Arity analysis confused by abbreviation

[mtzguido]

type comparator_for (t:Type) = t -> t -> bool

val univ_eq : comparator_for int
let rec univ_eq (u1 u2 : int) : bool = false

This could be accepted, but F* complains that "Recursive functions must be introduced at arrow types", I believe due to taking bool as the annotation and ignoring the two binders. Dropping the annotations makes it work.