Can a multiclassed Warlock use a spell slot from the Spellcasting feature of a non-Warlock class to perform Eldritch Smites?
Eldritch Smite says:
Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra d8 force damage to a target, plus another d8 per level of the spel slot, and you knock the targert prone if it is huge or smaller.
The multiclassing rule for Spell Slots (PHB, page 164) says:
You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
Pact Magic (PHB, page 107) says:
If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
(All emphasis mine.)
It's pretty clear to me that spell slots are supposed to be interchangeable. You can't know which ones are paladin and which ones are sorcerer, for example. It is easy to tell warlock spell slots apart from the others however, but it seems you can cast paladin spells using warlock slots, so the intention here seems to be interchangeable as well.
Can you use a Spellcasting spell slot to Eldritch Smite?
I know from the question "If I multiclass into Paladin, can I use up any class's spell slots for Divine Smite?" that Divine Smite can use Warlock slots, but I want to know if it works the other way around as well.
Best Answer
Eldritch Smite is Warlock slot only
Only warlock spell slots from the Pact Magic feature can be expended to use Eldritch Smite; spell slots from the Spellcasting feature do not work with it.
An unofficial tweet by rules designer Jeremy Crawford in November 2017 confirms that this was intentional (unlike the paladin's Divine Smite, which had similar wording initially but was changed in errata later to work with any spell slot):