(*I'm not sure which behavior is better.  But if the change is intentional, it should be documented (I don't think it is), and it'd be nice if there were a flag for this, or if -compat 8.4 restored the old behavior.*)
Definition f (v : option nat) := match v with
| Some k => Some k
| None => None
end.
Axioms F G : (option nat -> option nat) -> Prop.

Axiom FG : forall f, f None = None -> F f = G f.