fix #13332: change accessibility in whyNotGenericSum

Author: bishabosha

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

@@ -109,7 +109,7 @@ object SymUtils:
         def problem(child: Symbol) = {
 
           def isAccessible(sym: Symbol): Boolean =
-            (self.isContainedIn(sym) && (companionMirror || declScope.isContainedIn(sym)))
+            (self.isContainedIn(sym) && (companionMirror || sym.isAccessibleFrom(declScope.thisType)))
             || sym.is(Module) && isAccessible(sym.owner)
 
           if (child == self) "it has anonymous or inaccessible subclasses"

tests/pos/i13332.scala

@@ -0,0 +1,15 @@
+import scala.deriving.Mirror
+
+class Scope extends IListDefn {
+
+  type Of = Mirror { type MirroredType[X] = IList[X]; type MirroredMonoType = IList[Any] ; type MirroredElemTypes[_] <: Tuple }
+
+  val M = summon[Of]
+}
+
+
+trait IListDefn {
+  sealed trait IList[A]
+  case class INil[A]() extends IList[A]
+  case class ICons[A](h: A, t: IList[A]) extends IList[A]
+}