The equality operators for _ExtPtr_allocator are defined as non-const
member functions, which causes ambiguities in C++20 due to the
synthesized operator!= candidates. They should always have been const.
The _Pointer_adapter class template has both value_type and element_type
members, which makes readable_traits<_Pointer_adapter<T>> ambiguous. The
intended workaround is to add a specialization of readable_traits.
* include/ext/extptr_allocator.h (_ExtPtr_allocator::operator==)
(_ExtPtr_allocator::operator!=): Add missing const qualifiers.
* include/ext/pointer.h (readable_traits<_Pointer_adapter<S>>): Add
partial specialization to disambiguate the two constrained
specializations.
From-SVN: r280067
