Add test case

Author: OlivierBlanvillain

tests/pos/9871.scala

@@ -0,0 +1,11 @@
+object Test {
+  type IsTypeInTuple[T, Tup <: Tuple] = Tup match {
+    case EmptyTuple => false
+    case T *: ts => true
+    case _ *: ts => IsTypeInTuple[T, ts]
+  }
+  summon[(Int *: String *: EmptyTuple) =:= (Int, String)] //they are the same
+  summon[IsTypeInTuple[String, Int *: String *: EmptyTuple] =:= true] //compiles
+  summon[IsTypeInTuple[String, (Int, String)] =:= true] //doesn't compile
+  summon[IsTypeInTuple[String, compiletime.Widen[Int *: String *: EmptyTuple]] =:= true] // doesn't compiles
+}