sixbrx

    type Fish = { swim: () => void };
    type Bird = { fly: () => void };
    type Human = { swim?: () => void; fly?: () => void };

    function move(animal: Fish | Bird | Human) {
      if ("swim" in animal) {
        // TSC infers wrongly here the presence of "swim" implies animal must be a Fish or Human
        onlyForFishAndHumans(animal); 
      } else {
        animal;
      }
    }

    function onlyForFishAndHumans(animal: Fish | Human) {
      if (animal.swim) {
        animal.swim(); // Error: attempt to call "not-callable".
      }
      // (receives bird which is not a Fish or Human)
    }

    const someObj = { fly: () => {}, swim: "not-callable" };
    const bird: Bird = someObj;

    move(bird);

    // runtime error: [ERR]: animal.swim is not a function