nested concatMaps no longer next values to destination in 6.3.0

[benlesh]

Bug Report

Current Behavior
Currently if you nest a concatMap inside of a concatMap, and the source completes, the inner most concatMap will not forward along values.

Reproduction

import { timer, from } from 'rxjs';
import { concatMap, tap } from 'rxjs/operators';

function wait(ms){
  return new Promise(res => {
    setTimeout(() => res('poop'), ms);
  })
}

timer(10).pipe(
  concatMap(() => from(wait(20)).pipe(
    concatMap(() => {
      console.log('here');
      return from(wait(20)).pipe(
        tap(x => console.log('>', x))
      );
    })
  ))
)
.subscribe(x => console.log('>', x));

Expected behavior
It should work like it did in 6.2

Environment

• Runtime: all
• RxJS version: 6.3.0

Possible Solution

None

[benlesh]

cc @hansl @IgorMinar

[benlesh]

This seems to specifically have to do with promises, FYI:

    const results: any[] = [];

    of(1).pipe(
      concatMap(() =>
        from(Promise.resolve(2)).pipe(
          concatMap(() => Promise.resolve(3))
        )
      )
    )
    .subscribe({
      next(value) { results.push(value); },
      complete() { results.push('done'); }
    });

    setTimeout(() => {
      expect(results).to.deep.equal([3, 'done']);
      done();
    }, 0);

[cartant]

Specifically, it seems to be due to the outermost promise.

If you comment out the from, replacing it with of(2, asapScheduler), it works fine:

of(1).pipe(
  concatMap(() =>
    from(Promise.resolve(2))
    // of(2, asapScheduler)
    .pipe(
      concatMap(() => defer(() => {
        console.log('sub');
        return of(3, asapScheduler).pipe(
          tap(() => console.log('next')),
          finalize(() => console.log('unsub'))
        );
      }))
    )
  )
)
.subscribe({
  next(value: any) { results.push(value); },
  complete() { results.push('done'); }
});

When the promise is used, it appears to subscribe to the innermost observable and then immediately unsubscribe. And then it seemingly doesn't complete, as there's no done in results.

I can look further into this later, if you've not already resolved the regression in the interim.

[cartant]

This fails, too, and does not use a promise:

of(1).pipe(
  concatMap(() =>
    defer(() => {
      console.log('outer-sub');
      return of(2, asapScheduler).pipe(
        tap(() => console.log('outer-next')),
        finalize(() => console.log('outer-unsub'))
      );
    })
    .pipe(
      concatMap(() => defer(() => {
        console.log('inner-sub');
        return of(3, asapScheduler).pipe(
          tap(() => console.log('inner-next')),
          finalize(() => console.log('inner-unsub'))
        );
      }))
    )
  )
)
.subscribe({
  next(value: any) { results.push(value); },
  complete() { results.push('done'); }
});

setTimeout(() => {
  console.log(results);
}, 0);

It emits a value, but does not complete. Works under 6.2.2.