fix(defer) allow passing () => any to observableFactory

spec-dtslint/observables/defer-spec.ts

@@ -16,6 +16,10 @@ 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 function return any', () => {
+  const a = defer(() => 3 as any); // $ExpectType Observable<unknown>
+});
+
 it('should error with void functions', () => {
   const a = defer(() => {}); // $ExpectError
 });
@@ -28,8 +32,8 @@ it('should error if function returns undefined', () => {
   const a = defer(() => undefined); // $ExpectError
 });
 
-it('should error if function returns never', () => {
-  const a = defer(() => { throw new Error(); }); // $ExpectError
+it('should infer if function returns never', () => {
+  const a = defer(() => { throw new Error(); }); // $ExpectType Observable<never>
 });
 
 
@@ -43,6 +47,6 @@ it('should infer correctly with function that sometimes error', () => {
   });
 });
 
-it('should infer correctly with functions that sometimes do not return an ObservableInput', () => {
+it('should error with functions that sometimes do not return an ObservableInput', () => {
   const a = defer(() => { if (Math.random() < 0.5) { return of(42); } }); // $ExpectError
 });

src/internal/observable/defer.ts

@@ -51,9 +51,7 @@ import { from } from './from'; // lol
  * @name defer
  * @owner Observable
  */
-export function defer<R extends ObservableInput<any>>(
-  observableFactory: [R] extends [undefined] | [void] ? never : () => R
-): Observable<ObservedValueOf<R>> {
+export function defer<R extends ObservableInput<any>>(observableFactory: () => R): Observable<ObservedValueOf<R>> {
   return new Observable<ObservedValueOf<R>>(subscriber => {
     let input: R;
     try {