运算符重载后无法识别转换构造函数转换后的形参

运算符重载实现两个复数相加，同时定义了转换构造函数将int转化为复数类型，但在执行complex+complex（int）时报错(c6），编译无法找到相关转化

#include <iostream>
#include <cmath>
using namespace std;
class complex
{
public:
    complex(int a,int b):real(a),imag(b){}
    complex() { real = 0; imag = 0; }
    friend istream& operator>>(istream&,complex&);
    friend ostream& operator<<(ostream&, const complex&);
    complex operator++();
    complex operator++(int);
    friend complex operator+(complex&, complex&);
    complex(int a) { real = a; imag = 0; }
private:
    int real;
    int imag;
};
istream& operator>>(istream& input,complex& c)
{
    cout << "enter the real and imag" << endl;
    input >> c.real >> c.imag;
    return input;
}
complex operator +(complex& c1, complex& c2)
{
    return complex(c1.real + c2.real, c1.imag + c2.imag);
}
ostream& operator<<(ostream& output, const complex& c)
{
    if (c.imag < 0)
        output << c.real << "-" << abs(c.imag) << "i" << endl;
    else if (c.imag == 0)
        output << c.real << endl;
    else
        output << c.real << "+" << c.imag << "i" << endl;
    return output;
}
complex complex::operator++()
{
    complex temp(this->real+1,this->imag+1);
    return temp;
}
complex complex::operator++(int)
{
    complex temp(*this);
    *this = complex(this->real + 1, this->imag + 1);
    return temp;
}
int main()
{
    complex c1(1, 2), c2(3, -1), c4, c5,c6,c8;
    cout << c1 << '\t' << c2 << endl;
    c4 = c1++;
    cout << c4 << endl;
    c5 = ++c2;
    int i = 3;
    cout << c5 << endl;
    cout << c1 << endl;
    cout << complex(i) << endl;
    c8 = complex(i);
    c6 =complex(i)+c2;
    cout << c6;
    complex c7;
    cout << "enter c7" << endl;
    cin >> c7;
    cout << c7 << endl;
    system("pause");
    return 0;
}

错误(活动) E0349 没有与这些操作数匹配的 "+" 运算符
错误 C2678 二进制“+”: 没有找到接受“complex”类型的左操作数的运算符(或没有可接受的转换)