为什么会出现这种报错  怎么弄

#include<iostream>
using namespace std;
class Complex
    {public:
        Complex(){real=0;imag=0;}
        Complex(double r){real=r;imag=0;}
        Complex(double r,double i){real=r;imag=i;}
        friend Complex operator+(Complex c1,Complex c2);
        void display();
     private:
         double real;
         double imag;
    };
Complex operator+(Complex c1,Complex c2)
    {return Complex(c1.real+c2.real,c1.imag+c2.imag);
    }
void Complex::display()
    {cout<<"("<<real<<","<<imag<<"i)"<<endl;
    }
int main()
    {Complex c1(3,4),c2(5,-10),c3;
     c3=c1+2.5;
     c3.display();
     return 0;
    }