Opening an issue because currently this is just being ignored.

This issue is not about DEPRECATIONS... enough traction is already being made on that.

This issue is about PHPUnit 8 not being reliable for the future and some investment work is needed to move to PHPUnit 9 asap.

Every single build fails (silently) because the unit tests for PHP 8.1 fail.

This is because PHP unit 8 doesn't support PHP 8/8.1 Union Types (Expected behaviour)

PHP 8.1.x is the current PHP version. All other prior versions are now "old, dead dying". PHP 8.0 only has 11 months of life left.

Joomla needs to migrate to PHPUnit 9 and refactor any failing tests to ensure that the test suite passes again.

Simply upgrading the PHPUnit to PHPUnit 9 is NOT ENOUGH.

At the moment JOOMLA IS NOT RELYING ON UNIT TESTS ON PHP 8.1 as they are just failing. But this is an untenable position to be in. This needs fixing the sooner the better.

Yes I know there is only limited test coverage anyway, but some tests are better than no tests.

PHPUnit 9 requires PHP 7.3 (or newer), Joomla 4 requires PHP 7.2 - OR NEWER, without tests passing on PHP 8.1 Joomla cannot categorically state that Joomla runs on PHP 8.1

https://thephp.cc/articles/migrating-to-phpunit-9