unchecked

final
class unchecked extends Annotation

An annotation to designate that the annotated entity should not be considered for additional compiler checks. Specific applications include annotating the subject of a match expression to suppress exhaustiveness and reachability warnings, and annotating a type argument in a match case to suppress unchecked warnings.

Such suppression should be used with caution, without which one may encounter scala.MatchError or java.lang.ClassCastException at runtime. In most cases one can and should address the warning instead of suppressing it.

object Test extends App {
  // This would normally warn "match is not exhaustive"
  // because `None` is not covered.
  def f(x: Option[String]) = (x: @unchecked) match { case Some(y) => y }
  // This would normally warn "type pattern is unchecked"
  // but here will blindly cast the head element to String.
  def g(xs: Any) = xs match { case x: List[String @unchecked] => x.head }
}
Source
unchecked.scala
class Object
trait Matchable
class Any