Tempest Domain clerics (PHB, p. 62) get the Thunderbolt Strike feature at 6th level:
At 6th level, when you deal lightning damage to a Large or smaller creature, you can also push it up to 10 feet away from you.
What happens if a Level 6+ Tempest Cleric deals lightning damage to a lightning-immune Large (or smaller) creature? Can the cleric still push the creature, or not?
As I read the feature and understand the mechanics, an immune creature cannot take damage from whatever it is immune to. Therefore, effects which depend on damage cannot take place against said creatures.
I think this would be a fine interpretation if the description read "when you hit a creature with a spell that deals Lightning Damage", because there's no requirement in that description that actually requires the creature to take damage, but here, I don't think that's the case.
The alternative reading is to infer that Immune Creatures "reduce damage by 100%", implying that the creature takes "0 damage", and thus still qualifies. But I think that's a weird reading, and I think my overall conclusion, that immune creatures do not trigger "when damaged" effects, stands. My interpretation is also supported by Crawford, who asserted that taking 0 damage means taking no damage, and specifically used this feature as an example of an effect that would not trigger on a creature immune to the damage type.
The spell specifically says when you do damage, you can also push it away, meaning the damage has to be dealt to the creature in order for the pushback to happen.
However...
If the player was trying to say, push the creature off a cliff by using the pushback feature, I'd probably allow it as one-off rule-of-cool type thing!
