运算符重载为友元函数的问题

#include <iostream>
using namespace std;
class Complex
{
public:
    Complex(double r=0.0, double i=0.0):real(r), imag(i) {}
    friend Complex operator+(const Complex &c1, const Complex &c2);
    friend Complex operator-(const Complex &c1, const Complex &c2);
    friend ostream &operator<<(ostream &out, const Complex &c);
private:
    double real;
    double imag;
};

Complex operator+(const Complex &c1, const Complex &c2)
{
    return Complex(c1.real+c2.real, c1.imag+c2.imag);
}
Complex operator-(const Complex &c1, const Complex &c2)
{
    return Complex(c1.real-c2.real, c1.imag-c2.imag);
}
ostream &operator<<(ostream &out, const Complex &c)
{
    out<<"("<<c.real<<","<<c.imag<<")";
    return out;
}
int main()
{
    Complex c1(5, 4), c2(2, 10), c3;
    cout<<"c1="<<c1<<endl;
    cout<<""c2="<<c2<<endl;
    c3=c1-c2;
    cout<<"c3=c1-c2"<<c3<<endl;
    c3=c1+c2;
    cout<<"c3=c1+c2"<<c3<<endl;
    return 0;
}
为什么运行时会出现错误：fatal error C1001: INTERNAL COMPILER ERROR
怎么回事？