problem with dialyzer

argon2_test.ex

defmodule Argon2Test do

  @hash_for_foo "$argon2i$v=19$m=65536,t=6,p=1$dxSBP76R4icBDzu06CJRBQ$q4v8CePvqsdbSNBjBS0RRibryQI3Xo6ahYSqUZbdNaI"

  @spec test_pass(String.t) :: (:failed | {:ok, map()})
  def test_pass(pass) do
    db_user = %{password_hash: @hash_for_foo}
    with {:ok, ^db_user} <- Comeonin.Argon2.check_pass(db_user, pass) do
      {:ok, db_user}
    else
      _ -> :failed
    end
  end
end

mix.exs

defmodule Argon2Test.MixProject do
  use Mix.Project

  def project do
    [
      app: :argon2_test,
      version: "0.1.0",
      elixir: "~> 1.6",
      start_permanent: Mix.env() == :prod,
      deps: deps(),
      dialyzer: [
        plt_add_deps: :transitive,
      ]
    ]
  end

  # Run "mix help compile.app" to learn about applications.
  def application do
    [extra_applications: [:logger]]
  end

  # Run "mix help deps" to learn about dependencies.
  defp deps do
    [
      {:comeonin, "~> 4.1"},
      {:argon2_elixir, "~> 1.2"},
      {:dialyxir, "~> 0.5", only: [:dev], runtime: false},
    ]
  end
end

I came here from your Stack Overflow question. Did you ever find a way around this? I'm having a very similar error.