Fix-#4867 Update union tests to improve inference when ascribed

Splits union.scala test from negative to negative and positive tests,
exhibiting the preservation of the union when the union has been
explicitly ascribed as per fix #4867.

Author: landerlo

tests/neg/union.scala

@@ -3,26 +3,14 @@ object Test {
   class A
   class B extends A
   class C extends A
-  class D extends A
 
   val b = true
-  val x = if (b) new B else new C
-  val y: B | C = x  // error
-}
-
-object O {
-  class A
-  class B
-  def f[T](x: T, y: T): T = x
-
-  val x: A = f(new A { }, new A)
-
-  val y1: A | B = f(new A { }, new B) // error
-  val y2: A | B = f[A | B](new A { }, new B) // ok
 
-  val z = if (???) new A{} else new B
+  val x: B | C = if (b) new B else new C
 
-  val z1: A | B = z // error
+  val y:  B | C = x
+  val ok: B | C = y  // ok, as x and y are ascribed with unions
 
-  val z2: A | B = if (???) new A else new B // ok
+  val z = x
+  val error: B | C = z // error, as z is not ascribed with an union
 }

tests/pos/union.scala

@@ -0,0 +1,27 @@
+object Test {
+
+  class A
+  class B extends A
+  class C extends A
+  class D extends A
+
+  val b = true
+  val x: B | C = if (b) new B else new C
+  val y: B | C = x
+}
+
+object O {
+  class A
+  class B
+  def f[T](x: T, y: T): T = x
+
+  val x: A = f(new A { }, new A)
+
+  val y1: A | B = f(new A { }, new B)
+  val y2: A | B = f[A | B](new A { }, new B)
+
+  val z: A | B = if (???) new A{} else new B
+
+  val z1: A | B = z
+  val z2: A | B = if (???) new A else new B
+}