Can you say more about natively supporting discriminated unions?

You can already do this:

    type MyUnion = { type: "foo"; foo: string } | { type: "bar"; bar: string };

And this will compile:

    (u: MyUnion) => {
      switch (u.type) {
        case "foo":
          return u.foo;
        case "bar":
          return u.bar;
      }
    };

Whereas this wont:

    (u: MyUnion) => {
      switch (u.type) {
        case "foo":
          return u.bar;
        case "bar":
          return u.foo;
      }
    };