combineLatest doesn't not work correctly with zero input streams

[trotyl]

Bug Report

Current Behavior
A clear and concise description of the behavior.

combineLatest takes an array of input streams, namely, zero or more input streams. But in the case of zero input streams, it hangs infinitely instead of emit an empty Array/Tuple ([]).

Reproduction

• REPL or Repo link:

https://stackblitz.com/edit/rxjs-a961ph?devtoolsheight=60

// Promise

Promise.all([Promise.resolve(1), Promise.resolve(2)]) // => [1, 2]
Promise.all([Promise.resolve(1)]) // => [1]
Promise.all([]) // => []

// RxJS

combineLatest([of(1), of(2)])  // => [1, 2]
combineLatest([of(1)]) // => [1]
combineLatest([]) // Never produce a value

Expected behavior

combineLatest([]) // => []

As per the doc:

Combines multiple Observables to create an Observable whose values are calculated from the latest values of each of its input Observables.

RxJS should follow the conventional definition of each/every in algorithms, for a zero length sequence, its .every(criteria) is always true.

Environment

• Runtime: [e.g. Node vUnrelavant, Chrome vUnrelavant]
• RxJS version: 6.5.2
• (If bug is related) Loader, build configuration: [e.g webpack, angular-cli version, config]

Possible Solution

Additional context/Screenshots
Add any other context about the problem here. If applicable, add screenshots to help explain.

Relates to #3189

[cartant]

[] is the input stream though. And it is empty.

Arrays are valid ObservableInput values and combineLatest accepts rest parameters: combineLatest(of(1), of(2)).

I'm not saying that this isn't confusing and cannot be improved, though. There is special-case shenanigans going on to figure out if it was passed an array of streams or multiple arguments - where each argument is a stream.

rxjs/src/internal/observable/combineLatest.ts

Lines 236 to 244 in b713d39

	if (typeof observables[observables.length - 1] === 'function') {
	resultSelector = observables.pop() as (...values: Array<any>) => R;
	}
	// if the first and only other argument besides the resultSelector is an array
	// assume it's been called with `combineLatest([obs1, obs2, obs3], resultSelector)`
	if (observables.length === 1 && isArray(observables[0])) {
	observables = observables[0] as any;
	}

[trotyl]

[] is the input stream though. And it is empty.

@cartant Changing to combineLatest() would have the same result:

https://stackblitz.com/edit/rxjs-qu7wtj?file=index.ts

Is there any reason for "no arguments at all" being considered an input stream?

[cartant]

Yeah. I'm not saying it's not inconsistent. The inconsistency is one reason I wrote this. It's just not the sort of 'bug' that can be fixed without being deemed a breaking change. IMO any changes of this nature are going to have to wait for v8.

[dobesv]

I guess the workaround here is to use rxjs-etc combineLatestArray if you want to pass an array to combineLatest that could be empty.