声明“ operator（）”为无效

Question

我正在努力修复和理解编译器报告为错误的简单代码，该代码应为Klass定义调用运算符：

template<typename T>
class Klass{
    T operator(){ return T; }
};

g ++ 7.3报告：

a.cpp:3:26: error: declaration of ‘operator()’ as non-function
  T operator(){ return T; }
                          ^
a.cpp:3:26: error: expected ‘;’ at end of member declaration

怎么了？