[partly confirmed]Mimic Vat vs. Terror

[Korath]

Describe the bug:
Mimic Vat doesn't let me exile cards that were buried, just ones that were destroyed or sacrificed.

Also, it only lets me imprint one card if more than one are simultaneously destroyed with something like Pyroclasm, so Rakshasa Vizier can only get at most two +1/+1 counters at a time. (It can only get even that many if Mimic Vat works right with meld cards, which are broken badly enough in at least dev that I can't test with them.)

Which card behaved improperly?
is_this_dying().

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Dev 4721c01.

What exactly should be the correct behavior/interaction?
It only cares about cards going to the graveyard from the battlefield, not whether they can regenerate.

Are any other cards possibly affected by this bug?
This method of checking whether a card will actually go to a graveyard is so fragile as to be useless - it's incompatible with anything that replaces cards going to the graveyard with going somewhere else, like Leyline of the Void. Mimic Vat tries to fix this by searching the graveyard for a card with the same iid as the dying one, so it mostly isn't a problem in practice; but it'll incorrectly find a different copy of the same card that was put in a graveyard before the Leyline of the Void or whatever started exiling cards.

[Aswan jaguar]

In public version Mimic Vat works fine with bury cards.

Confirmed, you can imprint any of the cards that died simultaneously to exile but not imprint them all one by one to work with Rakshasa Vizier.

Meld cards have not yet been released in public so I can't test them certainly a dev only bug.

[Korath]

Looks like the is_this_dying() call was added to card_mimic_vat() in January, in commit 8dd0c9b9.

is_this_dying() was apparently introduced just after EMNv2, and it also excludes cards put into a graveyard as an explicit state-based action (though I think those get rewritten into sacrifices before cards with death triggers can see them). There's another function that's also been around for quite a while, is_dying(), that's exactly the same except it correctly excludes only exiled cards. It wouldn't work any better in combination with replace-death-with-exile effects on Mimic Vat or other cards with death triggers, though. Currently the ignore-buried-cards version is used in about 60 cards, some of which later verify the card really did end up in graveyard and some of which don't.