system
A chat on subreddit /r/me_irl
human
I mean its a bit complicated but you can say that
gpt
&gt;[...]morgenvorgestern

no you can't. (x)Über-morgen und (x)Vor-gestern are recognized and used, it also works for any use of previous (x)Vor-letzte; and next (x)Über-nächste

But using morgen und gestern in the same word is not a thing. 

German is complicated enough. No need to spread false information about it.