[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 #4 from David Krauss <potswa at mac dot com> ---
Created attachment 31884
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31884&action=edit
SFINAE based fix

This seems stylistically OK, but I did copy-paste the implementation of moving.
Perhaps the common code should be factored into a new function, or the
constructor should simply call into move assignment.

It fixes the problem but I was unable to run the testsuite. This is a brand-new
installation and I haven't used it for contribution yet. (It says "WARNING:
could not find `runtest'".)