“It is de fide that the sin of heresy alone, committed as an external, public act severs the person from the body of the Church, without any Church judgment or any canonical censure, and would have the effect of severing the heretic even if there were no canonical penalty attached to the sin.”
Kramer, Paul. To Deceive the Elect: The Catholic Doctrine on the Question of a Heretical Pope. Kindle Edition.
To hold the contradictory proposition is to hold a heretical doctrine.