Assigns is not assigning attribute in custom core component

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After a lot of digging around, i realized that all of the input functions incore_components.ex modify assigns. The best example of this is this intermediary function, called before any input component functions:

  def input(%{field: %Phoenix.HTML.FormField{} = field} = assigns) do
    assigns
    |> assign(field: nil, id: assigns.id || field.id)
    |> assign(:errors, Enum.map(field.errors, &translate_error(&1)))
    |> assign_new(:name, fn -> if assigns.multiple, do: field.name <> "[]", else: field.name end)
    |> assign_new(:value, fn -> field.value end)
    |> input()
  end

This means I do not fully control what is going on. I decided to move away from using/modifying the input functions and created my own component from scratch:

@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 :name, :any

  attr :field, Phoenix.HTML.FormField,
    doc: "a form field struct retrieved from the form, for example: @form[:genres]"

  attr :required, :boolean, default: false

  attr :options, :list,
    default: [],
    doc: "the options to pass to Phoenix.HTML.Form.options_for_select/2"

  attr :disabled, :list, default: [], doc: "the list of options that are disabled"

  attr :selected, :list,
    default: [],
    doc: "the currently selected options, to know which boxes are checked"

  def checkgroup(%{field: %Phoenix.HTML.FormField{} = field} = assigns) do
    assigns =
      assigns
      |> assign(:name, "#{field.name}[]")

    ~H"""
    <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}
              class={
                if opt in @disabled do
                  "h-4 w-4 rounded border-gray-300 text-gray-300 focus:ring-indigo-600"
                else
                  "h-4 w-4 rounded border-gray-300 text-indigo-600 focus:ring-indigo-600"
                end
              }
              checked={opt in @selected}
              disabled={opt in @disabled}
            />
          </div>
          <div class="text-sm leading-6">
            <label
              for={opt.id}
              class={
                if opt in @disabled do
                  "text-base font-semibold text-gray-300"
                else
                  "text-base font-semibold text-gray-900"
                end
              }
            >
              <%= opt.name %>
            </label>
          </div>
        </div>
      <% end %>
    </div>
    """
  end

which works as expected and all the assigns not have the expected values.

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