diff --git a/akka-docs-dev/rst/java/code/docs/MigrationsJava.java b/akka-docs-dev/rst/java/code/docs/MigrationsJava.java
index 1a61b92a3b..a2c3b0eefe 100644
--- a/akka-docs-dev/rst/java/code/docs/MigrationsJava.java
+++ b/akka-docs-dev/rst/java/code/docs/MigrationsJava.java
@@ -1,6 +1,7 @@
 package docs;
 
 import akka.japi.Pair;
+import akka.japi.function.Function;
 import akka.stream.*;
 import akka.stream.javadsl.*;
 import scala.Option;
@@ -64,7 +65,7 @@ public class MigrationsJava {
 
             FlowGraph.create(builder -> {
                 //...
-                return new FlowShape(inlet, outlet);
+                return new FlowShape<>(inlet, outlet);
             });
             //#graph-create
         }
@@ -117,9 +118,14 @@ public class MigrationsJava {
         Flow<Integer, Integer, BoxedUnit> emptyFlow2 = Flow.of(Integer.class);
         //#empty-flow
 
-        //#flattenConcat
-        Flow.<Source<Integer, BoxedUnit>>create().flattenConcat();
-        //#flattenConcat
+        //#flatMapConcat
+        Flow.<Source<Integer, BoxedUnit>>create().
+          <Integer>flatMapConcat(new Function<Source<Integer, BoxedUnit>, Source<Integer, ?>>(){
+            @Override public Source<Integer, ?> apply(Source<Integer, BoxedUnit> param) throws Exception {
+              return param;
+            }
+          });
+        //#flatMapConcat
     }
 
 }
diff --git a/akka-docs-dev/rst/java/migration-guide-1.0-2.x-java.rst b/akka-docs-dev/rst/java/migration-guide-1.0-2.x-java.rst
index c83b7e565c..e081d33f65 100644
--- a/akka-docs-dev/rst/java/migration-guide-1.0-2.x-java.rst
+++ b/akka-docs-dev/rst/java/migration-guide-1.0-2.x-java.rst
@@ -263,12 +263,13 @@ should be replaced by
 ====================================================================
 
 To simplify type inference in Java 8 and to make the method more discoverable, ``flatten(FlattenStrategy.concat)``
-has been removed and replaced with the alternative method ``flatten(FlattenStrategy.concat)``.
+has been removed and replaced with the alternative method ``flatMapConcat(f)``.
 
 Update procedure
 ----------------
 
-1. Replace all occurences of ``flatten(FlattenStrategy.concat)`` with ``flattenConcat()``
+1. Replace all occurrences of ``flatten(FlattenStrategy.concat)`` with ``flatMapConcat(identity)``
+2. Consider replacing ``map(f).flatMapConcat(identity)`` with ``flatMapConcat(f)``
 
 Example
 ^^^^^^^
@@ -279,7 +280,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/MigrationsJava.java#flattenConcat
+.. includecode:: code/docs/MigrationsJava.java#flatMapConcat
 
 FlexiMerge an FlexiRoute has been replaced by GraphStage
 ========================================================
diff --git a/akka-docs-dev/rst/scala/code/docs/Migrations.scala b/akka-docs-dev/rst/scala/code/docs/MigrationsScala.scala
similarity index 97%
rename from akka-docs-dev/rst/scala/code/docs/Migrations.scala
rename to akka-docs-dev/rst/scala/code/docs/MigrationsScala.scala
index 5c17569633..12f770abe6 100644
--- a/akka-docs-dev/rst/scala/code/docs/Migrations.scala
+++ b/akka-docs-dev/rst/scala/code/docs/MigrationsScala.scala
@@ -9,7 +9,7 @@ import scala.concurrent.{ Future, ExecutionContext, Promise }
 import scala.concurrent.duration._
 import scala.util.{ Failure, Success, Try }
 
-class Migrations extends AkkaSpec {
+class MigrationsScala extends AkkaSpec {
 
   "Examples in migration guide" must {
     "compile" in {
@@ -110,10 +110,9 @@ class Migrations extends AkkaSpec {
         val ticks = Source(1.second, 3.seconds, "tick")
         //#source-creators
 
-        //#flatten
-        // Please note that the parenthesis is mandatory due to implicit parameters
-        Flow[Source[Int, Any]].flattenConcat()
-        //#flatten
+        //#flatMapConcat
+        Flow[Source[Int, Any]].flatMapConcat(identity)
+        //#flatMapConcat
 
         //#port-async
         class MapAsyncOne[In, Out](f: In ⇒ Future[Out])(implicit ec: ExecutionContext)
diff --git a/akka-docs-dev/rst/scala/migration-guide-1.0-2.x-scala.rst b/akka-docs-dev/rst/scala/migration-guide-1.0-2.x-scala.rst
index 4758055cbe..7b516ed45d 100644
--- a/akka-docs-dev/rst/scala/migration-guide-1.0-2.x-scala.rst
+++ b/akka-docs-dev/rst/scala/migration-guide-1.0-2.x-scala.rst
@@ -71,7 +71,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#flow-wrap
+.. includecode:: code/docs/MigrationsScala.scala#flow-wrap
 
 and
 
@@ -90,7 +90,7 @@ and
 
 Should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#bidiflow-wrap
+.. includecode:: code/docs/MigrationsScala.scala#bidiflow-wrap
 
 FlowGraph builder methods have been renamed
 ===========================================
@@ -123,7 +123,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#graph-create
+.. includecode:: code/docs/MigrationsScala.scala#graph-create
 
 Methods that create Source, Sink, Flow from Graphs have been removed
 ====================================================================
@@ -180,7 +180,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#graph-create-2
+.. includecode:: code/docs/MigrationsScala.scala#graph-create-2
 
 Several Graph builder methods have been removed
 ===============================================
@@ -213,7 +213,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#graph-edges
+.. includecode:: code/docs/MigrationsScala.scala#graph-edges
 
 Source constructor name changes
 ===============================
@@ -249,7 +249,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#source-creators
+.. includecode:: code/docs/MigrationsScala.scala#source-creators
 
 ``flatten(FlattenStrategy)`` has been replaced by named counterparts
 ====================================================================
@@ -260,7 +260,8 @@ has been removed and replaced with the alternative method ``flatten(FlattenStrat
 Update procedure
 ----------------
 
-1. Replace all occurences of ``flatten(FlattenStrategy.concat)`` with ``flattenConcat()``
+1. Replace all occurrences of ``flatten(FlattenStrategy.concat)`` with ``flatMapConcat(identity)``
+2. Consider replacing all occurrences of ``map(f).flatMapConcat(identity)`` with ``flatMapConcat(f)``
 
 Example
 ^^^^^^^
@@ -272,7 +273,7 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#flatten
+.. includecode:: code/docs/MigrationsScala.scala#flatMapConcat
 
 FlexiMerge an FlexiRoute has been replaced by GraphStage
 ========================================================
@@ -408,4 +409,4 @@ Example
 
 should be replaced by
 
-.. includecode:: code/docs/Migrations.scala#port-async
\ No newline at end of file
+.. includecode:: code/docs/MigrationsScala.scala#port-async
\ No newline at end of file
diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala
index 950ef41e14..ccd0af28a3 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala
@@ -60,8 +60,7 @@ private[http] object OutgoingConnectionBlueprint {
 
     val requestRendering: Flow[HttpRequest, ByteString, Unit] = Flow[HttpRequest]
       .map(RequestRenderingContext(_, hostHeader))
-      .via(Flow[RequestRenderingContext].map(requestRendererFactory.renderToSource).named("renderer"))
-      .flattenConcat()
+      .via(Flow[RequestRenderingContext].flatMapConcat(requestRendererFactory.renderToSource).named("renderer"))
 
     val methodBypass = Flow[HttpRequest].map(_.method)
 
diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
index ea12116193..71346f4b4c 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
@@ -20,6 +20,7 @@ import akka.http.impl.util._
 import akka.http.scaladsl.Http
 import akka.http.scaladsl.model._
 import akka.stream._
+import akka.stream.impl.ConstantFun
 import akka.stream.io._
 import akka.stream.scaladsl._
 import akka.stream.stage._
@@ -108,7 +109,7 @@ private[http] object HttpServerBluePrint {
       Flow[ResponseRenderingContext]
         .via(Flow[ResponseRenderingContext].transform(() ⇒ new ErrorsTo500ResponseRecovery(log)).named("recover")) // FIXME: simplify after #16394 is closed
         .via(Flow[ResponseRenderingContext].transform(() ⇒ responseRendererFactory.newRenderer).named("renderer"))
-        .flattenConcat()
+        .flatMapConcat(ConstantFun.scalaIdentityFunction)
         .via(Flow[ResponseRenderingOutput].transform(() ⇒ errorLogger(log, "Outgoing response stream error")).named("errorLogger"))
 
     BidiFlow.fromGraph(FlowGraph.create(requestParsingFlow, rendererPipeline, oneHundredContinueSource)((_, _, _) ⇒ ()) { implicit b ⇒
diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala
index 707db8a92b..4bc7f6191a 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala
@@ -27,11 +27,11 @@ private[http] object MessageToFrameRenderer {
         Source.single(FrameEvent.emptyLastContinuationFrame)
 
     Flow[Message]
-      .map {
+      .flatMapConcat {
         case BinaryMessage.Strict(data) ⇒ strictFrames(Opcode.Binary, data)
         case bm: BinaryMessage          ⇒ streamedFrames(Opcode.Binary, bm.dataStream)
         case TextMessage.Strict(text)   ⇒ strictFrames(Opcode.Text, ByteString(text, "UTF-8"))
         case tm: TextMessage            ⇒ streamedFrames(Opcode.Text, tm.textStream.transform(() ⇒ new Utf8Encoder))
-      }.flattenConcat()
+      }
   }
 }
diff --git a/akka-http-core/src/main/scala/akka/http/impl/util/package.scala b/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
index c2acb0aae7..fdff173f60 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
@@ -49,12 +49,11 @@ package object util {
 
   private[http] def headAndTailFlow[T]: Flow[Source[T, Any], (T, Source[T, Unit]), Unit] =
     Flow[Source[T, Any]]
-      .map {
+      .flatMapConcat {
         _.prefixAndTail(1)
           .filter(_._1.nonEmpty)
           .map { case (prefix, tail) ⇒ (prefix.head, tail) }
       }
-      .flattenConcat()
 
   private[http] def printEvent[T](marker: String): Flow[T, T, Unit] =
     Flow[T].transform(() ⇒ new PushPullStage[T, T] {
diff --git a/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala b/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala
index a58d8e13df..cff907f653 100644
--- a/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala
+++ b/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala
@@ -7,6 +7,7 @@ package akka.http.scaladsl.model
 import java.io.File
 
 import akka.event.{ NoLogging, LoggingAdapter }
+import akka.stream.impl.ConstantFun
 
 import scala.collection.immutable.VectorBuilder
 import scala.concurrent.duration.FiniteDuration
@@ -40,7 +41,7 @@ sealed trait Multipart {
     val chunks =
       parts
         .transform(() ⇒ BodyPartRenderer.streamed(boundary, charset.nioCharset, partHeadersSizeHint = 128, log))
-        .flattenConcat()
+        .flatMapConcat(ConstantFun.scalaIdentityFunction)
     HttpEntity.Chunked(mediaType withBoundary boundary, chunks)
   }
 }
diff --git a/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala b/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala
index ad2ed0f687..4656912798 100644
--- a/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala
+++ b/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala
@@ -531,7 +531,7 @@ class RequestParserSpec extends FreeSpec with Matchers with BeforeAndAfterAll {
             Right(HttpRequest(method, uri, headers, createEntity(entityParts), protocol))
           case (x @ (MessageStartError(_, _) | EntityStreamError(_)), _) ⇒ Left(x)
         }
-        .map { x ⇒
+        .flatMapConcat { x ⇒
           Source {
             x match {
               case Right(request) ⇒ compactEntity(request.entity).fast.map(x ⇒ Right(request.withEntity(x)))
@@ -539,7 +539,6 @@ class RequestParserSpec extends FreeSpec with Matchers with BeforeAndAfterAll {
             }
           }
         }
-        .flattenConcat()
         .map(strictEqualify)
         .grouped(100000).runWith(Sink.head)
         .awaitResult(awaitAtMost)
diff --git a/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala b/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala
index 1fa849f442..94fe7192be 100644
--- a/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala
+++ b/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala
@@ -3,6 +3,7 @@
  */
 package akka.stream.tck
 
+import akka.stream.impl.ConstantFun
 import akka.stream.scaladsl.Sink
 import akka.stream.scaladsl.Source
 import org.reactivestreams.Publisher
@@ -12,7 +13,7 @@ class FlattenTest extends AkkaPublisherVerification[Int] {
   def createPublisher(elements: Long): Publisher[Int] = {
     val s1 = Source(iterable(elements / 2))
     val s2 = Source(iterable((elements + 1) / 2))
-    Source(List(s1, s2)).flattenConcat().runWith(Sink.publisher)
+    Source(List(s1, s2)).flatMapConcat(ConstantFun.scalaIdentityFunction).runWith(Sink.publisher)
   }
 
 }
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
index b309b5c56e..8c47d6ac1c 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
@@ -10,6 +10,7 @@ import akka.japi.JavaPartialFunction;
 import akka.japi.Pair;
 import akka.japi.function.*;
 import akka.stream.*;
+import akka.stream.impl.ConstantFun;
 import akka.stream.javadsl.FlowGraph.Builder;
 import akka.stream.stage.*;
 import akka.stream.testkit.AkkaSpec;
@@ -481,12 +482,12 @@ public class FlowTest extends StreamTest {
     final Iterable<Integer> input1 = Arrays.asList(1, 2, 3);
     final Iterable<Integer> input2 = Arrays.asList(4, 5);
 
-    final List<Source<Integer, BoxedUnit>> mainInputs = new ArrayList<Source<Integer,BoxedUnit>>();
+    final List<Source<Integer, ?>> mainInputs = new ArrayList<Source<Integer,?>>();
     mainInputs.add(Source.from(input1));
     mainInputs.add(Source.from(input2));
 
-    final Flow<Source<Integer, BoxedUnit>, List<Integer>, BoxedUnit> flow = Flow.<Source<Integer, BoxedUnit>>create().
-      <Integer>flattenConcat().grouped(6);
+    final Flow<Source<Integer, ?>, List<Integer>, ?> flow = Flow.<Source<Integer, ?>>create().
+      flatMapConcat(ConstantFun.<Source<Integer, ?>>javaIdentityFunction()).grouped(6);
     Future<List<Integer>> future = Source.from(mainInputs).via(flow)
         .runWith(Sink.<List<Integer>>head(), materializer);
 
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
index 2f19c93dd5..d41715bc9c 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
@@ -15,6 +15,7 @@ import akka.stream.Graph;
 import akka.stream.OverflowStrategy;
 import akka.stream.StreamTest;
 import akka.stream.UniformFanInShape;
+import akka.stream.impl.ConstantFun;
 import akka.stream.stage.*;
 import akka.stream.testkit.AkkaSpec;
 import akka.stream.testkit.TestPublisher;
@@ -348,12 +349,13 @@ public class SourceTest extends StreamTest {
     final Iterable<Integer> input1 = Arrays.asList(1, 2, 3);
     final Iterable<Integer> input2 = Arrays.asList(4, 5);
 
-    final List<Source<Integer, BoxedUnit>> mainInputs = new ArrayList<Source<Integer,BoxedUnit>>();
+    final List<Source<Integer, ?>> mainInputs = new ArrayList<Source<Integer,?>>();
     mainInputs.add(Source.from(input1));
     mainInputs.add(Source.from(input2));
 
     Future<List<Integer>> future = Source.from(mainInputs)
-      .<Integer>flattenConcat().grouped(6)
+      .<Integer>flatMapConcat(ConstantFun.<Source<Integer,?>>javaIdentityFunction())
+      .grouped(6)
       .runWith(Sink.<List<Integer>>head(), materializer);
 
     List<Integer> result = Await.result(future, probe.dilated(FiniteDuration.create(3, TimeUnit.SECONDS)));
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala
index 9789071563..4fcdd95652 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala
@@ -3,6 +3,8 @@
  */
 package akka.stream.scaladsl
 
+import akka.stream.impl.ConstantFun
+
 import scala.concurrent.duration._
 import scala.util.control.NoStackTrace
 import akka.stream.ActorMaterializer
@@ -31,7 +33,7 @@ class FlowConcatAllSpec extends AkkaSpec {
       val main = Source(List(s1, s2, s3, s4, s5))
 
       val subscriber = TestSubscriber.manualProbe[Int]()
-      main.flattenConcat().to(Sink(subscriber)).run()
+      main.flatMapConcat(ConstantFun.scalaIdentityFunction).to(Sink(subscriber)).run()
       val subscription = subscriber.expectSubscription()
       subscription.request(10)
       for (i ← 1 to 10)
@@ -42,7 +44,7 @@ class FlowConcatAllSpec extends AkkaSpec {
 
     "work together with SplitWhen" in {
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(1 to 10).splitWhen(_ % 2 == 0).flattenConcat().runWith(Sink(subscriber))
+      Source(1 to 10).splitWhen(_ % 2 == 0).flatMapConcat(ConstantFun.scalaIdentityFunction).runWith(Sink(subscriber))
       val subscription = subscriber.expectSubscription()
       subscription.request(10)
       for (i ← (1 to 10))
@@ -54,7 +56,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on onError on master stream cancel the current open substream and signal error" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
+      Source(publisher).flatMapConcat(ConstantFun.scalaIdentityFunction).to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -74,7 +76,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on onError on master stream cancel the currently opening substream and signal error" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
+      Source(publisher).flatMapConcat(ConstantFun.scalaIdentityFunction).to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -94,10 +96,27 @@ class FlowConcatAllSpec extends AkkaSpec {
       subUpstream.expectCancellation()
     }
 
+    "on onError on opening substream, cancel the master stream and signal error " in assertAllStagesStopped {
+      val publisher = TestPublisher.manualProbe[Source[Int, _]]()
+      val subscriber = TestSubscriber.manualProbe[Int]()
+      Source(publisher).flatMapConcat(_ ⇒ throw testException).to(Sink(subscriber)).run()
+
+      val upstream = publisher.expectSubscription()
+      val downstream = subscriber.expectSubscription()
+      downstream.request(1000)
+
+      val substreamPublisher = TestPublisher.manualProbe[Int]()
+      val substreamFlow = Source(substreamPublisher)
+      upstream.expectRequest()
+      upstream.sendNext(substreamFlow)
+      subscriber.expectError(testException)
+      upstream.expectCancellation()
+    }
+
     "on onError on open substream, cancel the master stream and signal error " in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
+      Source(publisher).flatMapConcat(ConstantFun.scalaIdentityFunction).to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -117,7 +136,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on cancellation cancel the current open substream and the master stream" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
+      Source(publisher).flatMapConcat(ConstantFun.scalaIdentityFunction).to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -138,7 +157,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on cancellation cancel the currently opening substream and the master stream" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
+      Source(publisher).flatMapConcat(ConstantFun.scalaIdentityFunction).to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -162,7 +181,11 @@ class FlowConcatAllSpec extends AkkaSpec {
       val up = TestPublisher.manualProbe[Source[Int, _]]()
       val down = TestSubscriber.manualProbe[Int]()
 
-      val flowSubscriber = Source.subscriber[Source[Int, _]].flattenConcat().to(Sink(down)).run()
+      val flowSubscriber = Source
+        .subscriber[Source[Int, _]]
+        .flatMapConcat(ConstantFun.scalaIdentityFunction)
+        .to(Sink(down))
+        .run()
 
       val downstream = down.expectSubscription()
       downstream.cancel()
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
index 539d0ab787..84a977cfbd 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
@@ -255,7 +255,7 @@ private[akka] object ActorProcessorFactory {
       case GroupBy(f, _)         ⇒ (GroupByProcessorImpl.props(settings, f), ())
       case PrefixAndTail(n, _)   ⇒ (PrefixAndTailImpl.props(settings, n), ())
       case Split(d, _)           ⇒ (SplitWhereProcessorImpl.props(settings, d), ())
-      case ConcatAll(_)          ⇒ (ConcatAllImpl.props(materializer), ())
+      case ConcatAll(f, _)       ⇒ (ConcatAllImpl.props(f, materializer), ())
       case DirectProcessor(p, m) ⇒ throw new AssertionError("DirectProcessor cannot end up in ActorProcessorFactory")
     }
   }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ConcatAllImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/ConcatAllImpl.scala
index b910184483..abbccb03a3 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ConcatAllImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ConcatAllImpl.scala
@@ -4,28 +4,27 @@
 package akka.stream.impl
 
 import akka.stream.ActorMaterializer
-import akka.stream.scaladsl.Sink
+import akka.stream.scaladsl.{ Source, Sink }
 import akka.actor.{ Deploy, Props }
 
 /**
  * INTERNAL API
  */
 private[akka] object ConcatAllImpl {
-  def props(materializer: ActorMaterializer): Props =
-    Props(new ConcatAllImpl(materializer)).withDeploy(Deploy.local)
+  def props(f: Any ⇒ Source[Any, _], materializer: ActorMaterializer): Props =
+    Props(new ConcatAllImpl(f, materializer)).withDeploy(Deploy.local)
 }
 
 /**
  * INTERNAL API
  */
-private[akka] class ConcatAllImpl(materializer: ActorMaterializer)
+private[akka] class ConcatAllImpl(f: Any ⇒ Source[Any, _], materializer: ActorMaterializer)
   extends MultiStreamInputProcessor(materializer.settings) {
 
   import akka.stream.impl.MultiStreamInputProcessor._
 
   val takeNextSubstream = TransferPhase(primaryInputs.NeedsInput && primaryOutputs.NeedsDemand) { () ⇒
-    val Extract.Source(source) = primaryInputs.dequeueInputElement()
-    val publisher = source.runWith(Sink.publisher)(materializer)
+    val publisher = f(primaryInputs.dequeueInputElement()).runWith(Sink.publisher)(materializer)
     // FIXME we can pass the flow to createSubstreamInput (but avoiding copy impl now)
     val inputs = createAndSubscribeSubstreamInput(publisher)
     nextPhase(streamSubstream(inputs))
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ConstantFun.scala b/akka-stream/src/main/scala/akka/stream/impl/ConstantFun.scala
index f65bec90b6..ee4d0dcd47 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ConstantFun.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ConstantFun.scala
@@ -6,7 +6,7 @@ package akka.stream.impl
 import akka.japi.function.{ Function ⇒ JFun, Function2 ⇒ JFun2 }
 import akka.japi.{ Pair ⇒ JPair }
 
-private[stream] object ConstantFun {
+private[akka] object ConstantFun {
   private[this] val JavaIdentityFunction = new JFun[Any, Any] {
     @throws(classOf[Exception]) override def apply(param: Any): Any = param
   }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/Extract.scala b/akka-stream/src/main/scala/akka/stream/impl/Extract.scala
deleted file mode 100644
index 1cce40f847..0000000000
--- a/akka-stream/src/main/scala/akka/stream/impl/Extract.scala
+++ /dev/null
@@ -1,32 +0,0 @@
-/**
- * Copyright (C) 2014 Typesafe Inc. <http://www.typesafe.com>
- */
-package akka.stream.impl
-
-import akka.stream.{ scaladsl, javadsl }
-
-/**
- * INTERNAL API
- *
- * Unapply methods aware of both DSLs.
- * Use these instead of manually casting to [[scaladsl.Source]].
- */
-private[akka] object Extract {
-
-  object Source {
-    def unapply(a: Any): Option[scaladsl.Source[Any, _]] = a match {
-      case s: scaladsl.Source[_, _] ⇒ Some(s)
-      case s: javadsl.Source[_, _]  ⇒ Some(s.asScala)
-      case _                        ⇒ None
-    }
-  }
-
-  object Sink {
-    def unapply(a: Any): Option[scaladsl.Sink[Nothing, _]] = a match {
-      case s: scaladsl.Sink[_, _] ⇒ Some(s)
-      case s: javadsl.Sink[_, _]  ⇒ Some(s.asScala)
-      case _                      ⇒ None
-    }
-  }
-
-}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
index bc1196ba6d..6a0e9d9fcc 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
@@ -10,6 +10,7 @@ import akka.stream.Supervision.Decider
 import akka.stream._
 import akka.stream.impl.SplitDecision.{ Continue, SplitAfter, SplitBefore, SplitDecision }
 import akka.stream.impl.StreamLayout._
+import akka.stream.scaladsl.Source
 import akka.stream.stage.AbstractStage.PushPullGraphStage
 import akka.stream.stage.Stage
 import org.reactivestreams.Processor
@@ -232,7 +233,7 @@ private[stream] object Stages {
     def after(f: Any ⇒ Boolean) = Split(el ⇒ if (f(el)) SplitAfter else Continue, name("splitAfter"))
   }
 
-  final case class ConcatAll(attributes: Attributes = concatAll) extends StageModule {
+  final case class ConcatAll(f: Any ⇒ Source[Any, _], attributes: Attributes = concatAll) extends StageModule {
     override def withAttributes(attributes: Attributes) = copy(attributes = attributes)
   }
 
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
index 513ceb285c..3d7bce587a 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -839,8 +839,10 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
     new Flow(delegate.splitAfter(p.test).map(_.asJava))
 
   /**
-   * Transforms a stream of streams into a contiguous stream of elements using the provided flattening strategy.
-   * This operation can be used on a stream of element type `Source[U]`.
+   * Transform each input element into a `Source` of output elements that is
+   * then flattened into the output stream by concatenation,
+   * fully consuming one Source after the other.
+   *
    * '''Emits when''' a currently consumed substream has an element available
    *
    * '''Backpressures when''' downstream backpressures
@@ -850,8 +852,8 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * '''Cancels when''' downstream cancels
    *
    */
-  def flattenConcat[U](): javadsl.Flow[In, U, Mat] =
-    new Flow(delegate.flattenConcat[U]()(conforms[U].asInstanceOf[Out <:< scaladsl.Source[U, _]]))
+  def flatMapConcat[T](f: function.Function[Out, Source[T, _]]): Flow[In, T, Mat] =
+    new Flow(delegate.flatMapConcat[T](x ⇒ f(x).asScala))
 
   /**
    * Concatenate the given [[Source]] to this [[Flow]], meaning that once this
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
index 68d96219fa..4a54467dd2 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
@@ -840,11 +840,21 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
     new Source(delegate.splitAfter(p.test).map(_.asJava))
 
   /**
-   * Transforms a stream of streams into a contiguous stream of elements using the provided flattening strategy.
-   * This operation can be used on a stream of element type `Source[U]`.
+   * Transform each input element into a `Source` of output elements that is
+   * then flattened into the output stream by concatenation,
+   * fully consuming one Source after the other.
+   *
+   * '''Emits when''' a currently consumed substream has an element available
+   *
+   * '''Backpressures when''' downstream backpressures
+   *
+   * '''Completes when''' upstream completes and all consumed substreams complete
+   *
+   * '''Cancels when''' downstream cancels
+   *
    */
-  def flattenConcat[U](): javadsl.Source[U, Mat] =
-    new Source(delegate.flattenConcat[U]()(conforms[U].asInstanceOf[Out <:< scaladsl.Source[U, _]]))
+  def flatMapConcat[T](f: function.Function[Out, Source[T, _]]): Source[T, Mat] =
+    new Source(delegate.flatMapConcat[T](x ⇒ f(x).asScala))
 
   /**
    * If the first element has not passed through this stage before the provided timeout, the stream is failed
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
index 3626101073..652744f6c7 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
@@ -995,7 +995,9 @@ trait FlowOps[+Out, +Mat] {
     deprecatedAndThen(Split.after(p.asInstanceOf[Any ⇒ Boolean]))
 
   /**
-   * Flattens a stream of [[Source]]s into a contiguous stream by fully consuming one stream after the other.
+   * Transform each input element into a `Source` of output elements that is
+   * then flattened into the output stream by concatenation,
+   * fully consuming one Source after the other.
    *
    * '''Emits when''' a currently consumed substream has an element available
    *
@@ -1006,7 +1008,8 @@ trait FlowOps[+Out, +Mat] {
    * '''Cancels when''' downstream cancels
    *
    */
-  def flattenConcat[U]()(implicit ev: Out <:< Source[U, _]): Repr[U, Mat] = deprecatedAndThen(ConcatAll())
+  def flatMapConcat[T](f: Out ⇒ Source[T, _]): Repr[T, Mat] =
+    deprecatedAndThen(ConcatAll(f.asInstanceOf[Any ⇒ Source[Any, _]]))
 
   /**
    * If the first element has not passed through this stage before the provided timeout, the stream is failed