template<typename T>
class A {
private:
  int a = 0;
public:
  void func(A<double> second) {
    cout << second.a;
  }
};


void main()
{
  A<int> first;
  A<double> second;
  first.func(second);
}

template<typename T>
  class A {
  private:
    int a = 0;
  public:
    friend void A::func(A<double> second);
    void func(A<double> second) {
      cout << second.a;
    }
  };

public:
  int a = 0;

template <typename> friend class A;

template <typename U> friend void A<U>::func(A<double>);

template<typename T> class A {
private:
  int a = 0;
public:
  void func(A<double>);
};

template<> class A<double> {
private:
  int a = 0;
public:
  template <typename U> friend void A<U>::func(A<double>);

  void func(A second) {
    cout << second.a;
  }
};

template<typename T>
inline void A<T>::func(A<double> second) {
    cout << second.a;
}

#include <iostream>

template<typename T>
class node {
public:
    explicit node(T _value)
        : m_value(_value)
    {}

public:
    template <typename U>
    void add_child(node<U>& _node) {
        // логика, не относящаяся к примеру
        std::clog << "добавлена дочерняя нода [value=" << _node.m_value << "]n";
    }

protected:
    T m_value;

    template <typename U>
    friend class node;
};

int main() {
    node<int>   node_i { 1 };
    node<long>  node_l { 2 };
    node<short> node_s { 3 };

    node_i.add_child(node_l);
    node_l.add_child(node_s);
    return 0;
}