defoverridable_bug.exs

defmodule Bug.Default do
  defmacro __using__(_) do
    quote do
      @impl Bug.Server
      def one(), do: "Fallback"
      def two(), do: "Fallback"

      defoverridable Bug.Server
    end
  end
end

defmodule Bug.Server do
  @callback one() :: String.t()
  @callback two() :: String.t()

  defmacro __using__(_) do
    quote do
      @behaviour unquote(__MODULE__)

      use Bug.Default
    end
  end
end

defmodule Bug.Plugin do
  defmacro __using__(_) do
    quote do
      @impl Bug.Server
      def one() do
        super() <> " + Plugin"
      end

      defoverridable Bug.Server
    end
  end
end

defmodule MyApp do
  use Bug.Server
  use Bug.Plugin

  def one() do
    super() <> " <- MyApp"
  end
end