How to check if an arbitrary type is an iterator?

Question

This is an exact duplicate of this question, except that accepted answer is wrong, so I ask it again:

How do you correctly check to see if

Answer 1

I implemented this one some time ago:

template 
struct is_iterator {  
    template 
    static char test(typename std::iterator_traits::pointer* x);

    template 
    static long test(U* x);

    static const bool value = sizeof(test(nullptr)) == 1;
};

It compiles fine using your example. I can't test it on VC though.

Demo here.