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WhosDaDungeonMaster
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As was pointed out to me a while back when I asked Crawford about the timing issue, there is no declaration step or phase, or whatever you want to call it, in 5E. That is a legacy of previous editions. So you attack or you do not attack. There is no saying "I am going to attack" and then get to do a bonus action before the attack is resolved. If the rules say "take the attack action" then you have to make an attack in order to trigger the bonus action. As for Shield Master itself, this in one of those feats that really feels like it should not be available at low levels, but I know they designed 5E feats to not have any prerequisites.
Actually if you use the variant of non-cyclical initiative, re-rolling every round, you must declare your actions before you roll. That is why I allow bonus actions to precede other actions required of them if player declares they are performing the required action (even if it comes after the bonus action). It hasn't been an issue at all.