discriminate unions based on return type

type Good = { is: true; _tag: "good" };
type Bad = { is: false; _tag: "bad" };

// I know ts can discriminate a union type
function x(y: Good | Bad) {
  if (y.is) return;
  // then we know _tag must be 'bad'
  y._tag;
}

type Fortunate = { getIs: () => true; _tag: "good" };
type Unlucky = { getIs: () => false; _tag: "bad" };

// but I want to discriminate based off the return type
function z(yy: Fortunate | Unlucky) {
  //  this what I want to achieve
  if (yy.getIs()) return;
  // but _tag is still unknown
  yy._tag;
}