模板类上的 C++ 模板复制构造函数

I have a template class that has a template copy constructor. The problem is when I instantiate this class using another instance of this class with the same template type, my template copy constructor is not called. Why doesn't it match?

这是代码片段:

#include <iostream>

template <typename T>
class MyTemplateClass
{
    public:
        MyTemplateClass()
        {
            std::cout << "default constructor" << std::endl;
        }

        /*
        MyTemplateClass(const MyTemplateClass<T>& other)
        {
            std::cout << "copy constructor" << std::endl;
        }
        */

        template <typename U>
        MyTemplateClass(const MyTemplateClass<U>& other)
        {
            std::cout << "template copy constructor" << std::endl;
        }
};

int main()
{
    MyTemplateClass<int> instance;
    MyTemplateClass<int> instance2(instance);
    return EXIT_SUCCESS;
}

输出是

default constructor

但是如果我显式地编写了默认的复制构造函数(通过取消注释)，那么输出就变成了

default constructor
copy constructor

MyTemplateClass(const MyTemplateClass&);
template <typename U> MyTemplateClass(const MyTemplateClass<U>&);

MyTemplateClass<int> instance2(instance);