+str #15960 Correction of FlowGraph variance

akka-stream-tests/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala

@@ -6,8 +6,16 @@ package akka.stream.scaladsl2
 import akka.stream.testkit.AkkaSpec
 import akka.stream.Transformer
 import akka.stream.OverflowStrategy
+import akka.stream.testkit.StreamTestKit.SubscriberProbe
+import akka.stream.testkit.StreamTestKit.PublisherProbe
+
+object FlowGraphCompileSpec {
+  class Fruit
+  class Apple extends Fruit
+}
 
 class FlowGraphCompileSpec extends AkkaSpec {
+  import FlowGraphCompileSpec._
 
   implicit val mat = FlowMaterializer()
 
@@ -273,5 +281,49 @@ class FlowGraphCompileSpec extends AkkaSpec {
       }.getMessage should include("at most 1 outgoing")
     }
 
+    "build with variance" in {
+      val out = SubscriberSink(SubscriberProbe[Fruit]())
+      FlowGraph { b ⇒
+        val merge = Merge[Fruit]
+        b.
+          addEdge(FlowFrom[Fruit](() ⇒ Some(new Apple)), merge).
+          addEdge(FlowFrom[Apple](() ⇒ Some(new Apple)), merge).
+          addEdge(merge, FlowFrom[Fruit].map(identity), out)
+      }
+    }
+
+    "build with implicits and variance" in {
+      PartialFlowGraph { implicit b ⇒
+        val inA = PublisherSource(PublisherProbe[Fruit]())
+        val inB = PublisherSource(PublisherProbe[Apple]())
+        val outA = SubscriberSink(SubscriberProbe[Fruit]())
+        val outB = SubscriberSink(SubscriberProbe[Fruit]())
+        val merge = Merge[Fruit]
+        import FlowGraphImplicits._
+        FlowFrom[Fruit](() ⇒ Some(new Apple)) ~> merge
+        FlowFrom[Apple](() ⇒ Some(new Apple)) ~> merge
+        inA ~> merge
+        inB ~> merge
+        inA ~> FlowFrom[Fruit].map(identity) ~> merge
+        inB ~> FlowFrom[Apple].map(identity) ~> merge
+        UndefinedSource[Apple] ~> merge
+        UndefinedSource[Apple] ~> FlowFrom[Fruit].map(identity) ~> merge
+        UndefinedSource[Apple] ~> FlowFrom[Apple].map(identity) ~> merge
+        merge ~> FlowFrom[Fruit].map(identity) ~> outA
+
+        FlowFrom[Apple](() ⇒ Some(new Apple)) ~> Broadcast[Apple] ~> merge
+        FlowFrom[Apple](() ⇒ Some(new Apple)) ~> Broadcast[Apple] ~> outB
+        FlowFrom[Apple](() ⇒ Some(new Apple)) ~> Broadcast[Apple] ~> UndefinedSink[Fruit]
+        inB ~> Broadcast[Apple] ~> merge
+
+        "UndefinedSource[Fruit] ~> FlowFrom[Apple].map(identity) ~> merge" shouldNot compile
+        "UndefinedSource[Fruit] ~> Broadcast[Apple]" shouldNot compile
+        "merge ~> Broadcast[Apple]" shouldNot compile
+        "merge ~> FlowFrom[Fruit].map(identity) ~> Broadcast[Apple]" shouldNot compile
+        "inB ~> merge ~> Broadcast[Apple]" shouldNot compile
+        "inA ~> Broadcast[Apple]" shouldNot compile
+      }
+    }
+
   }
 }

akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala

@@ -16,7 +16,7 @@ import akka.stream.impl2.Ast
  * Fan-in and fan-out vertices in the [[FlowGraph]] implements
  * this marker interface. Edges may end at a `JunctionInPort`.
  */
-sealed trait JunctionInPort[T] {
+sealed trait JunctionInPort[-T] {
   private[akka] def port: Int = FlowGraphInternal.UnlabeledPort
   private[akka] def vertex: FlowGraphInternal.Vertex
   type NextT
@@ -27,7 +27,7 @@ sealed trait JunctionInPort[T] {
  * Fan-in and fan-out vertices in the [[FlowGraph]] implements
  * this marker interface. Edges may start at a `JunctionOutPort`.
  */
-sealed trait JunctionOutPort[T] {
+sealed trait JunctionOutPort[+T] {
   private[akka] def port: Int = FlowGraphInternal.UnlabeledPort
   private[akka] def vertex: FlowGraphInternal.Vertex
 }
@@ -226,7 +226,7 @@ object UndefinedSink {
  * yet by using this placeholder instead of the real [[Sink]]. Later the placeholder can
  * be replaced with [[FlowGraphBuilder#attachSink]].
  */
-final class UndefinedSink[T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex {
+final class UndefinedSink[-T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex {
   override def minimumInputCount: Int = 1
   override def maximumInputCount: Int = 1
   override def minimumOutputCount: Int = 0
@@ -256,7 +256,7 @@ object UndefinedSource {
  * yet by using this placeholder instead of the real [[Source]]. Later the placeholder can
  * be replaced with [[FlowGraphBuilder#attachSource]].
  */
-final class UndefinedSource[T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex {
+final class UndefinedSource[+T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex {
   override def minimumInputCount: Int = 0
   override def maximumInputCount: Int = 0
   override def minimumOutputCount: Int = 1