Are you seriously asking what magisterial documents declare non-Catholics to be non-Catholic?It's essentially a question about mentevacantism.
No human can read the heart of any man. We cannot know what's in anyone's heart.
On the other hand, we can read the actions and statements of anyone who does or says something. If a man declares that he has Catholic Faith and his actions do not contradict that declaration, we must accept that person as a Catholic (assuming, of course, he has been baptized).
If, however, a man declares a faith contrary to the Catholic Faith and his actions (i.e., his teaching, his work, etc.) confirm that his faith is contrary to the Catholic Faith, then we must accept that person as an heretic or apostate (if he has been baptized).
Bishop Williamson has posited a theory that a pope may be Catholic but not actually confirm the faith in words or deeds and, in fact, work against the faith in words and deeds because of...confusion. (Please correct me if I have that wrong, but that is my understanding from reading his newsletter.) As far as I have ever read, Bishop Williamson has never supported that supposition with any evidence or with any authorities other than his own.
But such a theory is condemned by the history of the Church in crisis after crisis as heresies and heretics have been condemned for 19 centuries. Only in the twentieth Century does it seem that bishops (including the "Bishop of Rome") stopped condemning heresies or heretics.
In hindsight, it seems that the Catholic world was united against Arius or Nestorius. And, actually, it was. But the during the heydays of Arius and Nestorius the people who said they were Catholics but supported the heresies of these heresiarchs outnumbered the faithful Catholics. Bishop Williamson and people who follow his theories simply don't think such a thing could happen in our own day--yet it has.
You want a magisterial document? How about the Gospel of St. Matthew 12:30 and the Gospel of St. Luke 11:23.