(* -*- tuareg -*- *)

module Let_syntax = struct
  type 'a t = T of 'a
  let map (T x) ~f = T (f x)
  let both (T x) (T y) = T (x, y)
  module Open_on_rhs = struct
    let return x = T x
    let f x ~(doc:string) = T (x, doc)
  end
end

let _ = [%map_open let x = return 42 and y = f 42 in ()]
[%%expect{|
Line _, characters 45-49:
Error: This expression has type doc:string -> (int * string) Let_syntax.t
       but an expression was expected of type 'a Let_syntax.t
|}]