Make ClassicActorSystemProvider.classicSystem public API

See @28818

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/ActorSystemStub.scala

@@ -66,7 +66,7 @@ import org.slf4j.LoggerFactory
   override def provider: ActorRefProvider = throw new UnsupportedOperationException("no provider")
 
   // stream materialization etc. using stub not supported
-  override private[akka] def classicSystem =
+  override def classicSystem =
     throw new UnsupportedOperationException("no classic actor system available")
 
   // impl InternalRecipientRef

akka-actor-typed/src/main/scala/akka/actor/typed/internal/adapter/ActorSystemAdapter.scala

@@ -51,7 +51,7 @@ import org.slf4j.{ Logger, LoggerFactory }
 
   import ActorRefAdapter.sendSystemMessage
 
-  override private[akka] def classicSystem: classic.ActorSystem = system
+  override def classicSystem: classic.ActorSystem = system
 
   // Members declared in akka.actor.typed.ActorRef
   override def tell(msg: T): Unit = {
@@ -155,12 +155,5 @@ private[akka] object ActorSystemAdapter {
       new LoadTypedExtensions(system)
   }
 
-  def toClassic[U](sys: ActorSystem[_]): classic.ActorSystem =
-    sys match {
-      case adapter: ActorSystemAdapter[_] => adapter.classicSystem
-      case _ =>
-        throw new UnsupportedOperationException(
-          "Only adapted classic ActorSystem permissible " +
-          s"($sys of class ${sys.getClass.getName})")
-    }
+  def toClassic[U](sys: ActorSystem[_]): classic.ActorSystem = sys.classicSystem
 }

akka-actor-typed/src/main/scala/akka/actor/typed/scaladsl/adapter/package.scala

@@ -74,7 +74,7 @@ package object adapter {
    * Extension methods added to [[akka.actor.typed.ActorSystem]].
    */
   implicit class TypedActorSystemOps(val sys: ActorSystem[_]) extends AnyVal {
-    def toClassic: akka.actor.ActorSystem = ActorSystemAdapter.toClassic(sys)
+    def toClassic: akka.actor.ActorSystem = sys.classicSystem
 
     /**
      * INTERNAL API

akka-actor/src/main/scala/akka/actor/ActorSystem.scala

@@ -976,7 +976,7 @@ private[akka] class ActorSystemImpl(
   def /(actorName: String): ActorPath = guardian.path / actorName
   def /(path: Iterable[String]): ActorPath = guardian.path / path
 
-  override private[akka] def classicSystem: ActorSystem = this
+  override def classicSystem: ActorSystem = this
 
   // Used for ManifestInfo.checkSameVersion
   private def allModules: List[String] =

akka-actor/src/main/scala/akka/actor/ClassicActorSystemProvider.scala

@@ -15,9 +15,10 @@ import akka.annotation.InternalApi
 @DoNotInherit
 trait ClassicActorSystemProvider {
 
-  /** INTERNAL API */
-  @InternalApi
-  private[akka] def classicSystem: ActorSystem
+  /**
+   * Allows access to the classic `akka.actor.ActorSystem` even for `akka.actor.typed.ActorSystem[_]`s.
+   */
+  def classicSystem: ActorSystem
 }
 
 /**