C++: Why does numeric_limits work on types it does not know?

I have created my own type, without any comparator, and without a specialization of std::numeric_limits. Despite that, for some reason, std::numeric_limits<MyType> compiles fine. How come? Example code below:

#include <iostream>
#include <limits>
using namespace std;

// This is an int wrapper that defaults to 666 instead of 0
class A {
public:
    int x;
public:
    A() : x(666) {}
};

int main() {
    A a = std::numeric_limits<A>::max();
    A b = std::numeric_limits<A>::max();

    std::cout << a.x << "\n" << b.x;
    // your code goes here
    return 0;
}