fix(types): support union types passed to defer

Author: benlesh

spec-dtslint/observables/defer-spec.ts

@@ -3,11 +3,15 @@ import { of, defer } from 'rxjs';
 it('should enforce function parameter', () => {
   const a = defer(); // $ExpectError
 });
-  
+
 it('should infer correctly with function return observable', () => {
   const a = defer(() => of(1, 2, 3)); // $ExpectType Observable<number>
 });
 
 it('should infer correctly with function return promise', () => {
   const a = defer(() => Promise.resolve(5)); // $ExpectType Observable<number>
 });
+
+it('should support union type returns', () => {
+  const a = defer(() => Math.random() > 0.5 ? of(123) : of('abc')); // $ExpectType Observable<string | number>
+});

src/internal/observable/defer.ts

@@ -1,5 +1,5 @@
 import { Observable } from '../Observable';
-import { SubscribableOrPromise } from '../types';
+import { SubscribableOrPromise, ObservedValueOf, ObservableInput } from '../types';
 import { from } from './from'; // lol
 import { empty } from './empty';
 
@@ -50,9 +50,9 @@ import { empty } from './empty';
  * @name defer
  * @owner Observable
  */
-export function defer<T>(observableFactory: () => SubscribableOrPromise<T> | void): Observable<T> {
-  return new Observable(subscriber => {
-    let input: SubscribableOrPromise<T> | void;
+export function defer<O extends ObservableInput<any>>(observableFactory: () => O | void): Observable<ObservedValueOf<O>> {
+  return new Observable<ObservedValueOf<O>>(subscriber => {
+    let input: O | void;
     try {
       input = observableFactory();
     } catch (err) {
@@ -62,4 +62,4 @@ export function defer<T>(observableFactory: () => SubscribableOrPromise<T> | voi
     const source = input ? from(input) : empty();
     return source.subscribe(subscriber);
   });
-}
+}

src/internal/observable/iif.ts

@@ -94,5 +94,5 @@ export function iif<T, F>(
   trueResult: SubscribableOrPromise<T> = EMPTY,
   falseResult: SubscribableOrPromise<F> = EMPTY
 ): Observable<T|F> {
-  return defer<T|F>(() => condition() ? trueResult : falseResult);
+  return defer(() => condition() ? trueResult : falseResult);
 }