Overloading operator+() in global scope, for a template class inside a template class

Question

I am trying to overload operator+(ptrdiff_t, It) for a random access iterator, which is template class, inside a template class.

The following similar code refuses to work. But if make class B not a template, it works.

template<typename T>
struct A
{
    template<bool S>
    struct B // random_access_iterator
    {
        //friend B operator+<>(T, B);
    };
};


template<typename T, bool S>
typename A<T>::B<S> operator+(T a, typename A<T>::B<S> i) { return i; }

int main()
{
    5 + A<int>::B<true>{};  // Why argument deduction/substitution failed?
    return 0;
}