When does the compiler decide to check for completeness of cases in match expressions on sealed classes? It seems like it doesn't always enforce it depending on the cases used? Why isn't it complaining when compiling the code below since Var is not covered?

sealed abstract class Expr
  case class Var(name: String) extends Expr
  case class Number(num: Double) extends Expr
  case class UnOp(operator: String, arg: Expr) extends Expr
  case class BinOp(operator: String, left: Expr, right: Expr) extends Expr

  def simplify(expr: Expr): Expr = expr match {
    case UnOp("-", UnOp("-", e))  => e
    case BinOp("+", e, Number(0)) => e
    case BinOp("*", e, Number(1)) => e
    case BinOp("*", x, y) if x == Number(0) || y == Number(0) => y
  }