Wrong unused warnings in Elixir

Background

I have a test that defines modules which use a behaviour. To save some typing I am using an alias for the behaviour module when I use it in my tests. However the compiler doesn’t see it and plasters my screen with unused warnings.

Code

defmodule MyApp.DispatcherTest do
  use ExUnit.Case

  # Contract 
  alias MyApp.Dispatcher.Backend
  
  # SUT
  alias MyApp.Dispatcher

  test "calls request_fn with the correct parameters" do

    defmodule MyApp.Dispatcher.Backend.MissTest do
      @behaviour Backend

      @impl Backend
      def match(_params), do: false

      @impl Backend
      def url(_params), do: "/bfg_division"
    end

    defmodule MyApp.Dispatcher.Backend.HitTest do
      @behaviour Backend

      @impl Backend
      def match(_params), do: true

      @impl Backend
      def url(_params), do: "/family_jules"
    end

    defmodule MyApp.Dispatcher.Backend.HitTest2 do
      @behaviour Backend

      @impl Backend
      def match(_params), do: true

      @impl Backend
      def url(_params), do: "/comatose"
    end

    params = %{}
    test_pid = self()
    deps = [
      backends: [
        MyApp.Dispatcher.Backend.HitTest,
        MyApp.Dispatcher.Backend.HitTest2,
        MyApp.Dispatcher.Backend.MissTest
      ],
      request: fn(name, url) ->
        send(test_pid, {:fire, {name, url}})
        {:ok, :received}
      end
    ]

    Dispatcher.send(params, deps)

    expected_name = :hit_test
    expected_url = "/family_jules"
    assert_receive {:fire, {^expected_name, ^expected_url}}

    expected_name2 = :hit_test2
    expected_url2 = "/comatose"
    assert_receive {:fire, {^expected_name2, ^expected_url2}}
  end

end

Inside the test I have 3 test modules (MissTest, HitTest, HitTest2), which implement the behaviour Backend. Each module implements 2 functions to comply with the contract of the behaviour.

Problem

The problem is that every time I run mix test I get this warning plastered onto my screen:

warning: unused alias Backend
  test/myapp/dispatcher_test.exs:4

Which is not true. To prove it, if I actually remove the line alias MyApp.Dispatcher.Backend then I get even more warnings, stating that behaviour Backend doens’t exist or is not specified.

Questions

1. Why am I getting this warning?
2. How can I fix it?