I see, I had a wrong interpretation of ?Sized. So it removes any Sized bound relationship from the type parameter.
I have slightly edited my question, and now I see I saw ?Trait as does-not-implements Trait. edit: actually: never-implements-Trait.
I see. Actually, your reply made me see that I saw !Trait as forbid-Trait instead - so I slightly edited my question to better convey that - so it would be "positively implemented", as you indicated.