[Caml-list] confusing type error when using function with optional arguments

["Milan Stanojević" <milanst@gmail.com>]

I was playing with %revapply primitive and ran into the following
compiler behavior that I don't understand.

external (|!) : 'a -> ('a -> 'b) -> 'b = "%revapply"

let foo1 ?x ?y () = ()    (* val foo1 : ?x:'a -> ?y:'b -> unit -> unit
*)
let foo2 ?x ?y z = z     (* val foo2 : ?x:'a -> ?y:'b -> 'c -> 'c  *)

let x = () |! foo1   (* compiles just fine *)

let y = () |! foo2   (* fails with
                             Error: This expression has type ?x:'a ->
?y:'b -> 'c -> 'c
                             but an expression was expected of type
unit -> 'd   *)

I don't understand why foo1 is fine and foo2 isn't. I would have
thought that I can use foo2 wherever I can foo1 since it has a
strictly more general type.
Am I missing something obvious here?

%revapply is not important here I just wanted to include context. I
get the same behavior in 3.12.1 and 4.00.1