[Bug libstdc++/56922] set: the default constructor should be explicit

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56922

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-04-11 16:33:38 UTC ---
Yes, libstdc++ is already correct, there's nothing for us to do here