partial_application_fail.ml

let smth ?(m=5) ?(n=5) ?(o=5) ~p = m+n+o+p

let with_z_instead : z:int -> (?m:int -> ?n:int -> 'a) -> 'a = fun ~z fn ->
  fn ~m:z ~n:z

let _ =
  let first = smth ~m:1 ~n:2 (* no ~o *) ~p:3 in
  let second = with_z_instead ~z:4 smth (* no ~o *) ~p:123 in
  (first, second)

(*
494 |   let second = with_z_instead ~z:4 smth (* no ~o *) ~p:123 in
                                         ^^^^
Error: This expression has type ?m:int -> ?n:int -> ?o:int -> p:int -> int
       but an expression was expected of type ?m:int -> ?n:int -> p:'a -> 'b
       Type ?o:int -> p:int -> int is not compatible with type p:'a -> 'b
*)