Like I said upthread, there's nothing at stake for me in this ruling. I'm just intrigued by the discussion over interpretive method.
Page 69 gives me this relevant text on bonus actions:
You can take a bonus action only when a special ability, spell, or other feature of the game states that you can do something as a bonus action. . . . You choose when to take a bonus action during your turn, unless the bonus action’s timing is specified . . .
There is no mention there of scope, trigger or nesting. There is a description of the conditions that permit a bonus action, and the rule for when it can occur. The interpretive question around Shield Mastery is entirely about whether or not it's timing is specified - by an interpolated then as in If you take the attack action, then you can shove as a bonus action.
I think that the most natural reading is to take the "then" as implicit. But I also think that the most natural reading of taking the attack action is making an attack - not making all the attacks one is going to make in a turn.
Those interpretations are obviously open to dispute, but I don't see how brining in external notions of "trigger", "scope", "nesting" etc helps with that. Those notions aren't part of the rules and don't seem to be implied by them either.
Well, the closest thing in the rules I quoted to the idea of a "trigger" is that a game feature states that you can take a bonus action. And as far as Cunning Action is concerned, the game feature in question is being a 2nd level rogue who is taking a turn in combat. So if the notion of "trigger" is going to be deployed, it's hard for me to see that in this case it does not include taking a turn. Even accepting for the sake of discussion that you can't not take your turn, I don't see why that would debar it from being a "trigger", or as the rules put it, a game feature that permits a player to take a bonus action on his/her turn.
A Reaction trigger requires an observable condition. "Start of turn" is not an observable condition, though a Reaction could potentially be triggered with similar timing due to the manner in which it is worded, or a lenient DM may not give a damn and simply allow it.
I don't see how. Page 64 says that a bonus action is an additional action that you take on your turn. And taking the Attack Action - which is what enlivens the Shield Master bonus action - is something that happens on your turn. I don't see anything there that comes within cooee of implying that the bonus action is in fact a Reaction.
Whereas I'm prepared to accept that "taking the attack action" is open to being read as "having completed all your attacks from the attack action", although I myself don't think that's the most natural reading, I'm at a complete loss to see how anyone could read Shield Master as permitting a Reaction shove. And I don't see how settling on one rather than another reading of taking the attack action contributes to the avoidance of any such misreading.
- A trigger must be completed before a Reaction occurs, unless specified otherwise.
- Crawford made a similar ruling/clarification on Bonus action timing ("if a feature says you can do X as a bonus action if you do Y, you must do Y before you can do X").
- The bonus action granted via Shield Master, due to its specific wording with Attack Action, is affected by the clarification or else it could operate like a Reaction with "specified otherwise" timing (re: bonus actions and reactions that have triggers -- "No general rule allows you to insert a bonus action between attacks in a single action. You can interrupt a multiple-attack action with a bonus action/reaction only if the trigger of the bonus action/reaction is an attack, rather than the action.")
It's really that simple. We can wax philosophical all day about natural language, poor wording, original intent, unforeseen consequences, or whether or not the tweets of the lead designer should be ignored, but the logic behind the clarification is not obtuse or unprecedented within the framework of the rules.
I wish I could provide a more thorough response, but I'm swamped at work.
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