template<int N>
class A
{
    template<int N2> friend class A;
public:
    A() : i(N) {}

    template<int K>
    void foo(A<K> other)
    {
        i = other.i; // <-- other.i is private
    }

private:
    int i;
};

int main()
{
    A<1> a1;
    A<2> a2;
    a1.foo(a2);

    return 0;
}