When using an extension system and creating a class, XenForo checks that there would be a class in to_class. But it ignores in from_class
That being said, there is no such class. But when we specify extension class name and there is no such class then reports an error.
@Chris D, the bug report can be closed, in fact if the add-ons extend other functions related to the add-ons and there is no need to be an add-on then it will not work and the user won't be able to use it.
Yes you kind of beat us to it, we were going to close it.
We changed it at some point during development of 2.0 intentionally as attempting to validate the base class can be somewhat tricky or dangerous.
There are definitely use cases at least where an add-on may extend another but the user may not have that add-on installed or you may need to support multiple XF versions where there might be entirely different classes across different versions.