Fix warning 4 in sexp-derived code

src/lib/merkle_mask/masking_merkle_tree.ml

@@ -81,6 +81,15 @@ module Make (Inputs : Inputs_intf.S) = struct
           *)
     }
 
+  (* Manual definitions of sexp are needed to avoid warning 4 in autogenerated code *)
+  type unhashed_accounts_t = Account.t option * Location.t
+
+  let sexp_of_unhashed_accounts_t =
+    Tuple2.sexp_of_t (Option.sexp_of_t Account.sexp_of_t) Location.sexp_of_t
+
+  let unhashed_accounts_t_of_sexp =
+    Tuple2.t_of_sexp (Option.t_of_sexp Account.t_of_sexp) Location.t_of_sexp
+
   type t =
     { uuid : Uuid.Stable.V1.t
     ; mutable parent : Parent.t
@@ -93,7 +102,7 @@ module Make (Inputs : Inputs_intf.S) = struct
              This is used as a lookup cache. *)
     ; mutable accumulated : (accumulated_t[@sexp.opaque]) option
     ; mutable is_committing : bool
-    ; mutable unhashed_accounts : (Account.t option * Location.t) list
+    ; mutable unhashed_accounts : unhashed_accounts_t list
     }
   [@@deriving sexp]