[meta] Ensure compatibility of Drupal 8/9 with PHP 8.0 (as it evolves)

Problem/Motivation

Make sure Drupal keeps being compatible with PHP 8. Work with dependencies as needed. See #3086374: Make Drupal 8 & 9 compatible with PHP 7.4 for how we did it with PHP 7.4. PHP 8.0 is feature frozen and beta4 is out, see https://www.php.net/archive/2020.php#2020-09-17-1

Proposed resolution

The following task list has been determined by testing on #3156595: Make Drupal 9 installable on PHP8.

1. Dependency updates to support PHP 8

To produce a list of dependencies that are blocking PHP 8 support, you can do the following. Make sure you use PHP 7.4+ to run it.

$ git clone --branch 9.1.x https://git.drupalcode.org/project/drupal.git whynotphp8
$ cd whynotphp8/
$ composer install
$ composer update
$ composer why-not php:8

Additionally to the composer reported dependencies above:

• behat/mink-selenium2-driver - https://github.com/minkphp/MinkSelenium2Driver/pull/320

2. Drupal issues that can be addressed in Drupal 8 & 9 today

These issues can proceed without worrying about the how and if vendor support works for PHP 8 since the changes are all PHP 7 compatible. We can keep the changes PHP 7.0 compatible to make things easy for Drupal 8.

• #3156542: \ReflectionParamter::getClass() is deprecated in PHP 8.0
• #3174662: Encapsulate \PDOStatement instead of extending from it or #3156881: Various methods of \Drupal\Core\Database\StatementInterface have changed their signature for PHP 8

3. Drupal issues that need a largely working PHP 8 build to test

• #3156887: \Drupal\system\Plugin\ImageToolkit\GDToolkit needs to support \GdImage objects for PHP 8 compatibility
  
  Should be okay for PHP 7.0 because it's really only loosening what we expect in \Drupal\system\Plugin\ImageToolkit\GDToolkit::$resource but testing this will be harder without all the above fixes and the fixes to vendor.

DONE

• In/with dependencies:
  • symfony/*, twig/twig, composer/* #3156558: Upgrade dependencies prior to 9.1.0 and #3157296: Upgrade dependencies prior to 9.1.0
  • Update symfony contract php requirement - https://github.com/symfony/symfony/pull/37494
  • phpspec/prophecy - https://github.com/phpspec/prophecy/pull/470
  • phpdocumentor/reflection-docblock - https://github.com/phpDocumentor/ReflectionDocBlock/pull/240
  • Symfony CMF #3176576: Update to Symfony CMF 2.3.3
  • #3127141: Use PHPUnit 9 for PHP 7.4+, while keeping support for PHPUnit 8.4 in PHP 7.3
• Within Drupal itself:
  • #3156944: Support nullable typehints and return types in ProxyBuilder and as a result support PHP 8
  • #3156878: \Drupal\Component\Datetime\DateTimePlus should pass correct parameter types to checkdate()
  • #3156885: Change \Drupal\error_test\Controller\ErrorTestController::generateWarnings() to throw E_NOTICE error compatible with PHP 8
  • #3156879: \Drupal\Component\Utility\Bytes::toInt() - ensure $size is a number type
  • #3156882: \Drupal\Core\Render\Element\StatusReport::preRenderGroupRequirements() and \Drupal\user\PermissionHandler::sortPermissions() sorts return bools
  • #3156883: \Drupal\Core\Url ensure fragment is not an empty string
  • #3170648: CKEditorPluginManager::getEnabledButtons throws warnings on PHP 8.0.0 beta3
  • #3173958: PHP 8 error: method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given
  • #3173888: Error handler signature changed in PHP 8
  • #3174038: ZipArchive::open(): Using empty file as ZipArchive is deprecated in PHP 8
  • #3173991: Incorrect passby reference use in anonymous functions
  • #3174158: Division by zero is not warning since PHP 8.0 beta4
  • #3174022: call_user_func_array() and named arguments in PHP 8

Remaining tasks

See above.

User interface changes

None.

API changes

Data model changes

None.

Release notes snippet