[Bug libstdc++/52942] [4.7/4.8 Regression] using std::ref with a std::unordered_map fails to compile

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52942

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-04-12 
10:04:51 UTC ---
where else is _Select1st used?  Does it need argument_type and result_type
defined?  It doesn't matter if they come from unary_function, but it might
matter that they're defined, and if they need to be there then the bug will
still exist