Implement is_iterator, sfinae iterator_traits.

[EricWF]

My current method of checking if something is an iterator is to instantiate iterator_traits. Unfortunately iterator_traits does not support SFINAE, causing hard compile errors when instantiated with non-iterators.

I could just write my own iterator_traits, but I would lose all of the specializations of std::iterator_traits.

How can is_iterator be implemented?
(I think SFINAE iterator_traits are in C++14)