From Cum Ex Apostolatus Officio, Pope Paul IV, 1559...
In addition, [by this Our Constitution, which is to remain valid in perpetuity We enact, determine, decree and define:] that if ever at any time it shall appear that any Bishop, even if he be acting as an Archbishop, Patriarch or Primate; or any Cardinal of the aforesaid Roman Church, or, as has already been mentioned, any legate, or even the Roman Pontiff, prior to his promotion or his elevation as Cardinal or Roman Pontiff, has deviated from the Catholic Faith or fallen into some heresy: the promotion or elevation, even if it shall have been uncontested and by the unanimous assent of all the Cardinals, shall be null, void and worthless; it shall not be possible for it to acquire validity (nor for it to be said that it has thus acquired validity) through the acceptance of the office, of consecration, of subsequent authority, nor through possession of administration, nor through the putative enthronement of a Roman Pontiff, or Veneration, or obedience accorded to such by all, nor through the lapse of any period of time in the foregoing situation.
And yet, Cum Ex assumes a trial.
"(vii) if perchance they shall have been Judges, their judgements shall have no force, nor shall any cases be brought to their hearing.;"
So, an HEARING is presumed in Cum Ex. That means, this declaration is only a guideline for determining within an hearing that someone proven guilty of heresy, Pope included, must be treated as written. But an hearing is presumed.
Bellarmine’s thinking regarding this matter is perfectly consistent with the mind of the Church, as we see expressed in Canon 10 of the Fourth Council of Constantinople. In response to the schism of Photius, the Council attached the grave penalty of excommunication to any layman or monk who, in the future, separated himself from his patriarch (the Pope is Patriarch of the West) before a careful inquiry and judgment by a synod.
“As divine scripture clearly proclaims, ‘Do not find fault before you investigate, and understand first and then find fault’. And does our law judge a person without first giving him a hearing and learning what he does? Consequently this holy and UNIVERSAL SYNOD justly and fittingly declares and lays down that no lay person or monk or cleric should separate himself from communion with his own patriarch before a careful inquiry and judgment in synod, even if he alleges that he knows of some crime perpetrated by his patriarch, and he must not refuse to include his patriarch's name during the divine mysteries or offices. (…) If anyone shall be found defying this holy synod, he is to be debarred from all priestly functions and status if he is a bishop or cleric; if a monk or lay person, he must be excluded from all communion and meetings of the church [i.e. excommunicated] until he is converted by repentance and reconciled”.