add regression test

Author: Decel

tests/pos/i17186.scala

@@ -0,0 +1,5 @@
+type Second[X <: Tuple2[Any, Any]] = Tuple.Head[Tuple.Tail[X]]
+val a = implicitly[Second[Tuple2[Int, String]] =:= String]
+
+type Middle[X <: Tuple2[Any, Any]] = Tuple.Tail[Tuple.Tail[X]]
+val b = implicitly[Middle[Tuple2[Int, String]] =:= EmptyTuple]