[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 #3 from David Krauss <potswa at mac dot com> ---
Thanks, I'm working on it now.

Is there some precedent for this kind of SFINAE in libstdc++? The hard part is
getting the style right. Metaprogramming looks weird in 80 columns.

As for the defect report, my thoughts exactly. Maybe we can extend
__allocator_always_compares_equal to evaluate such an equality check in a
SFINAE, constant expression context. Then a user-defined

constexpr bool operator == ( my_alloc, my_alloc )
    { return true; }

will satisfy the condition, but anything else will default.