It just doesn't stop.  Sedevacantists hold that the teachings of the V2 Papal claimants are NOT Magisterium, and it is precisely due to the recognition that the Papal Magisterium cannot become corrupt, and that many R&R (including yourself) are the ones who claim that the Magisterium can become corrupt.  Of course, you have a bizarre definition of Magisterium where you exclude "false teaching" from being "Magisterium" and therefore the incorruption of the Magisterium becomes reduced to a tautology.  But no Catholic theologian has ever defined "Magisterium" as dependent upon subsequent examination by Stubborn et al.

You place way too high a value on your erroneous opinion.
You can "hold" the V2 papal claimants are NOT magisterium all you want, since popes never were Magisterium you're just proving #2 - but I'm the one with a bizarre definition.

 Do whatever ya gotta do for sedeism I guess. But you really should stop saying the Magisterium is corrupt. Once you know what you're talking about, you'll see the absurdity, if not the heresy in constantly saying that.

Repeat until you understand what you quoted: Pope Pius XI Divini Illius Magistri (#18) Dec. 31, 1929: "...God Himself has made the Church sharer in the divine magisterium and, by a special privilege, granted her immunity from error..."

A Filial Correction: "Your understanding of what the Church's Magisterium is, per the quotes you posted, is altogether wrong, if not heretical." Please accept this filial correction as a token of my appreciation for your frequent and most entertaining banter. :-)

Already used one "is", why add another?

Already used one "is", why add another?

Because the first is operates within the subordinate clause.

Perhaps try diagramming the sentence.

Your understanding of what the Church's Magisterium is, per the quotes you posted, altogether wrong, if not heretical.

Replace the phrase "what the Church's Magisterium is" with just "the Church's Magisterium", and what do you have left?

Your understanding of the Church's Magisterium, per the quotes you posted, altogether wrong, if not heretical.

There's no verb in the main sentence.  "what the Church's Magisterium is" as a whole servers as the object of the preposition "of".

And so "is" is what it is...

It depends on what the meaning of is, is. 😜

What I meant was, I didn't inhale. lol

Ah, those were the days.