Frink The code I've fixed is class_exists('Tags'). Flarum\Tags\Tag is imported above, but it doesn't appear class_exists with just a class name as string will correctly resolve the namespace (it would check for the class at the root or in the current namespace I guess). In my PR I use Tag::class to pass the full class name including namespace to class_exists, which does seem to be the way to do it (alternatively class_exists could be called with the full class name and namespace as a string, either solution would have the same effect)
