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, 4 insertions, 0 deletions

diff --git a/test/files/neg/unchecked-impossible.check b/test/files/neg/unchecked-impossible.check
new file mode 100644
index 0000000000..0ab371dbaa
--- /dev/null
+++ b/test/files/neg/unchecked-impossible.check
@@ -0,0 +1,4 @@
+unchecked-impossible.scala:5: error: fruitless type test: a value of type T2[Int,Int] cannot also be a Seq[A]
+    case Seq(x) =>
+            ^
+one error found