reproduce issue #13777

Author: bishabosha

sbt-test/scala3-backcompat/hierarchical-mirrors/app/Main.scala

@@ -0,0 +1,8 @@
+package app
+
+import scala.deriving.Mirror
+
+object Main:
+  def main(args: Array[String]): Unit =
+    val mirrorTop = summon[Mirror.SumOf[lib.Top]]
+    assert(mirrorTop.ordinal(lib.Middle()) == 0)

sbt-test/scala3-backcompat/hierarchical-mirrors/build.sbt

@@ -0,0 +1,7 @@
+lazy val lib = project.in(file("lib"))
+  .settings(
+    scalaVersion := "3.0.2"
+  )
+
+lazy val app = project.in(file("app"))
+  .dependsOn(lib)

sbt-test/scala3-backcompat/hierarchical-mirrors/lib/Top.scala

@@ -0,0 +1,7 @@
+package lib
+
+sealed trait Top
+object Top // companion is necessary
+
+case class Middle() extends Top with Bottom
+sealed trait Bottom extends Top

sbt-test/scala3-backcompat/hierarchical-mirrors/project/DottyInjectedPlugin.scala

@@ -0,0 +1,11 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := sys.props("plugin.scalaVersion")
+  )
+}

sbt-test/scala3-backcompat/hierarchical-mirrors/test

@@ -0,0 +1 @@
+> app/run