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Magic the Gathering is Turing Complete
This leads to the following edge case, which I believe is not directly covered by any existing rules, although judges would in practice obviously just prevent this from happening / add it to the rules due to similarity with other rules and common sense.
We now have a 2 player legacy-legal way to build an arbitrary Turing machine in Magic in which neither player can take any actions once the machine starts executing:
which is an improvement to an older 4-player setup: https://www.toothycat.net/~hologram/Turing/HowItWorks.html
What this means, is that you could setup a board state where:
This comes close to the "indefinite infinite loop" type rules, but I could not find an exact rule that covers this case in either:
The indefinite infinite loop idea was mentioned at: https://boardgames.stackexchange.com/questions/24435/is-magic-the-gathering-a-consistent-game/24438#24438 and one notable example were the forbidden probabilistic skips of Basalt Monolith / Mesmeric Orb in the Four Horsemen combo deck:
but the Turing Machine loop is of a different kind however, since you:
I would add a new tournament rule phrased as:
Of course, this is not ideal, because in general it relies on players estimating how much mathematics judges are likely know, and there could be more legitimate gray cases in the future, but we'll have to wait for those to come up.
Also in this particular example, the player who starts such a loop had infinite mana and drew their entire deck, so one may argue that winning the game would have been trivial before the machine was started, so some judges could consider this particular case to fall under slow play.
I would also like to see what happens if someone does this on MTGO, but once again they likely have or would add a trigger limit of some sort, and I'm not sure if it is possible to set things up to automatically pass over all your steps, otherwise players would run out of time eventually due to the seconds that pass between you clicking "move to the next step".
But who knows. Maybe this is just the motivation a smart judge was needing to finally solve Collatz? ;-)