How is the "Lucky" Feat affected by Advantage/Disadvantage?

The other answers address the case of advantage or disadvantage, but I feel the issue of “not expending the luck point” hasn’t quite been answered. When the rule says (emphasis yours):

Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.

“the die” means the d20 rolled for the original attack or check, not the extra die rolled for using the feat. I can see how this might seem ambiguous, but the wording is the same as for similar abilities that allow a player to choose when they will apply to a roll, like bardic inspiration.

So the process goes:

• Roll the original check (with an extra die for advantage or disadvantage if applicable)
• Having seen the result (or results) of the dice roll, but before it has an in-game effect, decide whether to spend a luck point
• If you spend a luck point, roll an extra die and choose one out of all dice rolled to be the one used as the result
• The DM accepts that roll and uses it to determine the outcome

To put it simply, the phrase used in the PHB is "Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20." This means you get to add a d20 to the amount rolled, e.g. if two d20s are being used, you roll three. "You choose which of the d20s is used for the attack roll, ability check, or saving throw." This means that the player is able to choose which of the three rolls is used to create the end result.

As noted in xanderh's answer, the way Lucky interacts with disadvantage (RAW) is:

1. You roll 3 dice.
2. You choose which one you want to use.

This effectively turns disadvantage into a huge advantage, except for the fact your action is still treated as done in disadvantage, as noted in this tweet by rules designer Jeremy Crawford. So, answering your question, no, you don't get to not expend the lucky dice, but you also can't ever get a worse outcome, because you still can choose the higher dice from before.

---

I'd like to add some additional references from Jeremy Crawford.

First, it should be noted that in what seems to be the first time this question came to surface, the answer was actually different. In this answer, you would take the middle dice (second best/second worst). This might indicate that RAI, originally, Lucky shouldn't be so good. It is also stated as a potential problem and other solutions have been proposed. I'd like to point out that Crawford has said:

Sage advice is for clarification, not redesign.

and also:

In Sage Advice, I explain how the game works, not how I wish it worked.

The Sage Advice Compendium has, since then, also included an extra paragraph on their clarification of the Lucky feat:

If a DM wants advantage and disadvantage to play their normal roles even when the Lucky feat is used, here’s a way to do so: roll two d20s for advantage/disadvantage, roll a third d20 for Lucky, eliminate one of the three dice, and then use the higher (for advantage) or lower (for disadvantage) of the two dice that remain.

With that in mind, as a player you should probably ask your DM how he's going to handle this interaction before choosing the feat, and as a DM it's up to you choosing between RAW and (possibly) RAI (certainly more balanced). Unlike RAW, as a DM you can play exactly how you wish it was played and how you think it should be played.

Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.

I don't know why Crawford would interpret the text that way, and I hope that if it is addressed in an official Errata, he will reconsider what he said (assuming he said it on Twitter). I think that El Suscriptor is correct.

For one, you do expend a luck point if you roll a luck die because "you can spend one luck point to roll an additional d20" whenever you make an attack, meaning that if you choose to use your luck point and roll, it will be expended. This is indicated by the phrase, "You can choose to spend one of your luck points after you roll the die, but before the outcome is determined," which isn't commenting on when you can choose to expend a luck point in and of itself. Rather, it refers to the time that you may expend your luck point as part of the mechanism that it entails. In other words, expending a luck point necessarily gives you the right to roll a die to give you the de facto advantage.

Two, the text unambiguously states that there is no "superadvantage." This is because die is singular, not plural:

You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.

It's saying, "You can spend a luck point [and therefore roll another die] after your attack roll [of one die]." If there were a "superadvantage," the previous sentence would say "die or dice," but it does not. This means that, as El Suscriptor said, you choose between two rolls. It is like doing the order of operations in a simple math equation. For example, if you look at 2*(5+3), you don't multiply 5 by 2 and then 3 by 2, you need to conclude the addition within the paraphrase. Using that same order of logic, you need to conclude the advantage or disadvantage procedure.* If you roll a 1 and a 20 with disadvantage, you take the 1. That is your die after the roll, and then a luck point can be expended (roll a die) before the outcome is determined (i.e. when the DM says you crit missed or whatever).

---

* Procedure:

When you have either advantage or disadvantage, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.

Notice that you must decide which of the two dice must be used. It must be concluded, and it's mutually exclusive of any other rolls or procedures. It is a bracketed section of a math equation.

It's simpler than that. A disadvantage roll rolls 2d20 and discards the higher one, leaving only the lower; once you know what number your single one die has rolled, you can choose to spend a Luck Point to roll a second 1d20 (this one without the disad) and then choose which of the two rolls you take.

TL;DR You don't choose among three dice, you choose between two rolls.

According to this question from the Sage Advice Compendium, if you roll with disadvantage and use a luck point, you get to choose which of the three dice you use, effectively turning disadvantage into super-advantage:

How does the Lucky feat interact with advantage and disadvantage?

The Lucky feat represents extraordinary luck that can help you when you need it most. It lets you spend a luck point; roll an extra d20 for an attack roll, ability check, or saving throw; and then choose which d20 to use. This is true no matter how many d20s are in the mix. For example, if you have advantage or disadvantage on your attack roll, you could spend a luck point, roll a third d20, and then decide which of the three dice to use. You still have advantage or disadvantage, since the feat doesn’t say it negates it, but you get to pick the die. The upshot of this fact is that a rogue, for instance, who has disadvantage on an attack roll couldn’t use Sneak Attack even if the rogue uses the Lucky feat to pick the die.

The Lucky feat is a great example of an exception to a general rule. The general rule in this case is the one that tells us how advantage and disadvantage work. The specific rule is the Lucky feat, and we know that a specific rule trumps a general rule if they conflict with each other.

If a DM wants advantage and disadvantage to play their normal roles even when the Lucky feat is used, here’s a way to do so: roll two d20s for advantage/disadvantage, roll a third d20 for Lucky, eliminate one of the three dice, and then use the higher (for advantage) or lower (for disadvantage) of the two dice that remain.

It does still preclude using things dependent on not having disadvantage, like Sneak Attack, which Jeremy Crawford reminds us of in this tweet. You can even choose to roll the third die after you've seen the result of the roll.