DefaultConfigTest.php

<?php

namespace Drupal\Tests\Core\Extension;

use Drupal\Tests\UnitTestCase;
use Symfony\Component\Yaml\Yaml;

/**
 * Tests default configuration of the Extension system.
 *
 * @group Extension
 */
class DefaultConfigTest extends UnitTestCase {
    
    /**
     * Tests that core.extension.yml is empty by default.
     *
     * The default configuration MUST NOT specify any extensions, because every
     * extension has to be installed in a regular way.
     *
     * Otherwise, the regular runtime application would operate with extensions
     * that were never installed. The default configuration of such extensions
     * would not exist. Installation hooks would never be executed.
     */
    public function testConfigIsEmpty() {
        $config = Yaml::parse(file_get_contents($this->root . '/core/config/install/core.extension.yml'));
        $expected = [
            'module' => [],
            'theme' => [],
            'profile' => '',
        ];
        $this->assertEquals($expected, $config);
    }

}