Protocol @fallback_to_any true not working as expected: making methods optional

Hello,

I want to have a protocol with 3 methods with 2 optional having the following logic:

defprotocol MyRepo.Render do
  @fallback_to_any true
  def to_string(data)
  def to_index_string(data)
  def to_show_string(data)
end

# some custom implementations

defimpl MyRepo.Render, for: Any do
  def to_index_string(data), do: MyRepo.Render.to_string(data)
  def to_show_string(data), do: MyRepo.Render.to_string(data)
  def to_string(data), do: MyRepo.Render.to_index_string(data) || "#{inspect data}"
end

So that I can have for some data structures:

  defimpl MyApp.Render, for: __MODULE__ do
    def to_index_string(data) do
      "get something"
    end

    def to_show_string(booklet_1) do
      "get something else"
    end
  end

And for others:

  defimpl MyApp.Render, for: __MODULE__ do
    def to_string(data) do
      "get something general"
    end
  end

It seems like for modules with no def impl is present, it works as expected, but when I define either to_index_string and to_show_string or to_string, the compiler complains that the other methods are missing:

** (UndefinedFunctionError) function MyRepo.Render.MyApp.MyDataStruct.to_show_string/1 is undefined or private, but the behaviour MyRepo.Render expects it to be present. Did you mean one of:

      * to_string/1

    (vae) MyRepo.Render.MyApp.MyDataStruct.to_show_string(%MyApp.MyDataStruct{})

How can I achieve what I want?

Thanks