Support having a *default* country for an *optional* address field

I tested this and it works properly. Maybe you can set the fields (name, city, street) as required so you'll always have a legit address. Can you be more specific on how you intend to use this? ( why allow the user to save an address with just the country filled)

I more or less had the same use case too. I solved it by creating my own widget, where I override the form element and massage form values to only save the address field when postal code and locality had a value.

I also override the field_overrides for delta 0 and a specific field to make that one required. If you don't need that, you can leave out that code of course.

namespace Drupal\project\Plugin\Field\FieldWidget;

use Drupal\address\Plugin\Field\FieldWidget\AddressDefaultWidget;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'address_default' widget.
 *
 * @FieldWidget(
 *   id = "project_address_default_country",
 *   label = @Translation("Default country - optional fields"),
 *   field_types = {
 *     "address"
 *   },
 * )
 */
class AddressDefaultCountryWidget extends AddressDefaultWidget {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);

    if (empty($element['address']['#default_value']['country_code'])) {
      $element['address']['#default_value']['country_code'] = 'BE';
    }

    if ($delta === 0 && $items->getFieldDefinition()->getName() == 'field_zone_location') {
      $field_overrides = $element['address']['#field_overrides'];
      $field_overrides['postalCode'] = 'required';
      $field_overrides['locality'] = 'required';
      $element['address']['#required'] = TRUE;
      $element['address']['#field_overrides'] = $field_overrides;
    }

    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {

    if ($this->isDefaultValueWidget($form_state)) {
      return parent::massageFormValues($values, $form, $form_state);
    }

    $new_values = [];
    $delta_custom = 0;
    foreach ($values as $value) {
      if (!empty($value['address']['locality']) && !empty($value['address']['postal_code'])) {
        $new_values[$delta_custom] = $value['address'];
        $delta_custom++;
      }
    }
    return $new_values;
  }

}

This is something I need a solution for as well. I'd like to have the field not be required, but also not make my fully domestic audience select the country when they want to add an address. If I select the country to default to United States, then the rest of the address field becomes required.

A custom solution like #6 is the only way.

We can't preselect the country without making the field required. We tried many hacks in the early addressfield days but nothing truly worked for everyone.