The Bull Cum Ex Apostolatus Officio provides that a heretic cannot become Pope. Some hold that this Bull is not infallible. However, even if it is not infallible it repeats the doctrine of the Ordinary Magisterium of the Church, which is infallible.
The Catholic Encyclopedia under Papal Elections states: “Of course, the election of a heretic, schismatic, or female would be null and void.” Under Conclave it states: “Strictly speaking, any male Christian who has reached the use of reason can be chosen — not, however, a heretic, a schismatic, or a notorious simonist.”