forkJoin(Promise) "You provided 'null' where a stream was expected"

[rubenprins]

Bug Report

Current Behavior
After #4640, forkJoin(promise) throws an error

You provided 'null' where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.

Note that this only happens if you provide forkJoin with a single argument of type Promise.

Reproduction

forkJoin(Promise.resolve('hello')).subscribe(result => console.log(result));

Expected behavior
forkJoin(promise) should behave the same as forkJoin([promise]), like it does for Observable.

Environment

• RxJS version: 6.5.1

Possible Solution
Don't just check for isObservable, but also isPromise (and strictly speaking, check for Iterable too).

rxjs/src/internal/observable/forkJoin.ts

Line 150 in cd5895d

if (isObject(first) && !isObservable(first)) {

should become

if (isObject(first) && !isObservable(first) && !isPromise(first) && !isIterable(first))