dune

(library
 (name ocaml_bug)
 (inline_tests)
 (preprocess
  (pps ppx_inline_test)))

lib.ml

module Oracle (SomeModule : Map.OrderedType) = struct
  let counter = ref 0

  module Map = Map.Make (SomeModule)

  let memory : int Map.t ref = ref Map.empty

  let to_bytes (t : SomeModule.t) : string =
    match Map.find_opt t !memory with
    | Some d -> string_of_int d
    | None ->
        incr counter;
        memory := Map.add t !counter !memory;
        string_of_int !counter
end

module A = struct
  type t = { a : int; b : int } [@@deriving compare]

  let to_bytes block =
    let module Temp = Oracle (struct
      type nonrec t = t
      let compare = compare
    end) in
    Temp.to_bytes block

  let%test_unit "to_bytes" =
    let block = { a = 1; b = 2 } in
    let b1 = to_bytes block in
    let b2 = to_bytes block in
    assert (b1 = "1");
    assert (b1 = b2);
    let other_block = { block with b = 3 } in
    let b3 = to_bytes other_block in
    assert (b3 = "2")
end