Проблема с перегрузкой оператора +

class Complex
{
private:
    double re, im;
    
public:
    Complex() {}
 
    Complex(double real, double imaginary)
        : re(real), im(imaginary) {}
        
    friend Complex operator+(const Complex&, const Complex&);
    friend Complex operator-(const Complex&, const Complex&);
    
    friend std::ostream& operator<<(std::ostream&, Complex&);
    friend std::istream& operator>>(std::istream&, Complex&);
};

std::ostream& operator<<(std::ostream& os, Complex& cpl)
{
    if (cpl.im < 0)
        os << cpl.re << '-' << cpl.im << 'i';
    else
        os << cpl.re << '+' << cpl.im << 'i';
    return os;
}
 
std::istream& operator>>(std::istream& is, Complex& cpl)
{
    is >> cpl.re >> cpl.im;
        return is;
}
 
Complex operator+(const Complex& c1, const Complex& c2)
{
    return Complex(c1.re + c2.re, c1.im + c2.im);
}
 
Complex operator-(const Complex& c1, const Complex& c2)
{
    return Complex(c1.re - c2.re, c1.im - c2.im);
}

Complex c1(2, 3);
Complex c2(3, 2);
Complex c3 = c1 + c2;
std::cout << c; // Вывод: 5+5i

Complex c1(2, 3);
Complex c2(3, 2);
std::cout << c1 + c2; // Ошибка

std::ostream& operator<<(std::ostream& os, const Complex& cpl)