delayWhen doesn't emit when the duration selector is empty()

[Airblader]

RxJS version: 6.1.0

Code to reproduce:

of(42).pipe(delayWhen(() => empty()))
  .subscribe(console.log);

Expected behavior:

This should log 42 immediately, as empty completes immediately which according to the docs should suffice even if the duration selector doesn't emit:

The source value is emitted on the output Observable only when the duration Observable emits a value or completes.

Actual behavior:

The observable completes without emission.

[Airblader]

of(42).pipe(delayWhen(() => empty().delay(0)))
  .subscribe(console.log);

does work as intended, so this seems to be an issue with the synchronicity.

[cartant]

Looking at the existing implementation of delayWhen I found another problem. See #3665 if you are interested in fixing it whilst you are working with that particular operator's implementation. The fix should go into its own PR, of course.

[Airblader]

Yeah, I thought about this when first looking at it as well and then just forgot about it. I think my first PR attempt is no good. We'll need a better solution.

[Airblader]

OK, subscribeToResult memorizes the outerValue, so I think we can use that and be good. Updating the PR.

[benlesh]

The OP here isn't valid. of(42).pipe(() => empty()) should return empty, effectively... so there would be no emission.

[Airblader]

@benlesh Sorry, I messed up my OP when editing it. It's missing the delayWhen.

The correct example was

of(42).pipe(delayWhen(() => empty()))
  .subscribe(console.log);