Guardian Authorization & Dializer

Hi,

I followed some tutorials and read Hex documentation on Guardian authorization but i get a warning on a basic implementation :

The @spec for the function does not match the success typing of the function.

Function:
CTAPI.Guardian.available_permissions/0

Success typing:
@spec available_permissions() :: %{:default => [:read_users | :write_users, ...]}

Here is my implementation :

defmodule CTAPI.Guardian do
  @moduledoc """
  Integration with Guardian
  """
  use Guardian, otp_app: :ct_api

  use Guardian.Permissions, encoding: Guardian.Permissions.BitwiseEncoding # <- warning on this line

  def subject_for_token(%{id: id}, _claims) do
    {:ok, to_string(id)}
  end

  def subject_for_token(_, _) do
    {:error, :no_resource_id}
  end

  def resource_from_claims(%{"sub" => sub}) do
    {:ok, CTAPI.Account.get_user!(sub)}
  end

  def resource_from_claims(_claims) do
    {:error, :no_claims_sub}
  end

  def build_claims(claims, _resource, opts) do
    claims =
      claims
      |> encode_permissions_into_claims!(Keyword.get(opts, :permissions))

    {:ok, claims}
  end
end

Erlang/OTP 23 [erts-11.0.2] [source] [64-bit] [smp:6:6] [ds:6:6:10] [async-threads:1] [hipe] [dtrace]
Elixir 1.10.3 (compiled with Erlang/OTP 22)

Can someone at least tell me if i have to fork and fix Gardian for this version of elixir or if i can fix my code please ?

Best regards,