Exhaustive whens without else and 'Nothing' as the result are considered 'implicit exhaustive'

compiler/testData/diagnostics/tests/sealed/TreeWhen.kt

@@ -4,10 +4,10 @@ sealed class Tree {
     class Node(val left: Tree, val right: Tree): Tree()
 
     fun max(): Int {
-        when(this) {
+        <!DEBUG_INFO_IMPLICIT_EXHAUSTIVE!>when(this) {
             is Empty -> return -1
             is Leaf -> return <!DEBUG_INFO_SMARTCAST!>this<!>.x
             is Node -> return <!DEBUG_INFO_SMARTCAST!>this<!>.left.max()
-        }
+        }<!>
     }
 }