refactor: use _ctorUnsubscribe flag

src/internal/Subscription.ts

@@ -39,6 +39,7 @@ export class Subscription implements SubscriptionLike {
    */
   constructor(unsubscribe?: () => void) {
     if (unsubscribe) {
+      (this as any)._ctorUnsubscribe = true;
       (this as any)._unsubscribe = unsubscribe;
     }
   }
@@ -56,7 +57,7 @@ export class Subscription implements SubscriptionLike {
       return;
     }
 
-    let { _parentOrParents, _unsubscribe, _subscriptions } = (this as any);
+    let { _parentOrParents, _ctorUnsubscribe, _unsubscribe, _subscriptions } = (this as any);
 
     this.closed = true;
     this._parentOrParents = null;
@@ -75,12 +76,15 @@ export class Subscription implements SubscriptionLike {
 
     if (isFunction(_unsubscribe)) {
       // It's only possible to null _unsubscribe - to release the reference to
-      // any teardown function passed in the constructor - if it is actually an
-      // instance property, as there are some classes that are derived from
-      // Subscriber (which derives from Subscription) that implement an
-      // _unsubscribe method as a mechanism for obtaining unsubscription
-      // notifications and some of those subscribers are recycled.
-      if (this.hasOwnProperty('_unsubscribe')) {
+      // any teardown function passed in the constructor - if the property was
+      // actually assigned in the constructor, as there are some classes that
+      // are derived from Subscriber (which derives from Subscription) that
+      // implement an _unsubscribe method as a mechanism for obtaining
+      // unsubscription notifications and some of those subscribers are
+      // recycled. Also, in some of those subscribers, _unsubscribe switches
+      // from a prototype method to an instance property - see notifyNext in
+      // RetryWhenSubscriber.
+      if (_ctorUnsubscribe) {
         (this as any)._unsubscribe = undefined;
       }
       try {