Различия компиляторов. В чем причина?

#include <iostream>
 
template <class T>
class complex
{
public:
    T re, im;
    complex(T re = 0, T im = 0)
    {
        this->re = re;
        this->im = im;
    }
    complex(complex<T> & c)
    {
        this->re = c.re;
        this->im = c.im;
    }
 
    complex<T> operator + (complex<T> & c)
    {
        return complex<T>(re + c.re,im + c.im);
    }
 
    complex<T> operator - (complex<T> & c)
    {
        return complex<T>(re - c.re,im - c.im);
    }
    complex<T> & operator = (complex<T> & c)
    {
        this->re = c.re;
        this->im = c.im;
        return *this;
    }
    complex<T> operator + (T x)
    {
        return complex<T>(re + x,im);
    }
};
 
using namespace std;
int main(int argc, char* argv[])
{
    complex<double> a(1,3), b, c;
    c = a;
    b = a + c + 10;
    cout<<b.re<<" "<<b.im<<endl;
    cin.get();
    return 0;
}

b = a + c + 10;

complex<T> & operator = (complex<T> c)

    complex<T> & operator + (complex<T> & c)
    {
        return complex<T>(re + c.re,im + c.im);
    }

complex(const complex<T> & c){}
complex<T> & operator = (const complex<T> & c){}

class X
{
public:
    int a;
    X() { a = 0; }
    X(X & x) { a = x.a; }
};
int main(int argc, char* argv[])
{
        X z;
        X z1(z);
                .......
}

b = a + c + 10;

complex<double> operator + (double x) {}