[v3] additions to ext/alloc_traits.h

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

        * include/ext/alloc_traits.h (__allocator_always_compares_equal): New
        trait, provide partial specializations for known allocators.
        (__alloc_traits::construct, __alloc_traits::destroy): Overload for
        non-standard pointer types.
        (__alloc_traits::_S_always_equal): New trait for use with noexcept.
        (__alloc_traits::_S_nothrow_move): Likewise.
        (__alloc_traits::_S_nothrow_swap): Likewise.

This adds some new traits to __gnu_ext::__alloc_traits which allow
compile-time detection of allocator types that always compare equal,
allowing container operations to be more efficient and marked noexcept
when appropriate.

Tested x86_64-linux, committed to trunk.

Attachment: allocs.txt
Description: Text document