Why is passing a class instance into its own class constructor right?

Question

I am curious about the following code which can execute correctly.

#include <iostream>
using namespace std;

class A
{
public:
    A(int a=1, int b=2){
        this->a = a;
        this->b = b;
    }
    void print_a(){
        cout<<a<<endl;
    }

private:
    int a;
    int b;
};

int main() {
    A aa = A(A());  //
    aa.print_a();
    return 0;
}

The output is 1 which is correct.

I am curious about the mechanism.

The difference between this question is that that question's constructor explicitly accepts an instance.