A 7th-level Evocation wizard hits a 7th level rogue with a cantrip that requires a Dex saving throw, like acid splash.
The wizard has the Potent Cantrip feature that means any creature that succeeds the Dex save will always take half damage:
Starting at 6th level, your damaging cantrips affect even creatures that avoid the brunt of the effect. When a creature succeeds on a saving throw against your cantrip, the creature takes half the cantrip’s damage (if any) but suffers no additional effect from the cantrip.
The rogue has the Evasion feature, which means a save for half damage becomes save for none:
Beginning at 7th level, you can nimbly dodge out of the way of certain area effects, such as a red dragon's fiery breath or an ice storm spell. When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.
Since both of these are passive abilities, which takes priority and how much damage does the rogue take if it succeeds the dex save?
Best Answer
Potent Cantrip makes a cantrip exactly the kind of effect that Evasion works against. Evasion causes "an effect that allows you to make a Dexterity saving throw to take only half damage" to instead deal no damage. Potent Cantrip means that your cantrips deal half damage on a successful saving throw. Which is to say, the target gets to make a saving throw to take only half damage.