修改非 mutable 常量对象

示例：

struct T {
    mutable int i;
    int j;
};

const T obj {};
T* p = const_cast<T*>(&obj);

obj.i++;   // Well-defined
obj.j++;   // Ill-formed, compile error

p->i = 0;  // Well-defined
p->j = 1;  // Undefined behavior