Control flow based type analysis not working inside an callback function

[goldenbearkin]

TypeScript Version: nightly (2.0.0)

Code
// Below I am returning Observable, but I've tried returning a pure callback function and it doesn't work either.

interface IIdProvider {
    readonly idToken$: Observable<string>;
    readonly providerURL: string;

}
public getLastIdProvider(): IIdProvider | null;

public isAuthenticated(): boolean | Observable<boolean> {
    let idProvider = this.getLastIdProvider();

    if (idProvider === null) return false;

    idProvider.idToken$ 
      .subscribe(x => console.log(x));   // OK

    return Observable.create(observer => {
      idProvider.idToken$ // TS2531: Object is possibly null
        .subscribe(x => console.log(x));
        ....

// for Pure callback function version:
 public isAuthenticated(): boolean | (() => void) {

    let idProvider = this.getLastIdProvider();

    if (idProvider === null) return false;

    idProvider.idToken$ //OK
      .subscribe(x => console.log(x));

    return () => {
      idProvider.idToken$
        .subscribe(x => console.log(x));  // TS2531: Object is possibly null
    };

Expected behavior:
both the idProvider should be null following control flow based type analysis.

Actual behavior:
The second one inside an observable creating function prompts warning of 'possibly null'

[ahejlsberg]

Duplicate of #7719. If you change your local variable declaration to const idProvider = ... it should work.

[goldenbearkin]

It works. Thanks.