A shield isn’t a weapon. Otherwise the Duelist style wouldn’t work with a shield, which Sage Advice says it can.
@mackenzie884 A shield certainly isn’t a weapon, but like many things, it can be used as an improvised weapon.
— Jeremy Crawford (@JeremyECrawford) November 17, 2015
An improvised weapon is a weapon. It’s right there in the name: Improvised WEAPON. If a shield is an IW then no Duelist
@mackenzie884 An improvised weapon is, indeed, a weapon, but only the moment it’s used as such. A chair/shield/etc isn’t a weapon otherwise.
— Jeremy Crawford (@JeremyECrawford) November 17, 2015
So now I can have Duelist +2 dmg, Shield +2 AC, AND a second IW attack? Yes/no?
@mackenzie884 In this scenario, how are you getting a second attack with the shield?
— Jeremy Crawford (@JeremyECrawford) November 17, 2015
The shield is an IW off-hand. Attack #1 longsword, bonus action attack #2 with shield. In attack #1, Duelist is satisfied?
During my Longsword attack, is the Duelist condition satisfied? Then I make an off-hand IW attack on my bonus with shield.
@mackenzie884 I’ll ask my question differently: what game feature is letting you attack with the shield as a bonus action?
— Jeremy Crawford (@JeremyECrawford) November 17, 2015
@JeremyECrawford
The feature that is built into the rules stating that you can use your bonus action to make an improvised weapon or unarmed attack with your bonus action if you have used your action to attack. Please clarify without asking a question to a question if I am incorrect but I feel it reads fairly clearly on this point.
As John Burns said also: a shield can’t be used for off-handed weapon attack (sans a Feat or feature that allows it to) because it’s not a Light weapon and neither is the long sword; even as an improvised weapon, it doesn’t meet the requirement for being used for BA attack because it lacks the requisite feature(s) required. That’s why JC responded the way he did — he’s prompting you to re-read the RAW more closely.
Ben, I doubt Jeremy will ever see your reply on here as this site automatedly grabs his twitter replies and put them on here, but replying on here does not reciprocally put them on twitter. I can say from hearing continued discussion since you tried to reply to this in other forums and twitter that your reply back – that the two weapon fighting base rule would let you use a bonus attack to make a shield attack… is incorrect.
That base feature needs the weapon being used for the normal attack action, as well as the bonus attack action, to be “light”. Your example used longsword for the attack action and while shield is in other hand during this attack action, qualifying for duelist +2 damage, because neither it nor the shield are “light”, two weapon fighting rule cannot allow a bonus action shield attack using the shield as an improvised weapon.
Individually, if the only weapon being used in your attact action was your choice to strike with IT as an improvised weapon, in the moment you are striking with it on your attack action it is an improvised melee weapon. Duelist wont apply to give bonus to your shield damage, because you DO have a weapon in your other hand, the longsword.
Using a shield “Captain America” style might allow duelist to add to its +2 damage because if one hand is open and the shield becomes an improvised weapon in that striking moment, then duelist is satisfied.
The only way to really enjoy both duelist for your longsword and then a two weapon fighting attack for a shield that will be used to do damage, is to get dual wielder, allowing two weapon fighting to be possible with non-light weapons in each hand. Then hypothetically after taking a longsword attack enjoying duelist’s +2 damage, your bonus action could be used to launch an improvised weapon attack with your shield, becoming an improvised weapon in this moment and striking for 1d4 bludgeoning damage. Note that as an improvised weapon, you are not getting proficiency bonus to hit (without proficiency in improvised weapons- tavern brawler), nor ability modifier to damage (requiring two weapon fighting fighter weapon style) on this shield attack.
I think I would add this to that:
Your “momentary rule transformation” of the shield into an IW spans your entire turn, and lasts until the start of your next turn. So you don’t satisfy Duelist fighting style with the longsword attack, and you no longer get your shield bonus until the start of your next turn. Basically, from that mental moment at the start of your turn when your character decides “I’m gonna dual wield my longsword and shield”, you stop having a shield on your arm, and now you have an off-hand IW on your arm.
And while you (player) may not have decided it until after the longsword attack finished, I would rule that it retcons as an intention of your character from the start of the turn. You get a shield, or you get an IW, not both. Changing your stance for that IW attack effects your longsword attack. And everyone else gets a chance to take advantage of your choice to open your defenses for that other attack.
How about if you sheathe your longsword (thus having one free hand and a shield), and then attack with your shield as an improvised weapon?