The changes in 75c6a925dab5b7af9ab47c10906cb0e140261cc2 were slightly
incorrect, because the converting constructor should be noexcept, and
the POCMA and is_always_equal traits should still be present in C++20.
This fixes it, and slightly refactors the preprocessor conditions and
order of members. Also add comments explaining things.
The non-standard construct and destroy members added for PR 78052 can be
private if allocator_traits<allocator<void>> is made a friend.
libstdc++-v3/ChangeLog:
* include/bits/allocator.h (allocator<void>) [C++20]: Add
missing noexcept to constructor. Restore missing POCMA and
is_always_equal_traits.
[C++17]: Make construct and destroy members private and
declare allocator_traits as a friend.
* include/bits/memoryfwd.h (allocator_traits): Declare.
* include/ext/malloc_allocator.h (malloc_allocator::allocate):
Add nodiscard attribute. Add static assertion for LWG 3307.
* include/ext/new_allocator.h (new_allocator::allocate): Add
static assertion for LWG 3307.
* testsuite/20_util/allocator/void.cc: Check that converting
constructor is noexcept. Check for propagation traits and
size_type and difference_type. Check that pointer and
const_pointer are gone in C++20.
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