Compiler crash in GenBCode referring to a class that extends an annotation

[smarter]

scala/bug#9393 also affects Dotty. The corresponding PR in scalac was scala/scala#4638

[odersky]

@allanrenucci Let's discuss at the next Dotty meeting how we should handle porting backend issues.

[smarter]

Note that as discussed in the linked scalac PR, while the fix for scalac was done in the backend it might be more appropriate to fix it in typer.