Nailed down the "impossible match" logic.

I will again defer to a comment.

/** Given classes A and B, can it be shown that nothing which is
 *  an A will ever be a subclass of something which is a B? This
 *  entails not only showing that !(A isSubClass B) but that the
 *  same is true of all their subclasses.  Restated for symmetry:
 *  the same value cannot be a member of both A and B.
 *
 *   1) A must not be a subclass of B, nor B of A (the trivial check)
 *   2) One of A or B must be completely knowable (see isKnowable)
 *   3) Assuming A is knowable, the proposition is true if
 *      !(A' isSubClass B) for all A', where A' is a subclass of A.
 *
 *  Due to symmetry, the last condition applies as well in reverse.
 */

1 files changed, 10 insertions, 0 deletions

diff --git a/test/files/neg/patmat-type-check.check b/test/files/neg/patmat-type-check.check
index e045841ce1..721217c314 100644
--- a/test/files/neg/patmat-type-check.check
+++ b/test/files/neg/patmat-type-check.check
@@ -1,3 +1,12 @@
+patmat-type-check.scala:11: warning: fruitless type test: a value of type Test.Bop4[T] cannot also be a Seq[A]
+  def s3[T](x: Bop4[T]) = x match { case Seq('b', 'o', 'b') => true }
+                                            ^
+patmat-type-check.scala:15: warning: fruitless type test: a value of type Test.Bop5[_$1,T1,T2] cannot also be a Seq[A]
+  def s4[T1, T2](x: Bop5[_, T1, T2]) = x match { case Seq('b', 'o', 'b') => true }
+                                                         ^
+patmat-type-check.scala:19: warning: fruitless type test: a value of type Test.Bop3[T] cannot also be a Seq[A]
+  def f4[T](x: Bop3[T]) = x match { case Seq('b', 'o', 'b') => true }
+                                            ^
 patmat-type-check.scala:22: error: scrutinee is incompatible with pattern type;
  found   : Seq[A]
  required: String
@@ -18,4 +27,5 @@ patmat-type-check.scala:30: error: scrutinee is incompatible with pattern type;
  required: Test.Bop3[Char]
   def f4[T](x: Bop3[Char]) = x match { case Seq('b', 'o', 'b') => true } // fail
                                                ^
+three warnings found
 four errors found