select2.module

<?php

/**
 * Implements hook_form_views_exposed_form_alter().
 *
 * Integrates select2 with autocomplete widgets from entity_views module.
 */
function select2_form_views_exposed_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  /** @var \Drupal\views\ViewExecutable $view */
  $view = $form_state->get('view');

  foreach ($view->filter as $filter) {
    if ($filter->isExposed() && $filter->getPluginId() == 'entity_views_id') {
      $element_name = $filter->exposedInfo()['value'];

      // Retrieve autocomplete service from controller, and use it to create the
      // list of default values with labels. We cannot simply use the service,
      // because it may have been overridden in a child controller.
      /** @var \Symfony\Component\Routing\Route $route */
      $route = \Drupal::service('router.route_provider')->getRouteByName(
        $form[$element_name]['#autocomplete_route_name'],
        $form[$element_name]['#autocomplete_route_parameters']
      );
      $controller = Drupal::service('controller_resolver')
        ->getControllerFromDefinition($route->getDefault('_controller'));
      if (!empty($controller[0]) && $controller[0] instanceof \Drupal\entity_views\Controller\EntityAutocompleteController) {
        $autocompleteService = $controller[0]->entityAutocomplete;
      }

      // Retrieve labels for submitted values so select2 can properly display
      // existing values.
      $default_labels = array();
      if ($autocompleteService && !empty($form_state->getUserInput()[$element_name])) {
        $values = \Drupal\Component\Utility\Tags::explode($form_state->getUserInput()[$element_name]);
        $entities = entity_load_multiple($filter->getEntityType(), $values);
        /** @var \Drupal\Core\Entity\EntityInterface $entity */
        foreach ($entities as $entity) {
          $entry = $autocompleteService->getMatchEntry($entity,
            $route->getDefault('value_callback'),
            $route->getDefault('label_callback')
          );
          $entry['text'] = $entry['label'];
          $entry['id'] = $entry['value'];
          unset($entry['label'], $entry['value']);
          $default_labels[] = $entry;
        }
      }

      $form[$element_name] = \Drupal\Component\Utility\NestedArray::mergeDeep($form[$element_name], array(
        '#attributes' => array(
          'data-select2-autocomplete-widget' => TRUE,
          'class' => array(
            'select2-widget',
          ),
          'data-default-value' => json_encode($default_labels),
        ),
        '#attached' => array(
          'library' => array(
            'select2/select2.widget',
          ),
        )
      ));
    }
  }
}