I ran into this issue the other day and it’s been bugging me ever since. Below is a basic behaviour/implementation, and function which returns a module at runtime. This is the recommended way to decouple your modules when using Mox. It seems to me that this breaks Dialyzer analysis, since no errors are produced with this clearly incorrect typing.

defmodule MyBehaviour do
  @callback get_integer() :: integer
end

defmodule MyImplementation do
  @behaviour MyBehaviour

  @impl MyBehaviour
  def get_integer, do: 100
end

defmodule DialyzerMoxBehaviours do
  @moduledoc """
  Using the recommended implementation of a behaviour for Mox, Dialyzer is unable to infer the type of `get_integer()`.
  By decoupling this module from `MyImplementation`, we lose the typespec information from the behaviour.
  """

  @spec dialyze_me() :: binary
  def dialyze_me, do: "hello" # valid type

  @spec dialyze_me_two() :: binary
  def dialyze_me_two, do: impl().get_integer() # invalid type, but dialyzer does not complain.

  @spec impl() :: MyBehaviour
  defp impl(), do: Application.get_env(:dialyzer_mox_behaviours, :impl, MyImplementation)
end

Am I missing something? I like using Mox but this seems like a major drawback to me. If anyone else has encountered this issue I’m interested in how you’ve solved it.