My question is not a duplicate of When should I activate Man Lands and Keyrunes in order to attack with them?, although anyone answering this question would need to understand at least that much in order to understand this one. That question only explains when a player would need to activate. It doesn't cover shortcuts or illegal actions, which is what my question is primarily about.
Does activating an ability when you have already decided to move to the declare attackers step imply that you are declaring zero attackers?
Anthony: Animate Mutavault?
Anthony: Declare attackers?
Noah: Hang on. You're already in the declare attackers phase, right? I thought you animated Mutavault after you decided to declare no attackers.
Anthony offers the shortcut "Combat?". According to the Tournament Rules, that means he is offering to pass priority until Noah has priority in the Beginning of Combat step.
A statement such as "I'm ready for combat" or "Declare attackers?" offers to keep passing priority until an opponent has priority in the beginning of combat step. Opponents are assumed to be acting then unless they specify otherwise.
Noah responds with "Sure." I believe this means that Noah is passing priority back to Anthony (don't get too hung up on this - it isn't the focus of the question). That means the game moves to the Declare Attackers step.
Anthony activates Mutavault, which is an action that could, if it is legal, only be taken after declaring attackers. Now the game state is potentially ambiguous.
508.1. First, the active player declares attackers. This turn-based action doesn’t use the stack
508.3. Third, the active player gets priority
Assume the tournament level is Competitive REL, and that if a judge is needed, one will be called. What is the resulting game state? I believe the resulting game state would be one of the following, but I am not sure which is correct (if either):
- The game is in the Declare Attackers phase, and Anthony does not have the opportunity to declare attackers.
- Activating Mutavault was an illegal action because Anthony didn't have priority. That action is reversed. The game is in the Declare Attackers phase, and Anthony still has the opportunity to declare attackers.