C++子类如何调用基类的友元操作符重载函数？

class base
{
public:
friend bool operator==(base & x, base & y) { ... }
};
class derived : public base
{
public:
friend bool operator==(derived & x, derived & y) { 此处如何先调用基类的==重载函数？ }
};
谢谢回答！