type 'par t = 'par

let ident v = v (* here adding : 'a . 'a a -> 'a changes nothing *)

class alias =
object
  method alias : 'a . 'a t -> 'a = ident (* NOT OK *)
end

(* but *)
let alias : 'a . 'a t -> 'a = ident (* OK *)
type r = { alias : 'a . 'a t -> 'a}
let alias = { alias = ident } (* OK *)