valuesOf.scala

import scala.deriving.*
import scala.compiletime.*

inline def valuesOf[T <: Tuple]: List[String] = inline erasedValue[T] match {
  case _: EmptyTuple => Nil
  case _: (h *: t)   =>
    erasedValue[h] match {
      case label: String =>
        label :: valuesOf[t]
    }
}

inline def derived[T](using s: Mirror.SumOf[T]): Labels[T] =
  a => valuesOf[s.MirroredElemLabels].toList(s.ordinal(a))

Better, but still with a cast:

  import scala.deriving.*
  import scala.compiletime.*

  inline def derived[T](using s: Mirror.SumOf[T]): Labels[T] =
    a => constValueTuple[s.MirroredElemLabels].toList.asInstanceOf[List[String]](s.ordinal(a))

Got it:

  import scala.deriving.*
  import scala.compiletime.*

  inline def derived[T](using s: Mirror.SumOf[T]): Labels[T] =
    a =>
      val labels = constValueTuple[s.MirroredElemLabels].toList
      val allStrings = summonInline[labels.type <:< List[String]]
      allStrings(labels)(s.ordinal(a))