to_radio_button_tag ignores :index options partially

When the form_builder method #radio_button is given the :index option it gets ignored for the id of the radio button. The :index option is necessary when using nested forms with new and existing attributes.

The consequence is that you get html elements with duplicate id's and labels won't work with those elements because they do get the index.

The cause is visible in the #to_radio_button:

      def to_radio_button_tag(tag_value, options = {})
        options = DEFAULT_RADIO_OPTIONS.merge(options.stringify_keys)
        options["type"]     = "radio"
        options["value"]    = tag_value
        if options.has_key?("checked")
          cv = options.delete "checked"
          checked = cv == true || cv == "checked"
        else
          checked = self.class.radio_button_checked?(value(object), tag_value)
        end
        options["checked"]  = "checked" if checked
        pretty_tag_value    = tag_value.to_s.gsub(/\s/, "_").gsub(/\W/, "").downcase
        options["id"]     ||= defined?(@auto_index) ?                       ## <--- PROBLEM IS IN HERE!
          "#{tag_id_with_index(@auto_index)}_#{pretty_tag_value}" :
          "#{tag_id}_#{pretty_tag_value}"
        add_default_name_and_id(options)
        tag("input", options)
      end

Normally the index option is processed in #add_default_name_and_id but because :id already has been set this is not working.

I dont understand the logic and when I have time I will try to see if I can find it from the additional tests.