[Vala] Invalid cast Warnings

[Daniel Brendle <grindhold skarphed org>]

Hi, fellow valaists.

At the moment I deal with a series of warnings like these:

(process:3325): GLib-GObject-WARNING **: invalid cast from 'OParlSystem'
to 'System'

I have a library that exposes a set of classes. I extend each of those
classes in the application that uses said library. When I fetch a new
Object of one of those library-classes, I downcast it to the matching
class of the application layer code.

I compiled a minimum example to trigger the issue here:

https://0bin.net/paste/J0sgx2b2NJp6lTj8#QBfz-GUhWagsfGsF7nUqEn/8XneAiL9NhYNIdo8GRt6

Note that the deriving application-layer class System is entirely empty
and only inherits from the library-level class.

Why is this considered an invalid cast?

Thanks in advance,

    Grindhold