$ cat Test.scala 
package test

sealed trait Tree
case class Let(sth: List[Any]) extends Tree
case class Lit(sth: Any) extends Tree

object Test {
  def wroong(tree: Tree) =
    tree match {
      case Let(_ :: rest) =>
        ???
      case Let(Nil) =>
        ???
      // no warning for missing Lit(_) in 2.10
    }
}

$ scalac210 -version
Scala compiler version 2.10.4-20140209-180020-b66a39653b -- Copyright 2002-2013, LAMP/EPFL

$ scalac210 Test.scala 

$ scalac210 -Xoldpatmat Test.scala #using the old pattern matcher
Test.scala:10: warning: match is not exhaustive!
missing combination            Lit

    tree match {
    ^
one warning found

$ scalac211 -version
Scala compiler version 2.11.0-20140227-142303-fabc9f1424 -- Copyright 2002-2013, LAMP/EPFL

$ scalac211 Test.scala 
Test.scala:10: warning: match may not be exhaustive.
It would fail on the following input: Lit(_)
    tree match {
    ^
one warning found

$ cat Test2.scala 
package test

sealed trait Tree
case class Let(sth: List[Any]) extends Tree
case class Lit(sth: Any) extends Tree

object Test {

  def ookay(tree: Tree) =
    tree match {
      case Let(_) =>
        ???
      // missing Lit(_) warning in both 2.10 and 2.11
    }
}

$ scalac Test2.scala 
Test2.scala:10: warning: match may not be exhaustive.
It would fail on the following input: Lit(_)
    tree match {
    ^
one warning found