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

[akim.demaille at gmail dot com]

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

--- Comment #1 from Akim Demaille <akim.demaille at gmail dot com> 2013-04-11 16:08:07 UTC ---
FWIW: http://cplusplus.github.io/LWG/lwg-active.html#2193