Unsound use of ClassTag with arrays in Reflection interface

[nicolasstucki]

scala.tasty.Reflection provides given instances of ClassTag to be able to pattern match (ClassTag.unapply).
But if they are used those as ClassTag (i.e. for arrays) we hit an unsoundness when type types in Reflection are implemented with the same class.
The solution is not to give implicit ClassTags for those types, or only give one for the root type (Tree, Type, Position, ...).
Therefore the ClassTag concept should be separate from the type testing logic (see #7555).

Here is a minimal self contained example of how this unsoundness can be triggered

package x

@main def Foo = {
  val refl: Refl = new ReflImpl
  import refl._
  val arr = Array[refl.Tree]()
  arr match
    case arr: Array[refl.Term] => throw new Exception() // some elements of the array may to be refl.Term
    case _ => // ok
}

trait Refl {
  type Tree
  type Term <: Tree
  def treeCT: scala.reflect.ClassTag[Tree]
  def termCT: scala.reflect.ClassTag[Term]
  given scala.reflect.ClassTag[Tree] = treeCT
  given scala.reflect.ClassTag[Term] = termCT
}


object ReflImpl {
  class Tree
  def treeCT: scala.reflect.ClassTag[Tree] = summon[scala.reflect.ClassTag[Tree]]
}
class ReflImpl extends Refl {
  type Tree = ReflImpl.Tree
  type Term = ReflImpl.Tree
  def treeCT: scala.reflect.ClassTag[Tree] = ReflImpl.treeCT
  def termCT: scala.reflect.ClassTag[Term] = ReflImpl.treeCT
}

[nicolasstucki]

Requires a fix for #7554. #7555 would allow fixing this issue

[nicolasstucki]

This was fixed