Why is this simple assignment undefined behaviour?

Question

I was refreshing my understanding of value-initialisation versus default-initialisation, and came across this:

struct C {
    int x;
    int y;
    C () { }
};

int main () {
    C c = C ();
}

Apparently this is UB because

In the case of C(), there is a constructor that is capable of initializing the x and y members, so no initialization takes place. Attempting to copy C() to c therefore results in undefined behavior.

I think I understand why, but I'm not certain. Can someone please elaborate?

Does that mean this is also UB?

int x; x = x;

Incidentally, with regards to value initialisation, is the following guaranteed to be zero?

int x = int ();