想重载一个读入结构体的istream>>运算符，但是CE了 代码（ostream部分没有问题）：

struct Complex
{
	double real, imaginary;
	Complex(const double& treal, const double& timaginary): real(treal), imaginary(timaginary)
	{
	}
	friend istream& operator>>(istream &is, const Complex &z);
	friend ostream& operator<<(ostream &os, const Complex z);
	inline void add(Complex o)
	{
		this->real += o.real;
		this->imaginary += o.imaginary;
	}
};

istream& operator>>(istream &is, const Complex &z)
{
	is>>z.real>>z.imaginary;
	return is;
}

ostream& operator<<(ostream &os, const Complex z)
{
	os<<z.real<<' '<<z.imaginary;
	return os;
}

错误信息：

cannot bind 'std::istream {aka std::basic_istream<char>}' lvalue to 'std::basic_istream<char>&&'