fix #10997: check accessible from sealed parent not companion

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

@@ -91,19 +91,18 @@ object SymUtils:
     *  It must satisfy the following conditions:
     *   - it has at least one child class or object
     *   - none of its children are anonymous classes
-    *   - all of its children are addressable through a path from its companion object
+    *   - all of its children are addressable through a path from the parent class
     *   - all of its children are generic products or singletons
     */
     def whyNotGenericSum(using Context): String =
       if (!self.is(Sealed))
         s"it is not a sealed ${self.kindString}"
       else {
         val children = self.children
-        val companion = self.linkedClass
         def problem(child: Symbol) = {
 
           def isAccessible(sym: Symbol): Boolean =
-            companion.isContainedIn(sym) || sym.is(Module) && isAccessible(sym.owner)
+            self.isContainedIn(sym) || sym.is(Module) && isAccessible(sym.owner)
 
           if (child == self) "it has anonymous or inaccessible subclasses"
           else if (!isAccessible(child.owner)) i"its child $child is not accessible"

tests/neg/i10997.scala

@@ -0,0 +1,9 @@
+sealed trait Parent
+
+trait Wrapper {
+
+  case class Foo(x: Int, y: Int, s: String) extends Parent
+  case class Bar(x: Int, y: Int) extends Parent
+
+  println(summon[deriving.Mirror.Of[Parent]]) // error
+}

tests/pos/i10997.scala

@@ -0,0 +1,24 @@
+class Test {
+
+  sealed trait Parent
+  case class Foo(x: Int, y: Int, s: String) extends Parent
+  case class Bar(x: Int, y: Int) extends Parent
+
+  println(summon[deriving.Mirror.Of[Parent]])
+}
+
+object Test2 {
+
+  case class Foo(x: Int, y: Int, s: String) extends i.Parent
+  case class Bar(x: Int, y: Int) extends i.Parent
+
+  val i = Inner()
+
+  class Inner {
+
+    sealed trait Parent
+
+    println(summon[deriving.Mirror.Of[Parent]])
+  }
+
+}

tests/run/deriving-constructor-order.scala

@@ -7,7 +7,7 @@ object Test extends App {
       case _: T => ()
     }
 
-  sealed trait Base1
+  sealed trait Base1 // Base1 MUST NOT have a companion here!
   case class Foo() extends Base1
   case object Bar extends Base1
   case class Qux(i: Int) extends Base1

tests/run/deriving.scala

@@ -4,7 +4,7 @@ object T
 case class A(x: Int, y: Int) extends T
 case object B extends T
 
-sealed trait U
+sealed trait U // U MUST NOT have a companion here!
 case class C() extends U
 
 object Test extends App {