PR review suggestions.

akka-actor-tests/src/test/scala/akka/actor/ActorPathSpec.scala

@@ -89,10 +89,15 @@ class ActorPathSpec extends WordSpec with Matchers {
       ActorPath.fromString("akka://mysys/user/boom/*")
     }
 
-    "fail fast if invalid chars in host names when not using AddressFromURIString, e.g. docker host given name" in {
+    "detect invalid chars in host names when not using AddressFromURIString, e.g. docker host given name" in {
       Address("akka", "sys", Some("valid"), Some(0)).hasInvalidHostCharacters shouldBe false
       Address("akka", "sys", Some("in_valid"), Some(0)).hasInvalidHostCharacters shouldBe true
       intercept[MalformedURLException](AddressFromURIString("akka://sys@in_valid:5001"))
     }
+
+    "fail fast if the check is called when invalid chars are in host names" in {
+      Address("akka", "sys", Some("localhost"), Some(0)).checkHostCharacters()
+      intercept[IllegalArgumentException](Address("akka", "sys", Some("in_valid"), Some(0)).checkHostCharacters())
+    }
   }
 }

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

@@ -69,12 +69,18 @@ final case class Address private (protocol: String, system: String, host: Option
    */
   def hostPort: String = toString.substring(protocol.length + 3)
 
-  /** INTERNAL API
+      /** INTERNAL API
    * Check if the address is not created through `AddressFromURIString`, if there
    * are any unusual characters in the host string.
    */
   @InternalApi
   private[akka] def hasInvalidHostCharacters: Boolean = host.exists(_.contains("_"))
+
+  /** INTERNAL API */
+  @InternalApi
+  private[akka] def checkHostCharacters(): Unit =
+    require(!hasInvalidHostCharacters, s"Using invalid host characters '$host' in the Address is not allowed.")
+
 }
 
 object Address {

akka-cluster-typed/src/main/scala/akka/cluster/typed/Cluster.scala

@@ -81,7 +81,9 @@ sealed trait ClusterCommand
  * The name of the [[akka.actor.ActorSystem]] must be the same for all members of a
  * cluster.
  */
-final case class Join(address: Address) extends ClusterCommand
+final case class Join(address: Address) extends ClusterCommand {
+  address.checkHostCharacters()
+}
 
 object Join {
 
@@ -100,6 +102,7 @@ object Join {
  * cluster or to join the same cluster again.
  */
 final case class JoinSeedNodes(seedNodes: immutable.Seq[Address]) extends ClusterCommand {
+  seedNodes.foreach(_.checkHostCharacters())
 
   /**
    * Java API: Join the specified seed nodes without defining them in config.

akka-cluster-typed/src/test/scala/akka/cluster/typed/ClusterApiSpec.scala

@@ -4,6 +4,7 @@
 
 package akka.cluster.typed
 
+import akka.actor.Address
 import akka.actor.typed.scaladsl.adapter._
 import akka.cluster.ClusterEvent._
 import akka.cluster.MemberStatus
@@ -43,6 +44,12 @@ class ClusterApiSpec extends ScalaTestWithActorTestKit(ClusterApiSpec.config) wi
 
   "A typed Cluster" must {
 
+    "fail fast in a join attempt if invalid chars are in host names, e.g. docker host given name" in {
+      val address = Address("akka", "sys", Some("in_valid"), Some(0))
+      intercept[IllegalArgumentException](Join(address))
+      intercept[IllegalArgumentException](JoinSeedNodes(scala.collection.immutable.Seq(address)))
+    }
+
     "join a cluster and observe events from both sides" in {
 
       val system2 = akka.actor.ActorSystem(system.name, system.settings.config)

akka-cluster/src/main/scala/akka/cluster/Cluster.scala

@@ -301,7 +301,7 @@ class Cluster(val system: ExtendedActorSystem) extends Extension {
    * cluster.
    */
   def join(address: Address): Unit = {
-    checkHostCharacters(address)
+    address.checkHostCharacters()
     clusterCore ! ClusterUserAction.JoinTo(fillLocal(address))
   }
 
@@ -320,7 +320,7 @@ class Cluster(val system: ExtendedActorSystem) extends Extension {
    * cluster or to join the same cluster again.
    */
   def joinSeedNodes(seedNodes: immutable.Seq[Address]): Unit = {
-    seedNodes.foreach(checkHostCharacters)
+    seedNodes.foreach(_.checkHostCharacters())
     clusterCore ! InternalClusterAction.JoinSeedNodes(seedNodes.toVector.map(fillLocal))
   }
 
@@ -337,11 +337,6 @@ class Cluster(val system: ExtendedActorSystem) extends Extension {
   def joinSeedNodes(seedNodes: java.util.List[Address]): Unit =
     joinSeedNodes(Util.immutableSeq(seedNodes))
 
-  private def checkHostCharacters(address: Address): Unit =
-    require(
-      !address.hasInvalidHostCharacters,
-      s"Joining the cluster using invalid host characters '${address.host}' in the Address is not allowed.")
-
   /**
    * Send command to issue state transition to LEAVING for the node specified by 'address'.
    * The member will go through the status changes [[MemberStatus]] `Leaving` (not published to