Unexpected erasure of type alias

[allanrenucci]

class TypeAlias {
  type T[X] = X
  def a(i: T[Int]): T[Int] = i
}

erases to

class TypeAlias {
  def a(i: Int): Object = scala.Int.box(i)
}

In scalac:

class TypeAlias {
  def a(i: Int): Int = i
}