Fix-scala#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 scala#4867.

Author: landerlo

tests/neg/union.scala

@@ -6,23 +6,12 @@ object Test {
   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  // Explicit union ascription is kept
+  val ok: B | C = y  // ok
 
-  val z2: A | B = if (???) new A else new B // ok
+  val z = x // We lose the Union ascription as it's not explicit
+  val error: B | C = z // error as z has lost union ascription
 }

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
+}