4.x: ElementAt don't create exception after finding the item

[akarnokd]

When ElementAt finds an item, it disposes the upstream, however, the upstream may still emit an OnCompleted if it is unable to detect this dispose call (or ignores it). In this case, the ElementAt's OnCompleted gets invoked which creates an exception that gets thrown away as the base observer is already the no-op observer, wasting resources creating the stacktrace in the process.

I don't know how to verify this via unit test, but this extra call can be seen when placing a breakpoint in ElementAt's OnCompleted method and running this flow:

Observable.Return(1).ElementAt(0).Subscribe();

[danielcweber]

Just to clarify, a stacktrace isn't created until the exception is actually thrown.