[Bug libstdc++/52942] [4.7 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 #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-04-12 
09:06:59 UTC ---
Or for the hash tables, which have lots of template parameters, it would be
even better to use std::tuple to get the EBO benefits