[Bug] Typings startWith

[niklas-wortmann]

It seems like there is a bug in the typing for startWith.
It seems like it is introduced with #5376

TS throws following error:

Type `...` is missing the following properties from type `SchedulerLike`...

This occurs when ts is falling back to the following type signature of startWith:

export function startWith<T>(scheduler: SchedulerLike): MonoTypeOperatorFunction<T>;

This could occur for example by doing the following:

of(true).pipe(
    startWith<boolean>(undefined)
);

In general, this problem occurred recently in G3 so it is at least some kind of breaking behavior

[cartant]

Not a bug, IMO.

Without the explicit type argument, it should effect the correct type:

of(true).pipe(startWith(undefined)); // Observable<boolean | undefined>

If the explicit type parameter is specified, there is no way this can be satisfied, as undefined is assignable to neither boolean nor SchedulerLike.

[niklas-wortmann]

It also doesn't work if you set the explicit type parameter to boolean. And for some reason this was just popping up at google, so I don't know, but it seems like a regression or something.

[cartant]

The answer is to not use an explicit type parameter - this is Google's own advice. Or, if one is to be specified, specify one that's compatible with the argument being passed:

of(true).pipe(startWith<boolean | undefined>(undefined));

Not a bug, IMO.

[leggechr]

Even of(true).pipe(startWith<boolean | undefined>(undefined)); is not working. It gives the same error Jan-Niklas mentioned above. The only way I could get it to work is with

startWith(undefined as boolean|undefined)

[Airblader]

This is particularly annoying on void observables, such as Subject<void>. I really just want startWith() which typechecks but doesn't work, and because of this issue startWith(undefined) works but shows as a deprecated signature. The only way to truly have it work is then something like startWith(undefined as void) which is slightly insane. :-)

[cartant]

Addressed in #6114