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#include <iostream>
using namespace std;
 
template<class T>
class Test;
 
 
class BeMyFriend
{
public:
template<class T> 
int checkprivacy(const Test<T> areyoumyfriend)
{	
	std::cout << "lemme check " << areyoumyfriend.mything << std::endl;
 
};
BeMyFriend& operator=(const BeMyFriend& );
BeMyFriend(){};
};
class Imnotyourfriend 
{
public:
template<class T> 
int checkprivacy(const Test<T> areyoumyfriend){
	areyoumyfriend.mything;
};
Imnotyourfriend(){std::cout << "works " << std::endl;};
};
 
template<class T>
class Test
{
 private:
 string mything= "here it is";
 public:
 friend T& T::operator=(const T&); 
 friend BeMyFriend&::BeMyFriend(const BeMyFriend&); 
 friend T&::T(const T&); 
 template<class D=T>
 friend int T::checkprivacy(const Test<D>);
 
};
 
 
 
 
 
int main()
{
 Test<BeMyFriend> hmm;
 Imnotyourfriend wannabefriend;
 wannabefriend.checkprivacy(hmm);
 
 return 0;
}
 

prog.cpp:38:23: warning: friend declaration ‘T& T(const T&)’ declares a non-template function [-Wnon-template-friend]
  friend T&::T(const T&);
                       ^
prog.cpp:38:23: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) 
prog.cpp: In instantiation of ‘int Imnotyourfriend::checkprivacy(Test<T>) [with T = BeMyFriend]’:
prog.cpp:52:32:   required from here
prog.cpp:25:17: error: ‘std::__cxx11::string Test<BeMyFriend>::mything’ is private within this context
  areyoumyfriend.mything;
  ~~~~~~~~~~~~~~~^~~~~~~
prog.cpp:34:18: note: declared private here
  string mything= "here it is";
                  ^~~~~~~~~~~~

Standard output is empty