type _ t =
    IntLit : int -> int list t
  | BoolLit : bool -> bool list t;;

let eval = fun (type a) ~(u:a t) ->
  let result : a =
    match u with
      IntLit x -> [x]
    | BoolLit x -> [x] in
  List.map (fun x -> x, x) result


File "gadt.ml", line 10, characters 27-33:
Error: This expression has type a but an expression was expected of type
         'a list