Fix warnings and tests on Elixir/OTP master

Author: José Valim

lib/gen_stage.ex

@@ -1108,8 +1108,8 @@ defmodule GenStage do
   @doc """
   The same as `c:GenServer.format_status/2`.
   """
-  @callback format_status(:normal | :terminate, [pdict :: {term, term} | state :: term, ...]) ::
-              status :: term
+  @callback format_status(:normal | :terminate, [pdict :: {term, term} | (state :: term), ...]) ::
+              (status :: term)
 
   @optional_callbacks [
     # GenStage

lib/gen_stage/streamer.ex

@@ -2,6 +2,10 @@ defmodule GenStage.Streamer do
   @moduledoc false
   use GenStage
 
+  def start_link({_, opts} = pair) do
+    GenStage.start_link(__MODULE__, pair, opts)
+  end
+
   def init({stream, opts}) do
     continuation =
       &Enumerable.reduce(stream, &1, fn

test/gen_stage_test.exs

@@ -1672,7 +1672,7 @@ defmodule GenStageTest do
                [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 
       ref = Process.monitor(producer)
-      assert_received {:DOWN, ^ref, _, _, _}
+      assert_receive {:DOWN, ^ref, _, _, _}
     end
 
     test "raises on bad options" do