test.ts

type A = {
  x: string;
};
type B = {
  x?: string;
  z: string;
};

type C = A | B;

function isA(c: C): c is A {
  return (c as A) !== undefined;
}

function complicatedButDoesWork(c: C): string {
  if (isA(c)) {
    return c.x;
  }
  if (c.x) {
    return c.x;
  }
  return c.z;
}

function simplerButDoesntWork(c: C): string {
  if (c.x) {
    return c.x;
  }
  return c.z;
}