[Backport] Fix 13777: Tighten check to cache Mirror.Sum in companion

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

@@ -112,7 +112,19 @@ object SymUtils:
           self.isCoDefinedGiven(res.typeSymbol)
       self.isAllOf(Given | Method) && isCodefined(self.info)
 
-    def useCompanionAsMirror(using Context): Boolean = self.linkedClass.exists && !self.is(Scala2x)
+    def useCompanionAsSumMirror(using Context): Boolean =
+      self.linkedClass.exists
+      && !self.is(Scala2x)
+      && (
+        // If the sum type is compiled from source, and `self` is a "generic sum"
+        // then its companion object will become a sum mirror in `posttyper`. (This method
+        // can be called from `typer` when summoning a Mirror.)
+        // However if `self` is from a prior run then we should check that its companion subclasses `Mirror.Sum`.
+        // e.g. before Scala 3.1, hierarchical sum types were not considered "generic sums", so their
+        // companion would not cache the mirror. Companions from TASTy will already be typed as `Mirror.Sum`.
+        self.isDefinedInCurrentRun
+        || self.linkedClass.isSubClass(defn.Mirror_SumClass)
+      )
 
     /** Is this a sealed class or trait for which a sum mirror is generated?
     *  It must satisfy the following conditions:
@@ -129,7 +141,7 @@ object SymUtils:
         s"it is not an abstract class"
       else {
         val children = self.children
-        val companionMirror = self.useCompanionAsMirror
+        val companionMirror = self.useCompanionAsSumMirror
         assert(!(companionMirror && (declScope ne self.linkedClass)))
         def problem(child: Symbol) = {
 
@@ -144,7 +156,7 @@ object SymUtils:
             val s = child.whyNotGenericProduct
             if (s.isEmpty) s
             else if (child.is(Sealed)) {
-              val s = child.whyNotGenericSum(if child.useCompanionAsMirror then child.linkedClass else ctx.owner)
+              val s = child.whyNotGenericSum(if child.useCompanionAsSumMirror then child.linkedClass else ctx.owner)
               if (s.isEmpty) s
               else i"its child $child is not a generic sum because $s"
             } else i"its child $child is not a generic product because $s"

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -279,7 +279,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
   private def sumMirror(mirroredType: Type, formal: Type, span: Span)(using Context): Tree =
     val cls = mirroredType.classSymbol
-    val useCompanion = cls.useCompanionAsMirror
+    val useCompanion = cls.useCompanionAsSumMirror
 
     if cls.isGenericSum(if useCompanion then cls.linkedClass else ctx.owner) then
       val elemLabels = cls.children.map(c => ConstantType(Constant(c.name.toString)))

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