pattern matching works strange for an O'Caml programmer

Hello.

At first, I had this:
abstract class Expr {
  def eval: int = this match {
    case Number(n) => n
    case Sum(e1, e2) => e1.eval + e2.eval
  }
}
case class Number(n: int) extends Expr
case class Sum(e1: Expr, e2: Expr) extends Expr

It works great and makes perfect sense to me.

Later, I made it into this:
abstract class Expr {
  def eval: int = this match {
    case Number(n) => n
    //case Sum(e1, e2) => e1.eval + e2.eval
  }
}
case class Number(n: int) extends Expr
case class Sum(e1: Expr, e2: Expr) extends Expr

The warning I expected to receive didn't show up, that was
first thing that upset me.

Even later, I made it into this:
abstract class Expr {
  def eval: int = this match {
    case Number(n) => n
    case Sum(x, 0) => 0
  }
}
case class Number(n: int) extends Expr
case class Sum(e1: Expr, e2: Expr) extends Expr

The type error didn't showed up either.

These two errors are one of the most common mistakes that
I make during development, that's why I do love O'Caml.

Is there possibility that those errors/warnings will be
implemented in Scala? I really need them.

-- 
WBR, Alexander Sergeev.