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 a given type T is an iterator?

My attempt at solving it:

// Assume enable_if and is_same are defined
// (hoping for a solution that works for C++03 too)

template<class T>
class is_iterator
{
    static char (&test(...))[2];
    template<class U>
    static typename std::enable_if<
        !std::is_same<
            typename std::iterator_traits<T>::value_type,
            void
        >::value,
        char
    >::type test(U);
public:
    static bool const value = sizeof(test(0)) == 1;
};

struct Foo { };

int main()
{
    return is_iterator<Foo>::value;
}

on Visual C++ happens to fail with:

...\vc\include\xutility(373):
error C2039: 'iterator_category': is not a member of 'Foo'

because iterator_traits is looking for a definition of value_type in Foo, which (obviously) doesn't exist.

I am aware that __if_exists is a possibility on Visual C++, but I'm looking for a portable solution.