Fix Kernel.LexicalTracker not to discard alias directives on import

Closes #4871.

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: lexmag

lib/elixir/lib/kernel/lexical_tracker.ex

@@ -148,17 +148,15 @@ defmodule Kernel.LexicalTracker do
 
   def handle_cast({:add_import, module, fas, line, warn}, state) when is_atom(module) do
     directives =
-      for {{:import, {import_module, _, _}}, _} = directive <- state.directives,
-          module != import_module,
-          do: directive,
-          into: %{}
-
-    directives = add_directive(directives, module, line, warn, :import)
+      state.directives
+      |> Enum.reject(&match?({{:import, {^module, _, _}}, _}, &1))
+      |> :maps.from_list
+      |> add_directive(module, line, warn, :import)
 
     directives =
-      Enum.reduce fas, directives, fn {function, arity}, directives ->
+      Enum.reduce(fas, directives, fn {function, arity}, directives ->
         add_directive(directives, {module, function, arity}, line, warn, :import)
-      end
+      end)
 
     {:noreply, %{state | directives: directives}}
   end

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -310,6 +310,20 @@ defmodule Kernel.WarningTest do
     purge Sample
   end
 
+  test "unused alias when also import" do
+    assert capture_err(fn ->
+      Code.compile_string """
+      defmodule Sample do
+        alias :lists, as: List
+        import MapSet
+        new()
+      end
+      """
+    end) =~ "unused alias List"
+  after
+    purge Sample
+  end
+
   test "unused inside dynamic module" do
     import List, only: [flatten: 1], warn: false