Add explanation for code

tests/init-global/warn/Color.scala

@@ -4,7 +4,25 @@ enum Color:
 enum Player:
   case Black, White
 
+  // Explanation: See the desugaring below
   val color: Color =
     if this == Player.Black  // warn
     then Color.Black
     else Color.White
+
+// From the desugaring of Player, we can see the field `Player.Black` is not yet
+// initialized during evaluation of the first `new Player`:
+//
+//     class Player:
+//         val color: Color =
+//            if this == Player.Black ...
+//
+//     object Player:
+//          val Black: Player = new Player     // <--- problem
+//          val White: Player = new Player
+//
+//
+//  The complex desugaring makes it difficult to see the initialization
+//  semantics and it is prone to make such hard-to-spot mistakes.
+//
+// Note: The desugaring above is simplified for presentation.