test: add in a regression test for scala#14642 (scala#17512)

[skip community_build]

closes scala#14642

Author: nicolasstucki

tests/pos/i14642.scala

@@ -0,0 +1,16 @@
+// https://github.com/lampepfl/dotty/issues/14642
+case object A
+case class B()
+case class C()
+type Union = A.type | B | C
+val a: List[A.type] = ???
+val b: List[B] = ???
+val c: List[C] = ???
+val l1: List[Union] = a ++ b
+val l2: List[Union] =
+  a ++ b ++ c
+val l3: List[Union] =
+  (a: List[
+    Union
+  ]) ++ b ++ c
+val l4: List[Union] = (a: List[Union]) ++ (b ++ c)