe_container&&)):
	Remove assertion check for safe move and make it well-defined.
	* include/debug/safe_iterator.h (operator=(_Safe_iterator&&)):
	Remove assertion check for self-move.
	* include/debug/safe_local_iterator.h
	(operator=(_Safe_local_iterator&&)): Likewise.
	* testsuite/21_strings/basic_string/cons/char/self_move.cc: New test.
	* testsuite/23_containers/deque/cons/self_move.cc: New test.
	* testsuite/23_containers/forward_list/cons/self_move.cc: New test.
	* testsuite/23_containers/list/cons/self_move.cc: New test.
	* testsuite/23_containers/set/cons/self_move.cc: New test.
	* testsuite/23_containers/unordered_set/cons/self_move.cc: New test.
	* testsuite/23_containers/vector/cons/self_move.cc: New test.

Tested powerpc64le-linux. Committed to trunk.