[Bug libstdc++/59872] Cannot move std::map with move-only mapped_type

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59872

--- Comment #1 from David Krauss <potswa at mac dot com> ---
It looks like the allocator-aware update introduced a copying branch to the
move constructor, to handle the case of transferring an object from one
stateful allocator to another.

The branch on _Alloc_traits::_S_always_equal(), presumably a constant value,
should be split into two functions. I'll continue digging in, I don't yet
understand _Alloc_trats or why _S_always_equal is a private interface.