C++ partial template specialization: how to specialize std::iterator_category

I want to write template function void foo() which takes 2 iterators or pointers.

I want to have a specialized version for random_access_iterator. I try to write like below:

// generic version
template<class Iter, typename std::iterator_traits<Iter>::iterator_category>
void foo(Iter first, Iter last);

// Specialized version for random_access_iterator
template<class RandomIter>
void foo<RandomIter, std::random_access_iterator_tag>(RandomIter first, RandomIter last)
{
}

It gives error: illegal use of explicit template arguments

Please let me know what is correct way of defining this. Thanks

Based on "tag dispatching" pattern, I've written a solution to my question:

// Using "= delete" on un-supported categories to get concise compile-time error information
template<typename Iter> void _foo(Iter first, Iter last, std::bidirectional_iterator_tag) = delete;

template<typename RandomIter>
void _foo(RandomIter first, RandomIter last, std::random_access_iterator_tag)
{ ... }

template<class Iter>
void foo(Iter first, Iter last)
{
    typename std::iterator_traits<Iter>::iterator_category category;
    _foo(first, last, category);
}

It works now. Please leave comment if it has bug or need improvement. Thanks.