Revert to old definition if isSelfSym

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -702,8 +702,21 @@ object SymDenotations {
       flagsUNSAFE.is(Provisional) // do not force the info to check the flag
 
     /** Is this the denotation of a self symbol of some class?
-     */
-    final def isSelfSym(using Context): Boolean = is(SelfName)
+     *  This is the case if one of two conditions holds:
+     *  1. It is the symbol referred to in the selfInfo part of the ClassInfo
+     *     which is the type of this symbol's owner.
+     *  2. This symbol is owned by a class, it's selfInfo field refers to a type
+     *     (indicating the self definition does not introduce a name), and the
+     *     symbol's name is "_".
+     *  TODO: Find a more robust way to characterize self symbols, maybe by
+     *       spending a Flag on them?
+     */
+    final def isSelfSym(using Context): Boolean = owner.infoOrCompleter match {
+      case ClassInfo(_, _, _, _, selfInfo) =>
+        selfInfo == symbol ||
+          selfInfo.isInstanceOf[Type] && name == nme.WILDCARD
+      case _ => false
+    }
 
     /** Is this definition contained in `boundary`?
      *  Same as `ownersIterator contains boundary` but more efficient.