SI-7294 Treat TupleN as final under -Xfuture

For the purposes of checkability warnings. This will
warn in case of:

	scala> (1, 2) match { case Seq() => 0; case _ => 1 }
	res9: Int = 1

Given how often Tuples are used as scrutinees, this is
a highly desirable place to warn.

I was orginally going to unlock this under -Xlint, and
could be easily convinced to go that way, given that
-Xfuture is a less popular option.

1 files changed, 6 insertions, 0 deletions

diff --git a/test/files/pos/t7294.scala b/test/files/pos/t7294.scala
new file mode 100644
index 0000000000..ccac2b1400
--- /dev/null
+++ b/test/files/pos/t7294.scala
@@ -0,0 +1,6 @@
+object Test {
+  // no fruitless warning as Tuple2 isn't (yet) final.
+  // The corresponding `neg` test will treat it as final
+  // for the purposes of these tests under -Xfuture.
+  (1, 2) match { case Seq() => 0; case _ => 1 }
+}