Root cause exception info gets lost

[MSACATS]

I'm having trouble getting proper stack traces / exception root cause info. Below is an example test:
I have a multi-value observable and I call "SingleAsync" on it, which raises an exception that there is more than one element in the sequence. However, no matter how I try, I can never get the real location of that exception, which makes it basically impossible to track down in our actual system. For example all I get is:

when I await the Observable:

System.InvalidOperationException: Sequence contains more than one element.
at System.Reactive.PlatformServices.ExceptionServicesImpl.Rethrow(Exception exception) in /_/Rx.NET/Source/src/System.Reactive/Internal/ExceptionServicesImpl.cs:line 16

when I await the Observable.ToTask:

System.InvalidOperationException: Sequence contains more than one element.
at TestAwaitOnMultipleElements() in \ReactiveTests.cs:line 29

(i.e. the await task line, not the actual SingleAsync() line)

when I use the Subscribe:

System.InvalidOperationException: Sequence contains more than one element.
(i.e. no stack trace at all)

This is the test code:

[TestMethod]
public async Task TestAwaitOnMultipleElements()
{
	IObservable<int> multiElementObservable = OtherMethodToVerifyWeCanFindTheCause();

	await multiElementObservable;

	var task = multiElementObservable.ToTask();
	await task;

	TaskCompletionSource<Unit> taskCompletion = new TaskCompletionSource<Unit>();
	multiElementObservable.Subscribe(
			i => Trace.TraceInformation("Element {0}", i.ToString()),
			e =>
			{
				Trace.TraceError(e.ToString());
				taskCompletion.SetException(e);
			},
			() => taskCompletion.SetResult(Unit.Default));
	await taskCompletion.Task;
}

private static IObservable<int> OtherMethodToVerifyWeCanFindTheCause()
{
	var multiElementObservable = Observable.Range(0, 2);

	multiElementObservable = multiElementObservable
		.Do(i => Trace.TraceInformation("Element {0}", i.ToString()))
		.SingleAsync(); //this will be an error because there are multiple elements
	return multiElementObservable;
}

[bartdesmet]

Some improvement can likely be made for operators that create an exception object out of nowhere. Those lack stack traces because they don't really got thrown; they're just exception objects flowing into OnError calls. Will have a look for the upcoming release.

[bartdesmet]

A small improvement has been made. The very nature of exception propagation in Rx limits the context provided in a stack trace in cases where an operator such as SingleAsync propagates an exception of its own. However, with the change made (see PR linked above), there will be at least a hint at the operator that caused the exception.