C++20 removes a number of std::allocator members that have correct
defaults provided by std::allocator_traits, so aren't needed.

Several extensions including __gnu_cxx::hash_map and tr1 containers are
no longer usable with std::allocator in C++20 mode. They need to be
updated to use __gnu_cxx::__alloc_traits in a follow-up patch.

	* include/bits/alloc_traits.h
	(allocator_traits<allocator<T>>::allocate): Ignore hint for C++20.
	(allocator_traits<allocator<T>>::construct): Perform placement new
	directly for C++20, instead of calling allocator<T>::construct.
	(allocator_traits<allocator<T>>::destroy): Call destructor directly
	for C++20, instead of calling allocator<T>::destroy.
	(allocator_traits<allocator<T>>::max_size): Return value directly
	for C++20, instead of calling std::allocator<T>::max_size().
	(__do_alloc_on_copy, __do_alloc_on_move, __do_alloc_on_swap): Do not
	define for C++17 and up.
	(__alloc_on_copy, __alloc_on_move, __alloc_on_swap): Use if-constexpr
	for C++17 and up, instead of tag dispatching.
	* include/bits/allocator.h (allocator<void>): Remove for C++20.
	(allocator::pointer, allocator::const_pointer, allocator::reference)
	(allocator::const_reference, allocator::rebind): Remove for C++20.
	* include/bits/basic_string.h (basic_string): Use __alloc_traits to
	rebind allocator.
	* include/bits/memoryfwd.h (allocator<void>): Remove for C++20.
	* include/ext/debug_allocator.h: Use __alloc_traits for rebinding.
	* include/ext/malloc_allocator.h (malloc_allocator::~malloc_allocator)
	(malloc_allocator::pointer, malloc_allocator::const_pointer)
	(malloc_allocator::reference, malloc_allocator::const_reference)
	(malloc_allocator::rebind, malloc_allocator::max_size)
	(malloc_allocator::construct, malloc_allocator::destroy): Do not
	define for C++20.
	(malloc_allocator::_M_max_size): Define new function.
	* include/ext/new_allocator.h (new_allocator::~new_allocator)
	(new_allocator::pointer, new_allocator::const_pointer)
	(new_allocator::reference, new_allocator::const_reference)
	(new_allocator::rebind, new_allocator::max_size)
	(new_allocator::construct, new_allocator::destroy): Do not
	define for C++20.
	(new_allocator::_M_max_size): Define new function.
	* include/ext/rc_string_base.h (__rc_string_base::_Rep): Use
	__alloc_traits to rebind allocator.
	* include/ext/rope (_Rope_rep_base, _Rope_base): Likewise.
	(rope::rope(CharT, const allocator_type&)): Use __alloc_traits
	to construct character.
	* include/ext/slist (_Slist_base): Use __alloc_traits to rebind
	allocator.
	* include/ext/sso_string_base.h (__sso_string_base::_M_max_size):
	Use __alloc_traits.
	* include/ext/throw_allocator.h (throw_allocator): Do not use optional
	members of std::allocator, use __alloc_traits members instead.
	* include/ext/vstring.h (__versa_string): Use __alloc_traits.
	* include/ext/vstring_util.h (__vstring_utility): Likewise.
	* include/std/memory: Include <bits/alloc_traits.h>.
	* testsuite/20_util/allocator/8230.cc: Use __gnu_test::max_size.
	* testsuite/20_util/allocator/rebind_c++20.cc: New test.
	* testsuite/20_util/allocator/requirements/typedefs.cc: Do not check
	for pointer, const_pointer, reference, const_reference or rebind in
	C++20.
	* testsuite/20_util/allocator/requirements/typedefs_c++20.cc: New test.
	* testsuite/23_containers/deque/capacity/29134.cc: Use
	__gnu_test::max_size.
	* testsuite/23_containers/forward_list/capacity/1.cc: Likewise.
	* testsuite/23_containers/list/capacity/29134.cc: Likewise.
	* testsuite/23_containers/map/capacity/29134.cc: Likewise.
	* testsuite/23_containers/multimap/capacity/29134.cc: Likewise.
	* testsuite/23_containers/multiset/capacity/29134.cc: Likewise.
	* testsuite/23_containers/set/capacity/29134.cc: Likewise.
	* testsuite/23_containers/vector/capacity/29134.cc: Likewise.
	* testsuite/ext/malloc_allocator/variadic_construct.cc: Do not run
	test for C++20.
	* testsuite/ext/new_allocator/variadic_construct.cc: Likewise.
	* testsuite/ext/vstring/capacity/29134.cc: Use __gnu_test::max_size.
	* testsuite/util/replacement_memory_operators.h: Do not assume
	Alloc::pointer exists.
	* testsuite/util/testsuite_allocator.h (__gnu_test::max_size): Define
	helper to call max_size for any allocator.

From-SVN: r277300