diff --git a/akka-http/src/main/scala/akka/http/scaladsl/server/directives/FormFieldDirectives.scala b/akka-http/src/main/scala/akka/http/scaladsl/server/directives/FormFieldDirectives.scala
index f9c8525f30..c027812a98 100644
--- a/akka-http/src/main/scala/akka/http/scaladsl/server/directives/FormFieldDirectives.scala
+++ b/akka-http/src/main/scala/akka/http/scaladsl/server/directives/FormFieldDirectives.scala
@@ -115,11 +115,11 @@ object FormFieldDirectives extends FormFieldDirectives {
     import akka.http.scaladsl.server.util.TupleOps._
     import akka.http.scaladsl.server.util.BinaryPolyFunc
 
-    implicit def forTuple[T](implicit fold: FoldLeft[Directive0, T, ConvertParamDefAndConcatenate.type]): FieldDefAux[T, fold.Out] =
+    implicit def forTuple[T](implicit fold: FoldLeft[Directive0, T, ConvertFieldDefAndConcatenate.type]): FieldDefAux[T, fold.Out] =
       fieldDef[T, fold.Out](fold(pass, _))
 
-    object ConvertParamDefAndConcatenate extends BinaryPolyFunc {
-      implicit def from[P, TA, TB](implicit fdef: FieldDefAux[P, Directive[TB]], ev: Join[TA, TB]) =
+    object ConvertFieldDefAndConcatenate extends BinaryPolyFunc {
+      implicit def from[P, TA, TB](implicit fdef: FieldDefAux[P, Directive[TB]], ev: Join[TA, TB]): BinaryPolyFunc.Case[Directive[TA], P, ConvertFieldDefAndConcatenate.type] { type Out = Directive[ev.Out] } =
         at[Directive[TA], P] { (a, t) ⇒ a & fdef(t) }
     }
   }
diff --git a/akka-http/src/main/scala/akka/http/scaladsl/server/directives/ParameterDirectives.scala b/akka-http/src/main/scala/akka/http/scaladsl/server/directives/ParameterDirectives.scala
index a9c3b64be0..0f007fa336 100644
--- a/akka-http/src/main/scala/akka/http/scaladsl/server/directives/ParameterDirectives.scala
+++ b/akka-http/src/main/scala/akka/http/scaladsl/server/directives/ParameterDirectives.scala
@@ -137,7 +137,7 @@ object ParameterDirectives extends ParameterDirectives {
       paramDef[T, fold.Out](fold(BasicDirectives.pass, _))
 
     object ConvertParamDefAndConcatenate extends BinaryPolyFunc {
-      implicit def from[P, TA, TB](implicit pdef: ParamDef[P] { type Out = Directive[TB] }, ev: Join[TA, TB]) =
+      implicit def from[P, TA, TB](implicit pdef: ParamDef[P] { type Out = Directive[TB] }, ev: Join[TA, TB]): BinaryPolyFunc.Case[Directive[TA], P, ConvertParamDefAndConcatenate.type] { type Out = Directive[ev.Out] } =
         at[Directive[TA], P] { (a, t) ⇒ a & pdef(t) }
     }
   }
diff --git a/akka-http/src/main/scala/akka/http/scaladsl/server/util/TupleOps.scala b/akka-http/src/main/scala/akka/http/scaladsl/server/util/TupleOps.scala
index 2df524f102..e6cf4c8fbf 100644
--- a/akka-http/src/main/scala/akka/http/scaladsl/server/util/TupleOps.scala
+++ b/akka-http/src/main/scala/akka/http/scaladsl/server/util/TupleOps.scala
@@ -42,20 +42,22 @@ object TupleOps {
     type Out
     def apply(prefix: P, suffix: S): Out
   }
+  type JoinAux[P, S, O] = Join[P, S] { type Out = O }
   object Join extends LowLevelJoinImplicits {
     // O(1) shortcut for the Join[Unit, T] case to avoid O(n) runtime in this case
-    implicit def join0P[T] =
+    implicit def join0P[T]: JoinAux[Unit, T, T] =
       new Join[Unit, T] {
         type Out = T
         def apply(prefix: Unit, suffix: T): Out = suffix
       }
     // we implement the join by folding over the suffix with the prefix as growing accumulator
     object Fold extends BinaryPolyFunc {
-      implicit def step[T, A](implicit append: AppendOne[T, A]) = at[T, A](append(_, _))
+      implicit def step[T, A](implicit append: AppendOne[T, A]): BinaryPolyFunc.Case[T, A, Fold.type] { type Out = append.Out } =
+        at[T, A](append(_, _))
     }
   }
   sealed abstract class LowLevelJoinImplicits {
-    implicit def join[P, S](implicit fold: FoldLeft[P, S, Join.Fold.type]) =
+    implicit def join[P, S](implicit fold: FoldLeft[P, S, Join.Fold.type]): JoinAux[P, S, fold.Out] =
       new Join[P, S] {
         type Out = fold.Out
         def apply(prefix: P, suffix: S): Out = fold(prefix, suffix)