At level 7, the Fey Wanderer ranger gets a feature called Beguiling Twist. The relevant part reads:
Whenever you or a creature you can see within 120 feet of you succeeds on a saving throw against being charmed or frightened, you can use your reaction to force a different creature you can see within 120 feet of you to make a Wisdom saving throw against your spell save DC. (TCoE, p. 59)
Also at level 7, the Oath of Devotion paladin gets the Aura of Devotion, which says:
You and friendly creatures within 10 feet of you can’t be charmed while you are conscious. (PHB, p. 86)
If a Fey Wanderer and a Devotion paladin are in the same party, and the paladin is targeted by an effect that would impose the charmed condition, does that trigger the ranger’s Beguiling Twist feature? Clearly one way or another the paladin won’t be charmed, but does its immunity function as an automatic success on the saving throw? Or should the paladin roll the saving throw even though it’s going to ignore the effect of a failure, just to determine whether or not the ranger’s ability will trigger?