template <typename T>
struct Foo
{
    struct Bar
    {
        template <typename V>
        friend void swap(typename Foo<V>::Bar&, typename Foo<V>::Bar&);
    };
};

template <typename T>
void swap(typename Foo<T>::Bar& b1, typename Foo<T>::Bar& b2) { }

template class Foo<int>;

int main()
{
    Foo<int>::Bar  bb1, bb2;
    swap<int>(bb1, bb2);
}