Try making `Elem` covariant in `N` — but this fails :-|

frames.scala

import scala.compiletime._
final case class Col[Key, Value](data: Array[Value])
object Frame {
  // Covariant version of Tuple.Elem
  /** Type of the element a position N in the tuple X */
  type Elem[+X <: Tuple, +N <: Int] = X match {
    case x *: xs =>
      N match {
        case 0 => x
        case S[n1] => Elem[xs, n1]
      }
  }
  // 
  extension {
    inline def [This <: NonEmptyTuple](t: This).mapply(n: Int): Elem[This, n.type] = {
      import scala.runtime.DynamicTuple
      DynamicTuple.dynamicApply[This, n.type](t, n).asInstanceOf[Elem[This, n.type]]
    }
  }

  object Types {
    type IndexOf[F <: Tuple, Key] <: Int =
      F match {
        case Col[Key, v] *: cols => 0
        case Col[k, v] *: cols => S[IndexOf[cols, Key]]
      }
    type Get[F <: Tuple, Key] = Elem[F, IndexOf[F, Key]]
  }

  inline def get[Key, F <: NonEmptyTuple](frame: F) : Types.Get[F, Key] =
    frame.mapply(constValue[Types.IndexOf[F, Key]])
}

This works in the presentation compiler, but fails in later phases when checking variance:

[info] Compiling 1 Scala source to /Users/pgiarrusso/git/dotty-staging-experiment/map-fusion/target/scala-0.23/classes ...
[error] -- Error: /Users/pgiarrusso/git/dotty-staging-experiment/map-fusion/src/main/scala/macro.scala:5:25
[error] 5 |  type Elem[X <: Tuple, +N <: Int] = X match {
[error]   |                        ^^^^^^^^^
[error]   |      covariant type parameter N occurs in invariant position in X match {
[error]   |        case
[error]   |          [x, xs <: Tuple] =>>
[error]   |            scala.internal.MatchCase[x *: xs,
[error]   |              N match {
[error]   |                case (0 : Int) => x
[error]   |                case
[error]   |                  [n1 <: Int] =>>
[error]   |                    scala.internal.MatchCase[scala.compiletime.S[n1],
[error]   |                      LazyRef(Frame.Elem[xs, n1])
[error]   |                    ]
[error]   |              }
[error]   |            ]
[error]   |      }
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 1 s, completed Mar 27, 2020 5:56:53 PM