Background

I have a custom component in my LiveView, which is basically a group of checkboxes.
I want to add a new attribute to my custom component, however no matter what I do, I always get nil when accessing said attribute.

Code

core_components.ex

  @doc """
  Generate a checkbox group for multi-select.

  ## Examples

    <.checkgroup
      field={@form[:genres]}
      label="Genres"
      options={[{"Fantasy", "fantasy"}, {"Science Fiction", "sci-fi"}]}
      selected={[{"Fantasy", "fantasy"}]}
    />

  """
  attr :id, :any
  attr :name, :any
  attr :label, :string, default: nil
  attr :field, Phoenix.HTML.FormField, doc: "a form field struct retrieved from the form, for example: @form[:genres]"
  attr :errors, :list
  attr :required, :boolean, default: false
  attr :rest, :global, include: ~w(form readonly)
  attr :class, :string, default: nil
  attr :options, :list, default: [], doc: "the options to pass to Phoenix.HTML.Form.options_for_select/2"
  attr :selected, :list, default: [], doc: "the currently selected options, to know which boxes are checked"

  attr :meow, :any, default: "meow", doc: "a very cool new attribute!"

  def checkgroup(assigns) do
    new_assigns =
      assigns
      |> assign(:multiple, true)
      |> assign(:type, "checkgroup")

    input(new_assigns)
  end


  def input(%{type: "checkgroup"} = assigns) do
    ~H"""
    <p> <%= "Cat says: #{@meow}" %></p> 

    <div class="mt-2">
      <%= for opt <- @options do %>
        <div class="relative flex gap-x-3">
          <div class="flex h-6 items-center">
            <input id={opt.id} name={@name} type="checkbox" value={opt.id} checked={opt in @selected} disabled={false} />
          </div>
          <div class="text-sm leading-6">
            <label  for={opt.id} ><%= opt.name %></label>
          </div>
        </div>

      <% end %>
    </div>
    """
  end

Here I am using a customer component to render a stylish checkbox group. It works fine, except for the :meow attribute, which somehow is always nil when inside the input function.

Error

Using this code as a sample:

<.checkgroup field={@form[:my_form]} label="SUper question!" options={@sounds} selected={@selected_sounds} meow={[1, 2]} required />

I get the following error upon rendering this code:

** (exit) an exception was raised:
    ** (KeyError) key :meow not found in: %{
  __changed__: nil,
  __given__: %{
    __changed__: nil,
    __given__: %{
      __changed__: nil,
      __given__: %{
        __changed__: nil,
        field: %Phoenix.HTML.FormField{
          id: "my_form",
          name: "my_form",
          errors: [],
          field: :my_form,
          form: %Phoenix.HTML.Form{
            source: %{"some_command" => []},
            impl: Phoenix.HTML.FormData.Map,
            id: nil,
            name: nil,
            data: %{},
            hidden: [],
            params: %{"some_command" => []},
            errors: [],
            options: [],
            index: nil,
            action: nil
          },
          value: nil
        },
        label: "SUper question!",
        meow: [1, 2],
        options: [1, 2, 3, 4],
        # .... more things follow

As you can see, meow: [1, 2], is clearly present. Yet, @meow returns nil and using assigns.meow outright crashes the application.

What am I doing wrong here and how can I fix it?