Opaque type with generic forgets its boundaries

[rcano]

minimized code

object opaque {
  opaque type Foo[X] <: String = String
}
object test {
  val s: String = ???.asInstanceOf[opaque.Foo[_]]
}

Compilation output

Found:    guarana.swing.opaque.Foo[?]
Required: String(7)

expectation

It should compile. Note that this does compile

object opaque {
  opaque type Foo <: String = String
}
object test {
  val s: String = ???.asInstanceOf[opaque.Foo]
}