fix(timeout): do not timeout if source emits synchronously when subsc…

…ribed

Closes ReactiveX#6862

spec/operators/timeout-spec.ts

@@ -691,6 +691,15 @@ describe('timeout operator', () => {
         expectSubscriptions(inner.subscriptions).toBe([]);
       });
     });
+
+    it('should not timeout if source emits synchronously when subscribed', () => {
+      rxTestScheduler.run(({ expectObservable, time }) => {
+        const source = new BehaviorSubject('a');
+        const t = time('  ---|');
+        const expected = 'a---';
+        expectObservable(source.pipe(timeout({ first: new Date(t) }))).toBe(expected);
+      });
+    });
   });
 
   it('should stop listening to a synchronous observable when unsubscribed', () => {

src/internal/operators/timeout.ts

@@ -360,6 +360,12 @@ export function timeout<T, O extends ObservableInput<any>, M>(
       );
     };
 
+    // Intentionally terse code.
+    // If `first` was provided, and it's a number, then use it.
+    // If `first` was provided and it's not a number, it's a Date, and we get the difference between it and "now".
+    // If `first` was not provided at all, then our first timer will be the value from `each`.
+    startTimer(first != null ? (typeof first === 'number' ? first : +first - scheduler!.now()) : each!);
+
     originalSourceSubscription = source.subscribe(
       createOperatorSubscriber(
         subscriber,
@@ -384,12 +390,6 @@ export function timeout<T, O extends ObservableInput<any>, M>(
         }
       )
     );
-
-    // Intentionally terse code.
-    // If `first` was provided, and it's a number, then use it.
-    // If `first` was provided and it's not a number, it's a Date, and we get the difference between it and "now".
-    // If `first` was not provided at all, then our first timer will be the value from `each`.
-    startTimer(first != null ? (typeof first === 'number' ? first : +first - scheduler!.now()) : each!);
   });
 }