#include <iostream>

template <typename T> class Matrix;
template <typename T> std::ostream& operator<<(
    std::ostream& ostr, const Matrix<T>& m);

template <typename T>
class Matrix {
private:
    int _rank;
    friend std::ostream& operator<< <> (
      std::ostream& ostr, const Matrix& m );
};

template <typename T> std::ostream& operator<<(
    std::ostream& ostr, const Matrix<T>& m )
{
    (void) m._rank; // test friendship
    return ostr;
}

void f(const Matrix<int>& m)
{
    std::cout << m;
}