OCaml bug: you might think that deriving {hash, eq} is enough to use a custom record in a hashtable. Think again!

dune

(library
 (name ocaml_bug)
 (inline_tests)
 (preprocess
  (pps ppx_inline_test ppx_deriving.eq ppx_deriving_hash)))

lib.ml

module Oracle (SomeModule : Hashtbl.HashedType) = struct
  let counter = ref 0

  module Hashtbl = Hashtbl.Make (SomeModule)

  let memory : int Hashtbl.t = Hashtbl.create 5

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

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

  let to_bytes block =
    let module Temp = Oracle (struct
      type nonrec t = t

      let hash = hash

      let equal = equal
    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

package.json

{
    "dependencies": {
        "ocaml": "~4.13.1",
        "@opam/dune": "*",
        "@opam/ppx_inline_test": "*",
        "@opam/ppx_deriving": "*",
        "@opam/ppx_deriving_hash": "*"
    },
    "devDependencies": {
        "@opam/merlin": "*"
    }
}