Fix #19808: Don't force to compute the owner of a symbol when there is no denotation (#19813)

Fix #19808: Don't force to compute the owner of a symbol when there is
no denotation.

Author: noti0na1

compiler/src/dotty/tools/dotc/typer/Nullables.scala

@@ -283,7 +283,7 @@ object Nullables:
     */
     def usedOutOfOrder(using Context): Boolean =
       val refSym = ref.symbol
-      val refOwner = refSym.owner
+      val refOwner = refSym.maybeOwner
 
       @tailrec def recur(s: Symbol): Boolean =
         s != NoSymbol

tests/explicit-nulls/pos/i19808.scala

@@ -0,0 +1,6 @@
+import scala.reflect.Selectable.reflectiveSelectable
+
+def saveRedir(what: {def validate: List[String]}) =
+  what.validate match
+    case Nil => ???
+    case xs  => ???

tests/pos/i19808.scala

@@ -0,0 +1,6 @@
+import scala.reflect.Selectable.reflectiveSelectable
+
+def saveRedir(what: {def validate: List[String]}) =
+  what.validate match
+    case Nil => ???
+    case xs  => ???