C++ 校验类型是否相互转换

#include 

template
class IsConvertable{

private:
    typedef char Small;
    class Big { char dummy[2];};
    static Small Test(U);
    static Big Test(...);
    static T MakeT(); // 防止T构造函数是private时构造失败

public:
    enum {exists = sizeof(Test(MakeT())) == sizeof(Small)};
    enum {exists2Way = exists && IsConvertable::exists};
    
};


struct Base{};
struct Derived: public Base{
    int a;
};
void Test(){
    std::cout<::exists<::exists<::exists<::exists<::exists<::exists2Way<::exists2Way<::exists2Way<::exists2Way<::exists2Way<
    Test();
}