type foo = [`Foo]
[@@deriving show]

type foobar = [ foo | `Bar ]

module M = struct
  let do_stuff_with_foo (param: foo) =
    Printf.printf "foo: %s"  (show_foo param);
    param

  let do_stuff (param: [> foo] as 'res): 'res =
    let res: 'res =
      match param with
      | `Foo as f   -> begin
          let foo' = do_stuff_with_foo f in
          (foo' :> 'res)
        end
      | _ as other  -> (other :> 'res)
    in
    res

end

let main () =
  let bar = `Bar in
  M.do_stuff bar