After namespacing the JFile and JFolder classes the administrator/components/com_joomlaupdate/restore_finalisation.php was attempted to be updated to reflect that change... “attempted” being the operative word. What it does and what it think it does are two different things :)

First, let's see what's broken.

Line 90 currently reads

if (!class_exists('File'))

but it should read

if (!class_exists('\Joomla\CMS\Filesystem\File'))

See the problem? You need to provide an FQN to class_exists. It won't do in-scope class resolution. The original class_exists('File') will always return false.

The other two problems on that file are of the exact same nature.

Line 131 currently reads

if (!class_exists('Folder'))

but it should read

if (!class_exists('\Joomla\CMS\Filesystem\Folder'))

Likewise for namespacting JText.

Line 174 reads

if (!class_exists('Text'))

but it should read

if (!class_exists('\Joomla\CMS\Language\Text'))

In normal operation of the updater over the web you will never experience it.

However, now that we have a CLI core updater it is possible that the respective File, Folder or Text class has already been loaded in the course of applying an update over CLI, e.g. when downloading the update file and/or displaying information to the user. As a result, when Joomla reaches the finalisation step it will fail with a PHP error about the class having already been defined. The consequence of that is that among other things leftover files are not deleted (which can be addressed from the CLI) and that the database schema is not updated. Okay, this is the important bit.

Within the 4.0 release you won't notice any major issues. However, as soon as you change anything substantial in the database you might end up with a prepared statement failing before you have the chance of logging in, preventing you from visiting your site's backend to fix the schema. Remember that there's no way to do that from CLI right now. That's a bit of a pickle to find yourselves in.

This actually did happen to me by adapting the Joomla 4 CLI updater to run under Joomla 3 so I could do an unattended update from Joomla 3.10 to Joomla 4.0.

So, while you cannot CURRENTLY reproduce a fatal error using just core code it's only a matter of time before you do. Please consider this an advance warning of a very nasty bug ready to strike at an unsuspecting time and a great opportunity for fixing what is in retrospect an obvious bug that went undetected for three years. The fix is literally three lines of code. If you want me to, I can send a PR as well.