How to show a menu link that is hidden because the role has no permission to see the linked page?
In your case you can use two menu entries in the same menu:


One pointing to /node/add/article.
Another one pointing to /user/login. 


First link is only visible to registered users, because anonymous users don't have permissions to add articles.

Second link is only visible to anonymous users because registered users don't have permissions to see the user login page (because they are already logged in).

The result is a menu with only one menu entry. Registered users see first link, anonymous see only second one.