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, 1 insertions, 0 deletions

diff --git a/test/files/neg/t7294.flags b/test/files/neg/t7294.flags
new file mode 100644
index 0000000000..3f3381a45b
--- /dev/null
+++ b/test/files/neg/t7294.flags
@@ -0,0 +1 @@
+-Xfuture -Xfatal-warnings