C++: the address of ‘X x()’ will never be NULL, using simple constructor?

I have this simple test where I'm researching use of old style () vs list style {} in constructors. Unfortunately, it has a compilation warning and prints incorrect results for the old style X x() construction case. Does anyone know why this particular case results in this warning?

My initial guess is X x(); is interpreted as calling a function. In that case X x; or X x{}; should be used.

#include <iostream>
using namespace std;

struct X {
    int x;
    X() : x(0) {}
    X(int xx) : x(xx) {}
    friend ostream& operator<<(ostream& os, const X& a) {
        return os << a.x;
    }
};

int main(int argc, char *argv[])
{
    X x();
    cout << x << endl; // addr of X x() will never be NULL
    X x2{};
    cout << x2 << endl; // ok, prints 0 correctly
    X x3(9);
    cout << x3 << endl; // ok, prints 9 correctly
}

Compilation and invocation:

g++ -pedantic -Wall -fno-elide-constructors test150.cc && ./a.out
test150.cc: In function ‘int main(int, char**)’:
test150.cc:17:13: warning: the address of ‘X x()’ will never be NULL [-Waddress]
     cout << x << endl; // addr of X x() will never be NULL
             ^
1
0
9