[Bug libstdc++/57263] std::set with user-defined allocator - compile error

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57263

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. the function that needs to be used is
std::pointer_traits<std::allocator_traits<Allocator>::pointer>::pointer_to