Union types that include Newtypes don't satisfy pattern matching exhaustivity check in Scala 3

[spavikevik]

Apologies in advance if the title is unclear.

Given the following example code:

object MyString extends Newtype[String]
type MyString = MyString.Type

class StringArray(val contents: Array[String])

def fn(s: MyString | StringArray) = s match
  case myS: MyString => println(myS)
  case arr: StringArray => println(arr.contents.mkString(","))

The compiler gives the following warning:

[warn]    |        match may not be exhaustive.
[warn]    |
[warn]    |        It would fail on pattern case: scala.reflect.ClassTag(_)

https://scastie.scala-lang.org/w509lJaCT3mtzUsnL4i2JA

I can see an implicit for ClassTag[Type] is defined in Newtype[A]. I am not sure where exactly this is used in practice, but I suppose it is there to help the compiler in situations like this one.

After playing around a bit, what I found to help is providing an instance for TypeTest. To my knowledge, TypeTest replaces ClassTag in Scala 3, and while ClassTag should still work, I guess there's something in this particular case that means it is not sufficient.
The example below compiles without warnings:

import zio.prelude.Newtype

import scala.reflect.TypeTest

trait MyNewtype[A] extends Newtype[A] {
given typeTest(using typeTest: TypeTest[Any, A]): TypeTest[Any, Type] = new TypeTest[Any, Type]:
    def unapply(x: Any): Option[x.type & Type] = x match
      case s: A => Some(s.asInstanceOf[x.type & Type])
      case _    => None
}

object MyString extends MyNewtype[String]
type MyString = MyString.Type

class StringArray(val contents: Array[String])

def fn(s: MyString | StringArray) = s match
  case myS: MyString => println(myS)
  case arr: StringArray => println(arr.contents.mkString(","))

https://scastie.scala-lang.org/B8Jhfu9eQ4S9wqyxtfkHbg