defaultIfEmpty does not give correct type for null or undefined when strictNullChecks is false in tsconfig

[shessafridi]

Describe the bug

defaultIfEmpty(null) or defaultIfEmpty(undefined) always returns the observable of type Observable<any>

Expected behavior

When strictNullChecks is off null type is ignored so it should just return the type of the source observable as it is.

Expected behaviour in strictNullChecks: false

const test$ = of(1).pipe(defaultIfEmpty(null)); // Observable<number>

Instead we get

const test$ = of(1).pipe(defaultIfEmpty(null)); // Observable<any>

Reproduction code

const test = of(1).pipe(defaultIfEmpty(null));

Reproduction URL

No response

Version

7.8.1

Environment

"typescript": "~5.0.4",
"rxjs": "^7.8.1"

Additional context

I am currently using a custom wrapper operator built over defaultIfEmpty to fix the issue, here is the code for it.

import { defaultIfEmpty, Observable } from 'rxjs';

export function defaultIfEmptyFixed<T, R extends null>(
  defaultValue: R
): (source: Observable<T>) => Observable<T | null>;

export function defaultIfEmptyFixed<T, R extends undefined>(
  defaultValue: R
): (source: Observable<T>) => Observable<T | undefined>;

export function defaultIfEmptyFixed<T, R>(
  defaultValue: R
): (source: Observable<T>) => Observable<T | R> {
  return (input: Observable<T>) =>
    input.pipe(defaultIfEmpty<T, R>(defaultValue));
}