Missing unchecked type argument warning

[lrytz]

//> using scala "3.7.3"

trait A[+T]:
  def x: T
trait B[+T] extends A[T]:
  def y: T

object Troll extends A[Int] with B[Any]:
  def x: Int = 0
  def y: Any = ""

def f[T](a: A[T]): T = a match {
  case b: B[T] => b.y  // compiles without warning
  case _ => a.x
}

@main def t(): Unit =
  val a: A[Int] = Troll
  val i: Int = f(a)    // ClassCastException
  println(i + 1)

in case b: B[T] the type argument T cannot be checked at runtime. The compiler does not report an unchecked warning.