fix(defer): use overload sig for void factory (#4810)

* test(defer): add failing dtslint test

* fix(defer): use overload sig for void factory

Closes #4804

Author: cartant

spec-dtslint/observables/defer-spec.ts

@@ -15,3 +15,7 @@ it('should infer correctly with function return promise', () => {
 it('should support union type returns', () => {
   const a = defer(() => Math.random() > 0.5 ? of(123) : of('abc')); // $ExpectType Observable<string | number>
 });
+
+it('should infer correctly with void functions', () => {
+  const a = defer(() => {}); // $ExpectType Observable<never>
+});

src/internal/observable/defer.ts

@@ -52,6 +52,8 @@ import { empty } from './empty';
  * @name defer
  * @owner Observable
  */
+export function defer<O extends ObservableInput<any>>(observableFactory: () => O): Observable<ObservedValueOf<O>>;
+export function defer(observableFactory: () => void): Observable<never>;
 export function defer<O extends ObservableInput<any>>(observableFactory: () => O | void): Observable<ObservedValueOf<O>> {
   return new Observable<ObservedValueOf<O>>(subscriber => {
     let input: O | void;