object test {
  sealed trait X[T]
  trait Iterable[T]
  case class I[T, C <: Iterable[T]](data: C) extends X[C]

  def t[T](i: X[T]): Any =
    i match {
      case i @ I(_) => i
    }
}

$ scala-cli compile -S 3.nightly gdat.scala -Wall
Compiling project (Scala 3.8.1-RC1-bin-20251127-fd2172d-NIGHTLY, JVM (21))
[warn] ./gdat.scala:7:12
[warn] the type test for test.I[T$1, T] cannot be checked at runtime because its type arguments can't be determined from test.X[T]
[warn]       case i @ I(_) => i
[warn]            ^
Compiled project (Scala 3.8.1-RC1-bin-20251127-fd2172d-NIGHTLY, JVM (21))