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, 42 insertions, 18 deletions

diff --git a/test/files/neg/unchecked2.check b/test/files/neg/unchecked2.check
index e37865928e..0ff2a249a8 100644
--- a/test/files/neg/unchecked2.check
+++ b/test/files/neg/unchecked2.check
@@ -1,19 +1,43 @@
-unchecked2.scala:2: error: non-variable type argument Int in type Option[Int] is unchecked since it is eliminated by erasure
-  Some(123).isInstanceOf[Option[Int]]
-                        ^
-unchecked2.scala:3: error: non-variable type argument String in type Option[String] is unchecked since it is eliminated by erasure
-  Some(123).isInstanceOf[Option[String]]
-                        ^
 unchecked2.scala:4: error: non-variable type argument List[String] in type Option[List[String]] is unchecked since it is eliminated by erasure
-  Some(123).isInstanceOf[Option[List[String]]]
-                        ^
-unchecked2.scala:5: error: non-variable type argument List[Int => String] in type Option[List[Int => String]] is unchecked since it is eliminated by erasure
-  Some(123).isInstanceOf[Option[List[Int => String]]]
-                        ^
-unchecked2.scala:6: error: non-variable type argument (String, Double) in type Option[(String, Double)] is unchecked since it is eliminated by erasure
-  Some(123).isInstanceOf[Option[(String, Double)]]
-                        ^
-unchecked2.scala:7: error: non-variable type argument String => Double in type Option[String => Double] is unchecked since it is eliminated by erasure
-  Some(123).isInstanceOf[Option[String => Double]]
-                        ^
-6 errors found
+  /*   warn */ Some(List(1)).isInstanceOf[Option[List[String]]]
+                                         ^
+unchecked2.scala:5: error: non-variable type argument Option[_] in type Option[Option[_]] is unchecked since it is eliminated by erasure
+  /*   warn */ Some(123).isInstanceOf[Option[Option[_]]]
+                                     ^
+unchecked2.scala:6: error: non-variable type argument String in type Option[String] is unchecked since it is eliminated by erasure
+  /*   warn */ Some(123).isInstanceOf[Option[String]]
+                                     ^
+unchecked2.scala:7: error: non-variable type argument List[String] in type Option[List[String]] is unchecked since it is eliminated by erasure
+  /*   warn */ Some(123).isInstanceOf[Option[List[String]]]
+                                     ^
+unchecked2.scala:8: error: non-variable type argument List[Int => String] in type Option[List[Int => String]] is unchecked since it is eliminated by erasure
+  /*   warn */ Some(123).isInstanceOf[Option[List[Int => String]]]
+                                     ^
+unchecked2.scala:9: error: non-variable type argument (String, Double) in type Option[(String, Double)] is unchecked since it is eliminated by erasure
+  /*   warn */ Some(123).isInstanceOf[Option[(String, Double)]]
+                                     ^
+unchecked2.scala:10: error: non-variable type argument String => Double in type Option[String => Double] is unchecked since it is eliminated by erasure
+  /*   warn */ Some(123).isInstanceOf[Option[String => Double]]
+                                     ^
+unchecked2.scala:14: error: non-variable type argument List[String] in type Option[List[String]] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(List(1)): Any).isInstanceOf[Option[List[String]]]
+                                                ^
+unchecked2.scala:15: error: non-variable type argument Int in type Option[Int] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(123): Any).isInstanceOf[Option[Int]]
+                                            ^
+unchecked2.scala:16: error: non-variable type argument String in type Option[String] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(123): Any).isInstanceOf[Option[String]]
+                                            ^
+unchecked2.scala:17: error: non-variable type argument List[String] in type Option[List[String]] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(123): Any).isInstanceOf[Option[List[String]]]
+                                            ^
+unchecked2.scala:18: error: non-variable type argument List[Int => String] in type Option[List[Int => String]] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(123): Any).isInstanceOf[Option[List[Int => String]]]
+                                            ^
+unchecked2.scala:19: error: non-variable type argument (String, Double) in type Option[(String, Double)] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(123): Any).isInstanceOf[Option[(String, Double)]]
+                                            ^
+unchecked2.scala:20: error: non-variable type argument String => Double in type Option[String => Double] is unchecked since it is eliminated by erasure
+  /*   warn */ (Some(123): Any).isInstanceOf[Option[String => Double]]
+                                            ^
+14 errors found