If I'm not allowed to choose to search my library, does that mean I'm not allowed to do that whole instruction from the Bloodlust? Or can I shuffle the library? IB's text doesn't fit either of the protocols from Shadow of Doubt's first two rulings.
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The "you may" refers to the entirety of actions that follow those two words in Infectious Bloodlust's last ability, including the shuffle. The situation would have been clearer, though, with wording like "You may search .... If you do, shuffle ..." (compare Infectious Bloodlust with Trinket Mage; see also C.R. 117.12). See also this thread.
I find only post #6 of that thread to be substantive, but there is no textual basis for Rezzahan's claim in it. As you note, the general form is "You may A, then B." If A is impossible, can you take the choice and end up doing B?
The reasoning I would use is to extend what is known about composite choices. They are few, but look at Demon of Death's Gate. If you can't sacrifice 3 creatures, you can't apply that cost. Also, if you can't pay life or your life total can't change, you can't apply that cost. But this is a cost, not a "choice made when applying an effect", so we're still a little lost.
Preferred Selection is a bit closer. It asks you to do a thing and pay a cost. Clearly, the check is whether you do both, and clearly, you aren't allowed to do either unless you do both.
But what if you're resolving Yavimaya Granger / Ondu Giant, and there's a Worms of the Earth? You can't put the card onto the battlefield this time. So is that reckoned as impossible to do the whole thing? That's an issue, because foreseeing that effect as impossible is now needed.
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The cases of Demon of Death's Gate and Preferred Selection involve costs because of C.R. 117.9 and C.R. 117.12, respectively. However, the form "You may A, then B", by itself, doesn't.
This can be resolved by treating effects of the form "You may A, B, then C" or "You may A, then B" no differently from the form "You may A, B, and C" or "You may A and B": that is, there is no more difference between "and" and "then" in the grouping of optional actions than in their behavior in separating sequential actions (outside of costs) (see also C.R. 608.2c).
However, as for your last paragraph (assuming Shadow of Doubt isn't involved), note that for both cards, a player "isn't required to find" a basic land card to put onto the battlefield while searching (C.R. 701.18b). If the comprehensive rules, however, don't clearly regulate whether an effect of the form "You may A, B, then C" (or "You may A, B, and C"), where a player chooses not to find one or more cards in a search (whether or not some or all of them could be put onto the battlefield), is nevertheless impossible, then that is a gap in those rules.And an effect of the form "You may search ..., then ..." still allows a player to search even if they don't find one or more cards in the search or the additional actions with the cards found are illegal or impossible (C.R. 701.18g).
EDIT (Dec. 9): Clarification.
EDIT (May 9): Edited to conform to rule changes in Dominaria.
An official ruling published in October's Magic Judge Monthly confirms the first sentence of my comment 2 and applies to all effects of the form "... may search ... then shuffle ..." (but using Ghost Quarter and Leonin Arbiter instead of Infectious Bloodlust and Shadow of Doubt, respectively).
Cool. So when divining what course of events the player has open to him or her, the instruction "may do A and B..." is allowed only if A and B are possible, and "may do A, (then) B, then C" is allowed if and only if A is possible. Of course, if B and C are impossible but A is not, then those won't happen, but it will count as the branch taken.
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If I'm not allowed to choose to search my library, does that mean I'm not allowed to do that whole instruction from the Bloodlust? Or can I shuffle the library? IB's text doesn't fit either of the protocols from Shadow of Doubt's first two rulings.
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The reasoning I would use is to extend what is known about composite choices. They are few, but look at Demon of Death's Gate. If you can't sacrifice 3 creatures, you can't apply that cost. Also, if you can't pay life or your life total can't change, you can't apply that cost. But this is a cost, not a "choice made when applying an effect", so we're still a little lost.
Preferred Selection is a bit closer. It asks you to do a thing and pay a cost. Clearly, the check is whether you do both, and clearly, you aren't allowed to do either unless you do both.
But what if you're resolving Yavimaya Granger / Ondu Giant, and there's a Worms of the Earth? You can't put the card onto the battlefield this time. So is that reckoned as impossible to do the whole thing? That's an issue, because foreseeing that effect as impossible is now needed.
Awesome avatar provided by Krashbot @ [Epic Graphics].
This can be resolved by treating effects of the form "You may A, B, then C" or "You may A, then B" no differently from the form "You may A, B, and C" or "You may A and B": that is, there is no more difference between "and" and "then" in the grouping of optional actions than in their behavior in separating sequential actions (outside of costs) (see also C.R. 608.2c).
However, as for your last paragraph (assuming Shadow of Doubt isn't involved), note that for both cards, a player "isn't required to find" a basic land card to put onto the battlefield while searching (C.R. 701.18b).
If the comprehensive rules, however, don't clearly regulate whether an effect of the form "You may A, B, then C" (or "You may A, B, and C"), where a player chooses not to find one or more cards in a search (whether or not some or all of them could be put onto the battlefield), is nevertheless impossible, then that is a gap in those rules.And an effect of the form "You may search ..., then ..." still allows a player to search even if they don't find one or more cards in the search or the additional actions with the cards found are illegal or impossible (C.R. 701.18g).EDIT (Dec. 9): Clarification.
EDIT (May 9): Edited to conform to rule changes in Dominaria.
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