TypeScript: Control flow analysis for destructured discriminated unions(github.com)

    const foo = { bar: 1, baz: "qux" };

    // Equivalent to
    // const bar = foo.bar;
    // const baz = foo.baz;
    const { bar, baz } = foo;

    type Foo = number | string;

    // Not helpful -- we can't tell from the value
    // whether it's supposed to be a username or an email.
    type UsernameOrEmail = string | string;

    type UsernameOrEmail = { type: "username", value: string }
                         | { type: "email", value: string };

    type ApiRequest = { type: "message", subject: string, contents: string }
                    | { type: "view-posts", user_id: number };

    const request: ApiRequest = getRequest();
    if (request.type === "message") {
        // Okay:
        console.log("Subject is", request.subject);
        // Rejected by compiler:
        console.log("User ID is", request.user_id);
    }

    type Action = { kind: "A", payload: number }
                | { kind: "B", payload: string };
    
    const { kind, payload } = getAction();
    if (kind === "B") {
        // Not currently allowed:
        payload.toUpperCase();
    }

    type Output = OutputExists | OutputDoesntExist;
    interface OutputExists { exists: true, url: string };
    interface OutputDoesntExist { exists: false };

    const f = ({ exists, url }: Output) => {}

    exists: true | false
    url: string | undefined

    interface OutputDoesntExist { exists: false, url: undefined };