Yes, it is possible to use class constants. Otherwise Joomla\CMS\Version::MAJOR_VERSION would fail, and that doesn't.

As is, this PR is invalid because there isn't actually a bug with the import of constants, plus you are defining these constants at the namespace level and not in the class. What is the specific bug (with error message(s) if available) you are seeing?

not when you are trying to get the value with constant() function

The error is that even if my joomla installation is on "stable" it finds updates that are in "dev" mode

To get a class constant you have to pass the fully qualified class name to the constant() function.

// Works
$major = constant('Joomla\\CMS\\Version::MAJOR_VERSION');

// Does not work, will give an undefined constant message
$major = constant('MAJOR_VERSION');

It does not on my PHP version 5.6

protected function stabilityTagToInteger($tag)
	{
		$constant = '\\Joomla\\CMS\\Update\\Updater::STABILITY_' . strtoupper($tag);

		if (defined($constant))
		{
			return constant($constant);
		}

		return Updater::STABILITY_STABLE;
	}

I have tested this item ✅ successfully on 24ec81b

Thanks at @johansundell, @csthomas too for this fix,
we had to remove multiple update tags (each of different stability level) because of the bug
i was going to look into this, but did not get time for it

I have tested this item ✅ successfully on 24ec81b

RTC after two successful tests.