Improvements to unchecked warnings.

Closes SI-6275, SI-5762.

The comment says is better than I can.

/** On pattern matcher checkability:
 *
 *  Consider a pattern match of this form: (x: X) match { case _: P => }
 *
 *  There are four possibilities to consider:
 *     [P1] X will always conform to P
 *     [P2] x will never conform to P
 *     [P3] X <: P if some runtime test is true
 *     [P4] X cannot be checked against P
 *
 *  The first two cases correspond to those when there is enough static
 *  information to say X <: P or that !(X <: P) for all X and P.
 *  The fourth case includes unknown abstract types or structural
 *  refinements appearing within a pattern.
 *
 *  The third case is the interesting one.  We designate another type, XR,
 *  which is essentially the intersection of X and |P|, where |P| is
 *  the erasure of P.  If XR <: P, then no warning is emitted.
 *
 *  Examples of how this info is put to use:
 *  sealed trait A[T] ; class B[T] extends A[T]
 *    def f(x: B[Int]) = x match { case _: A[Int] if true => }
 *    def g(x: A[Int]) = x match { case _: B[Int] => }
 *
 *  `f` requires no warning because X=B[Int], P=A[Int], and B[Int] <:< A[Int].
 *  `g` requires no warning because X=A[Int], P=B[Int], XR=B[Int], and B[Int] <:< B[Int].
 *      XR=B[Int] because a value of type A[Int] which is tested to be a B can
 *      only be a B[Int], due to the definition of B (B[T] extends A[T].)
 *
 *  This is something like asSeenFrom, only rather than asking what a type looks
 *  like from the point of view of one of its base classes, we ask what it looks
 *  like from the point of view of one of its subclasses.
 */

1 files changed, 5 insertions, 5 deletions

diff --git a/test/files/run/t576.scala b/test/files/run/t576.scala
index dc09d8dc98..756a241572 100644
--- a/test/files/run/t576.scala
+++ b/test/files/run/t576.scala
@@ -12,7 +12,7 @@ object Dingus {
 object Test {
   val x1 = new A
   val x2 = new A
-  
+
   val x3 = new { self =>
     override def equals(other : Any) = other match {
       case that: self.type    => true
@@ -20,7 +20,7 @@ object Test {
     }
   }
   val x4 = new { self =>
-    def f(x: Any) = x match {
+    def f(x: Any): Int = x match {
       case _: x1.type     => 1
       case _: x2.type     => 2
       case _: x3.type     => 3
@@ -35,11 +35,11 @@ object Test {
     assert(x1 != x2)
     assert(x1 != ())
     assert(x2 != x1)
-    
+
     assert(x3 == x3)
     assert(x3 != x2)
     assert(x2 != x3)
-    
+
     List(x1, x2, x3, x4, Dingus) map x4.f foreach println
   }
-}
\ No newline at end of file
+}