chore: fix iif typings to infer never (#5176)

* chore(typings): fix iif observable typings for optional parameters (#4494)

* test(dtslint): add dtslint test for iif observable static function (#4093)

* chore: remove unnecessary signature

* chore: fix dtslint expectations

Closes #4494

* chore: empty commit to kick CI

spec-dtslint/observables/iif-spec.ts

@@ -0,0 +1,17 @@
+import { iif, of } from 'rxjs';
+
+it('should accept function as first parameter', () => {
+  const a = iif(() => false); // $ExpectType Observable<never>
+});
+
+it('should infer correctly with 2 parameters', () => {
+  const a = iif(() => false, of(1)); // $ExpectType Observable<number>
+});
+
+it('should infer correctly with 3 parameters', () => {
+  const a = iif(() => false, of(1), of(2)); // $ExpectType Observable<number>
+});
+
+it('should infer correctly with 3 parameters of different types', () => {
+  const a = iif(() => false, of(1), of('a')); // $ExpectType Observable<string | number>
+});

src/internal/observable/iif.ts

@@ -90,8 +90,8 @@ import { SubscribableOrPromise } from '../types';
  * @static true
  * @name iif
  * @owner Observable
- */
-export function iif<T, F>(
+*/
+export function iif<T = never, F = never>(
   condition: () => boolean,
   trueResult: SubscribableOrPromise<T> = EMPTY,
   falseResult: SubscribableOrPromise<F> = EMPTY