fix(delayWhen): Emit source value if duration selector completes sync…

…hronously

This fixes an issue where delayWhen would not re-emit a source emission if the duration selector
completed synchronously.

ReactiveX#3663

spec/operators/delayWhen-spec.ts

@@ -1,6 +1,7 @@
 import * as Rx from 'rxjs/Rx';
 import { hot, cold, expectObservable, expectSubscriptions } from '../helpers/marble-testing';
 import { expect } from 'chai';
+import { EMPTY } from 'rxjs';
 
 declare function asDiagram(arg: string): Function;
 
@@ -104,6 +105,28 @@ describe('Observable.prototype.delayWhen', () => {
     expectObservable(result).toBe(expected);
     expectSubscriptions(e1.subscriptions).toBe(subs);
     expectSubscriptions(selector.subscriptions).toBe(selectorSubs);
+    });
+
+  it('should emit if the selector completes synchronously', () => {
+    const e1 =        hot('a--|');
+    const expected =      'a--|';
+    const subs =          '^  !';
+
+    const result = e1.delayWhen((x: any) => EMPTY);
+
+    expectObservable(result).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(subs);
+  });
+
+  it('should emit if the source completes synchronously and the selector completes synchronously', () => {
+    const e1 =        hot('(a|)');
+    const expected =      '(a|)';
+    const subs =          '(^!)';
+
+    const result = e1.delayWhen((x: any) => EMPTY);
+
+    expectObservable(result).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(subs);
   });
 
   it('should not emit if selector never emits', () => {

src/internal/operators/delayWhen.ts

@@ -140,12 +140,16 @@ class DelayWhenSubscriber<T, R> extends OuterSubscriber<T, R> {
   private tryDelay(delayNotifier: Observable<any>, value: T): void {
     const notifierSubscription = subscribeToResult(this, delayNotifier, value);
 
-    if (notifierSubscription && !notifierSubscription.closed) {
-      this.add(notifierSubscription);
-      this.delayNotifierSubscriptions.push(notifierSubscription);
+    if (notifierSubscription) {
+      if (notifierSubscription.closed) {
+        this.destination.next(value);
+        this.tryComplete();
+      } else {
+        this.add(notifierSubscription);
+        this.delayNotifierSubscriptions.push(notifierSubscription);
+        this.values.push(value);
+      }
     }
-
-    this.values.push(value);
   }
 
   private tryComplete(): void {