scala3-UnionTypes5.scala

// Covariance:

val t1s: Seq[T1] = Seq(new T1 {})
val t2s: Seq[T2] = Seq(new T2 {})
val t3s: Seq[T3] = Seq(new T3 {})
val t123s1: Seq[T1 | T2 | T3] = t1s
val t123s2: Seq[T1 | T2 | T3] = t2s
val t123s3: Seq[T1 | T2 | T3] = t3s

// Contravariance?

val t123s: Seq[T1 | T2 | T3] = Seq(t1, t2, t3)
val t1s1: Seq[T1] = t123s        // ERROR
val t2s1: Seq[T2] = t123s        // ERROR
val t3s1: Seq[T3] = t123s        // ERROR