[Bug libstdc++/57228] [c++11] constructing shared_ptr to enable_shared_from_this-derived class from raw pointers

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57228

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is http://cplusplus.github.io/LWG/lwg-active.html#2179 but if the
behaviour changes I'll eat my hat.