At 11th level the Ranger subclass Hunter can get Whirlwind Attack:
You can use your action to make a melee attack against any number of creatures within 5 feet of you, with a separate attack roll for each target.
When this is combined with Lunging Attack maneuver from the Battle Master:
When you make a melee weapon attack on your turn, you can expend one superiority die to increase your reach for that attack by 5 feet. If you hit, you add the superiority die to the attack’s damage roll.
Does the extra damage apply across all damage rolls (I'm ignoring the range change, that much seems like it would make no difference as Whirlwind Attack fixes the range)?
Whirlwind Attack is odd in that it's one attack, as per Sage Advice Compendium (page 4):
Q. Can a ranger move between the attack rolls of the Whirlwind Attack feature?
A. No. Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls. In most other instances, an attack has one attack roll. The rule on moving between attacks (PH, 190) lets you move between weapon attacks, not between the attack rolls of an exceptional feature like Whirlwind Attack.
The wording seems off, but I can't see any other reason to allow it? Every other maneuver used with an attack triggers on a hit or a roll, so this seems like a deliberate exception?
Best Answer
Whirlwind Attack's range does not depend on your reach
Whirlwind Attack says you can target "any number of creatures within 5 feet of you". It doesn't matter what the reach of your normal attacks is. You could be a bugbear wielding a halberd and using Lunging Attack, but Whirlwind Attack is still limited to targets within 5 feet of you.
The reasoning for this is obvious: if Whirlwind Attack worked based on the reach of your normal attacks, the number of potential targets would increase quadratically as your reach increases, which is a recipe for unbalanced combinations. Limiting the range to 5 feet also makes sense thematically: you are attacking everything around you in all directions, not reaching out in any specific direction.
Lunging Attack can add the superiority die to a damage roll
Just because Whirlwind Attack doesn't care about your weapon's reach doesn't mean you can't use Lunging Attack with it. If you do so, it increases your reach, which Whirlwind Attack then ignores. But then you still get to add the superiority die one of the damage rolls, because that is part of the maneuver's effect.
I say one of the damage rolls because the rules for making an attack say to roll damage on a hit. Hence, if an attack hits multiple targets, you roll damage separately for each hit. As for how Lunging Attack's damage bonus interacts with this, the exact wording is:
The ability is obviously written for what is by far the most common case: an attack with a single damage roll. It doesn't specify how to generalize this to the much rarer case of an attack with multiple damage rolls, so it's ultimately the DM's call how to apply it. However, rolls are rarely, if ever, used more than once, so the most likely intent is that the superiority die is added to one damage roll and then discarded. Still, a generous DM might allow you to add the bonus to the damage for every hit.
You should probably use another maneuver if you can
Note that many other maneuvers also add your superiority die to the damage roll and potentially have other useful effects as well, so you will most likely be better off using one of those with Whirlwind Attack instead of Lunging Attack. (Exactly how each of those manuevers interact with Whirlwind Attack is beyond the scope of this question.)