Unexpected startWith types behaviour inside pipe

[OliverJAsh]

Bug Report

Current Behavior
When using startWith inside of pipe, the argument type is not validated and/or the return type may be incorrect.

Reproduction

• REPL or Repo link: https://stackblitz.com/edit/typescript-v69rnb?file=index.ts

import { of } from 'rxjs';
import { startWith } from 'rxjs/operators';
import 'rxjs/Rx';

const ob2$ = of(1).startWith(
    // Expect error, got one :-)
    // Argument of type '"foo"' is not assignable to parameter of type 'number | SchedulerLike'.
    'foo',
);

const ob$ = of(1).pipe(
    startWith(
        // Expected error, but got none :-(
        'foo',
    ),
);
// OR, if we don't get an error, we should at least get a union:
// $ExpectType Observable<number | string>
// but got actual type Observable<{}>
ob$;

Expected behavior
When using startWith inside of pipe, I would expect either:

• the argument type must match the Observable's inner type
• the return type should be a union of the Observable's inner type and the argument type

Environment

• Runtime: no runtime, purely type level
• RxJS version: 6.2.2

Possible Solution

Additional context/Screenshots
Related #2544

[cartant]

This looks like it's the same problem that was reported in #3841 and fixed in #3945.

[OliverJAsh]

Amazing, thanks @cartant. I'll wait for the next release.