But what if you try to cast it anyway from exile? By the time you check for legality, it's no longer in exile but on the stack...
A friend is trying to convince me this is possible but I'm uncertain. I think 101.2, "When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence." is relevant - since you'er not allowed to cast the Griffin...
Under C.R. 601.2, in general, "[a] player must be legally allowed to cast the spell to begin" doing so., "ignoring any effect that would prohibit that spell from being cast based on information determined during that spell's proposal". The answer hinges on whether the effect of Godsend's last ability or that of Ixalan's Binding's last ability is an "effect that would prohibit that spell from being cast based on information determined during that spell's proposal".
If so, then that card's owner can begin to cast Misthollow Griffin (and the effects of both abilities will no longer refer to any "exiled cards" once it goes to the stack [C.R. 601.2a, 607.2a]), and casting it this way won't fail because the card is no longer in exile; that ability functions everywhere, even outside the game (C.R. 112.6e), so that it can check whether Misthollow Griffin was cast from exile at C.R. 601.2e.
If not, then that card's owner can't begin to cast Misthollow Griffin, so that card will remain in exile.
In general, if Misthollow Griffin's owner is kept from casting that card, that player can't begin to cast it (C.R. 101.2), so that card will remain in exile.
EDIT (Feb. 3): Adjust a rule citation.
EDIT (Jun. 8): C.R. 601.2 was materially changed with Battlebond.
EDIT (Jun. 10): Added a rule citation.
You indeed can't cast the Griffin, and yes, that's notably because can't beats can. You can't legally begin the process of casting a spell if an effect forbids it, the legality check isn't just made during the process, you can't begin it at all. See these rules:
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell includes proposal of the spell (rules 601.2a-d) and determination and payment of costs (rules 601.2f-h). To cast a spell, a player follows the steps listed below, in order. A player must be legally allowed to cast the spell to begin this process (see rule 601.3), ignoring any effect that would prohibit that spell from being cast based on information determined during that spell's proposal. (Such effects are considered during the check detailed in rule 601.2e.) If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before the casting of that spell was proposed (see rule 720, "Handling Illegal Actions").
601.3. A player can't begin to cast a spell unless a rule or effect allows that player to cast it. If that player is no longer allowed to cast that spell after completing its proposal, the casting of the spell is illegal and the game returns to the moment before the casting of that spell was proposed (see rule 720, "Handling Illegal Actions").
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
The recent rules update with Battlebond clarifies that comment 3 is correct (but note that the text of C.R. 601.2 has now changed); I have edited comment 2 accordingly. Under C.R. 601.2, in general, "[a] player must be legally allowed to cast the spell to begin" doing so; in all cases, this now takes into account all effects that forbid the spell from being cast, such as found in Godsend or Ixalan's Binding (C.R. 101.2).
But what if you try to cast it anyway from exile? By the time you check for legality, it's no longer in exile but on the stack...
A friend is trying to convince me this is possible but I'm uncertain. I think 101.2, "When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence." is relevant - since you'er not allowed to cast the Griffin...
, "ignoring any effect that would prohibit that spell from being cast based on information determined during that spell's proposal". The answer hinges on whether the effect of Godsend's last ability or that of Ixalan's Binding's last ability is an "effect that would prohibit that spell from being cast based on information determined during that spell's proposal".- If so, then that card's owner can begin to cast Misthollow Griffin (and the effects of both abilities will no longer refer to any "exiled cards" once it goes to the stack [C.R. 601.2a, 607.2a]), and casting it this way won't fail because the card is no longer in exile; that ability functions everywhere, even outside the game (C.R. 112.6e), so that it can check whether Misthollow Griffin was cast from exile at C.R. 601.2e.
- If not, then that card's owner can't begin to cast Misthollow Griffin, so that card will remain in exile.
In general, if Misthollow Griffin's owner is kept from casting that card, that player can't begin to cast it (C.R. 101.2), so that card will remain in exile.
EDIT (Feb. 3): Adjust a rule citation.
EDIT (Jun. 8): C.R. 601.2 was materially changed with Battlebond.
EDIT (Jun. 10): Added a rule citation.
EDIT (Jul. 28): Correctness edit.