Use classSymbols method

Co-authored-by: Jamie Thompson <bishbashboshjt@gmail.com>
[Cherry-picked 5b2ec61][modified]

Author: WojciechMazur

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -173,7 +173,7 @@ object SymUtils:
       else {
         val children = self.children
         val companionMirror = self.useCompanionAsSumMirror
-        val ownerScope = if pre.isInstanceOf[SingletonType] then pre.parentSymbols(_.isClass) else Nil
+        val ownerScope = if pre.isInstanceOf[SingletonType] then pre.classSymbols else Nil
         def problem(child: Symbol) = {
 
           def accessibleMessage(sym: Symbol): String =