That's a fair point. Maybe it would work to make the source of ErrorImpl<E> be E instead of E's source? That way you can still downcast by walking the sources. That also sort of makes sense if you squint, since you are taking some E and making a new sort of error out of it (in this case one with a backtrace attached).

Unfortunately that change may break existing code, though. But on the other hand you could also make that behavior only occur when it's converted to a boxed error to limit the extent of the change in behavior.