This really piqued my curiosity so I put together a little test and it seems you’re… half right? I’m not exactly known for being able to come up with the most accurate scenarios for these things but it appears you just need to use something from the dependency module.

With this test:

defmodule CoverTest do
  use ExUnit.Case

  test "foobars the world" do
    assert Cover.hello() == "foobar"
  end
end

…and this dependency module:

defmodule Cover.Dep do
  defmacro __using__(_) do
    quote do
      def foo, do: "foo"
      def bar, do: "bar"
    end
  end
end

…this gets 50% coverage:

defmodule Cover do
  use Cover.Dep

  def hello do
    "foobar"
  end
end

…and this gets 100%:

defmodule Cover do
  use Cover.Dep

  def hello do
    foo() <> bar()
  end
end

…which I thought proved your theory right until I tried this:

defmodule Cover do
  use Cover.Dep

  def hello do
    foo() <> "bar"
  end
end

…which also scores 100%

So ya, I know your post is only 5 hours old but BUMP for anyone who knows!

EDIT: Oh, perhaps it’s because it doesn’t consider strings executable so the fact that you are using anything from the module means it’s covered?