Mirror.SumOf not being provided when 0 inhabitants

Compiler version

3.0.0-RC1

Minimized code

import scala.deriving.*
sealed trait X
summon[Mirror.Of[X]]

Output

[error] 59 |  summon[Mirror.Of[X]]
[error]    |                      ^
[error]    |no implicit argument of type deriving.Mirror.Of[X] was found for parameter x of method summon in object Predef

Expectation

X is a sealed trait. I expect a Mirror.SumOf[X] { type MirroredElemTypes = EmptyTuple } to be provided.

[som-snytt]

An example would be, “Fool me once, shame on you. Fool me twice, shame on me.” There is no fool me zeroth. I guess we are always already fooled, which sounds about right to me.

[japgolly]

Hello. As I understand it, which ever library you’re talking about currently doesn’t support sum types with no inhabitants (both because Scala 3 isn’t providing a mirror and because they’ve used reduce instead of foldLeft), so fixing up the Scala 3 side of the equation will result in it going from “not working” (via compilation error due to no implicit mirror) to “not working” (due to a crash). This doesn’t seem like a great reason to hold back the entire language. If it didn’t work before and wont work immediately after this change, we’re not breaking anything, we’re just discussing what the break looks like.

I really think we should add this to a minor release. That some library used reduce instead of foldLeft shouldn’t hold back Scala itself. Plus it would be trivial to fix such a library, no? 0 inhabitants isn’t some special case here, it’s just a natural part of the mathematics. Expecting at least one inhabitant is the artificial special case here.