TypeScript Version: 2.1.0-dev.20160909

Code

type A = {t: "a", a: string};
type B = {t: "b", b: string};
type U = A | B;

const a: U = {t: "a", a: "a"} as U;

if (a.t === "b") {
    a.b; // works as expected
}

const {t} = a;
if (t === "b") {
    a.b; // error TS2339: Property 'b' does not exist on type 'U'.
}

Expected behavior:

The type of a.t should correctly transfer to t when assigning or using destructuring. t should be usable as discriminant.

Actual behavior:

The type of t is widened from "a" | "b" to string and therefore can not be used as discriminant any more.

As the same happens also for flow (See facebook/flow#2409), it may be my thinking that is a little bit off. But I think that the case should be valid. The programmer should not be forced to repeat base. all over, that’s what variables are for.