This extremely minimal example will fail to compile because `A<int>` cannot access the private member `i` in `A<double>`

```
template <class T>
class A {
    int i;
  public:
    template <class U>
    void copy_i_from( const A<U> & a ){
        i = a.i;
    }
};

int main(void) {
    A<int> ai;
    A<double> ad;
    ai.copy_i_from(ad);
    return 0;
}

```

I know that I can make all the template instances friends of each other (see: [How to access private members of other template class instances?](https://stackoverflow.com/questions/26422160/how-to-access-private-members-of-other-template-class-instances)), but since I have only one method that requires the access (like in the example) I would prefer to limit the friendship to that method. Is this possible?