Forbid combinations enum+attribute and enum+optional

[lars-reimann]

Another suggestion:
I think we should rethink allowing optional and enum annotations on the same parameter.
If we were to allow it, there needs to be a way to use a value from the enum inside the optional annotation and optional annotations need additional checking whether or not it matches the enum.
As it is currently implemented it's possible to have optional values, that are not part of the assigned enum.

Originally posted by @paul0314 in #386 (comment)