Add GADT test for upcasting

Author: Linyxus

tests/pos/gadt-upcast.scala

@@ -0,0 +1,14 @@
+trait TagA[A]
+trait TagB[B]
+trait TagC[C]
+trait TriTag[A, B, C] extends TagA[A] with TagB[B] with TagC[C]
+class IntStrCharTag extends TagA[Int] with TagB[String] with TagC[Char]
+
+def get[A, B, C]: TriTag[A, B, C] => (A, B, C) = {
+ case _: IntStrCharTag => (0, "zero", '0')
+}
+
+object GadtUpcast extends App {
+  val ret = get(new IntStrCharTag with TriTag[Int, String, Char])
+  println(ret)
+}