lettable toPromise?

RxJS version: 5.5.0-beta

Code to reproduce:

import { of } from "rxjs/observable/of";
import {
	map,
	toPromise
} from "rxjs/operators";

export function demoFn(): Promise<void> {
	return of([null])
		.pipe(
			map(() => {}),
			toPromise()
		);
}

Expected behavior: No error Actual behavior:

Error:(11, 4) TS2345: Argument of type 'UnaryFunction<Observable<void>, Promise<void>>' is not assignable to parameter of type 'UnaryFunction<Observable<void>, Observable<{}>>'.
  Type 'Promise<void>' is not assignable to type 'Observable<{}>'.
    Property '_isScalar' is missing in type 'Promise<void>'.

Additional information:

[jack-guy]

@mebibou Don’t pipe. It’s on the Observable object by default.

Observable.of('foo').toPromise(); // this
Observable.of('foo').pipe(toPromise()); // not this

[mebibou]

So what is the proper way of using toPromise on Observable now?