Issue #3074595 by Lendude, ravi.shankar, sime, jian he, Kristen Pol, longwave:...

      }

    }

    elseif (!empty($this->value) && ($this->operator == 'in' || $this->operator == 'not in')) {

      $errors[] = $this->t('The value @value is not an array for @operator on filter: @filter', ['@value' => var_export($this->value), '@operator' => $this->operator, '@filter' => $this->adminLabel(TRUE)]);

      $errors[] = $this->t('The value @value is not an array for @operator on filter: @filter', ['@value' => var_export($this->value, TRUE), '@operator' => $this->operator, '@filter' => $this->adminLabel(TRUE)]);

    }

    return $errors;

  }

<?php

namespace Drupal\Tests\views\Unit\Plugin\filter;

use Drupal\Tests\UnitTestCase;

use Drupal\views\Plugin\views\filter\InOperator;

/**

 * @coversDefaultClass \Drupal\views\Plugin\views\filter\InOperator

 * @group views

 */

class InOperatorTest extends UnitTestCase {

  /**

   * @covers ::validate

   */

  public function testValidate() {

    $definition = [

      'title' => 'Is InOperator Test',

      'group' => 'Test',

      'options callback' => '\Drupal\Tests\views\Unit\Plugin\filter\InOperatorTest::validate_options_callback',

    ];

    $filter = new InOperator([], 'in_operator', $definition);

    $filter->value = 'string';

    $filter->operator = 'in';

    $translation_stub = $this->getStringTranslationStub();

    $filter->setStringTranslation($translation_stub);

    $errors = $filter->validate();

    $this->assertSame('The value 'string' is not an array for in on filter: ' . $filter->adminLabel(TRUE), (string) $errors[0]);

  }

  /**

   * @return array

   */

  public static function validate_options_callback() {

    return ['Yes', 'No'];

  }

}