Problem with type inference when using Iterator.sliding on Union types

[hmf]

Minimized code

  type Numeric = Double | Int

  sealed trait VarValues[T, C <: VarValues[T,C]] {
    this: C =>
    val arr: Array[T]
  }
  final case class VarDoubles(arr: Array[Double]) extends VarValues[Double, VarDoubles]
  final case class VarInts(arr: Array[Int]) extends VarValues[Int, VarInts]
  final case class VarStrs(arr: Array[String]) extends VarValues[String, VarStrs]

  def check7(a: VarValues[_,_], b: VarValues[_,_]): Unit = {
    (a,b) match {
      case (x:(VarDoubles|VarInts), y:(VarDoubles|VarInts)) =>
        val x0: Iterator[Numeric] = x.arr.iterator
        val y0: Iterator[Numeric] = y.arr.iterator
        val l0: Iterator[(Numeric, Numeric)] = x0.zip(y0)
        val ll0: Iterator[(Numeric, Numeric)]#GroupedIterator[(Numeric, Numeric)] = x0.zip(y0).sliding(2,1)
        ???
      case _ => ???
    }

Output

The line:

        val ll0: Iterator[(Numeric, Numeric)]#GroupedIterator[(Numeric, Numeric)] = x0.zip(y0).sliding(2,1)

generates the error:

[error] -- [E007] Type Mismatch Error: /home/hmf/IdeaProjects/snol/splotly/src/gg/SlidingIssue.scala:115:102 
[error] 115 |        val ll0: Iterator[(Numeric, Numeric)]#GroupedIterator[(Numeric, Numeric)] = x0.zip(y0).sliding(2,1)
[error]     |                                                                                    ^^^^^^^^^^^^^^^^^^^^^^^
[error]     |Found:    Iterator[(gg.SlidingIssue.Numeric, AnyVal)]#GroupedIterator[(
[error]     |  gg.SlidingIssue.Numeric
[error]     |, AnyVal)]
[error]     |Required: Iterator[(gg.SlidingIssue.Numeric, gg.SlidingIssue.Numeric)]#GroupedIterator[(
[error]     |  gg.SlidingIssue.Numeric
[error]     |, gg.SlidingIssue.Numeric)]
[error] one error found

Expectation

Not an expert so may be wrong, but ....
I expected val ll0 to contain a tuple of the same type (Numeric) however the second element of the tuple is inferred to be AnyVal. This may be related to #8956 but it looks like type projection may also be causing problems.

[hmf]

Workaround:

  def check8(a: VarValues[_,_], b: VarValues[_,_]): Unit = {
    (a,b) match {
      case (x:(VarDoubles|VarInts), y:(VarDoubles|VarInts)) =>
        /*val x0: Iterator[Double|Int] = x.arr.iterator
        val y0: Iterator[Double|Int] = y.arr.iterator*/
        val x0: Iterator[Numeric] = x.arr.iterator
        val y0: Iterator[Numeric] = y.arr.iterator
        val l0: Iterator[(Numeric, Numeric)] = x0.zip(y0)
        val ll0: Iterator[(Numeric, Numeric)]#GroupedIterator[(Numeric, Numeric)] = l0.sliding(2,1)
        ???
      case _ => ???
    }

[bishabosha]

minimised to:

scala> type Numeric = Int | Double
// defined alias type Numeric = Int | Double

scala> val x1: Iterator[Numeric] = Array(1).iterator
val x1: Iterator[Numeric] = <iterator>

scala> val y1: Iterator[Numeric] = Array(1.0).iterator                                                               
val y1: Iterator[Numeric] = <iterator>

scala> x1.zip(y1).sliding(2,1)
val res0: Iterator[(Numeric, AnyVal)]#GroupedIterator[(Numeric, AnyVal)] = <iterator>