http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3386.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3582.html
Reviewed by EWG in Portland 2012, proceeding to CWG.
Accepted into the Working Draft in Bristol 2013, as N3638.