Observable<never> is not assignable to Subscribable<T> #3891
Description
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?