Observable<never> is not assignable to Subscribable<T>

[felixfbecker]

Bug Report

Current Behavior

Kinda related to #3890, when trying implement interop by accepting Subscribable as input, you cannot pass Observable<never> into any Subscribable<T>.
This is because Subscribable's signature for subscribe doesn't match that of Observable:

    subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void), error?: (error: any) => void, complete?: () => void): Unsubscribable;

vs

    subscribe(observer?: PartialObserver<T>): Subscription;
    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;

So you get this error:

Argument of type 'Observable<never>' is not assignable to parameter of type 'Subscribable<number>'.
  Types of property 'subscribe' are incompatible.
    Type '{ (observer?: NextObserver<never> | ErrorObserver<never> | CompletionObserver<never> | undefined)...' is not assignable to type '(observerOrNext?: NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | ((v...'.
      Types of parameters 'observer' and 'observerOrNext' are incompatible.
        Type 'NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | ((value: number) => v...' is not assignable to type 'NextObserver<never> | ErrorObserver<never> | CompletionObserver<never> | undefined'.
          Type '(value: number) => void' is not assignable to type 'NextObserver<never> | ErrorObserver<never> | CompletionObserver<never> | undefined'.
            Type '(value: number) => void' is not assignable to type 'CompletionObserver<never>'.
              Property 'complete' is missing in type '(value: number) => void'.

Reproduction

import { Observable, Subscribable, NEVER } from 'rxjs'

function foo(input: Subscribable<number>) {}

foo(NEVER)

Expected behavior

An Observable that never emits should be assignable to any Observable of any type, since the type doesn't matter if it never emits. This is the semantic of never.

Environment

• Runtime: any
• RxJS version: 6.2.1

Possible Solution
Change the signature of Subscribable.subscribe to use an overload, which in my eyes more accurately represents the actual interface.
With the current signature, this is possible, which it shouldn't:

subscribable.subscribe({ error: err => console.log('err 1') }, err => console.log('err 2'))

What would get logged?

[cartant]

This looks like a bit of a problem, as the signature for subscribe in the TC39 proposal - to which Symbol.observable refers - accepts only an observer. So typing it otherwise would be a lie, basically.

[felixfbecker]

Removing the positional parameters solves the problem too

[cartant]

@felixfbecker Ah, I see what you mean. It's already lying.

Also, regarding the WHATWG proposal, I've not (re)read the entire thread, but this comment suggests that its implementation of subscribe will also take only a single parameter: the observer.

The signature of Subscribable should probably represent that.