Dialyzer, who knows what's going on?

Timex.ex

defmodule Messages.Time do
  use Timex

  @spec timestamp() :: Timex.DateTime.t
  def timestamp, do: Timex.DateTime.now(:utc)

  @type ts :: Timex.Convertable.t | none
  @spec timestamp_iso8601(ts) :: binary
  def timestamp_iso8601(ts \\ __MODULE__.timestamp) do
    {:ok, iso8601} = Timex.format(ts, "{ISO:Extended}")
    String.replace(iso8601, "+00:00", "Z")
  end
end

# lib/messages/time.ex:13: Function timestamp_iso8601/0 has no local return
# lib/messages/time.ex:13: The call 'Elixir.Messages.Time':timestamp_iso8601(#{}) will never return since it differs in the 1st argument from the success typing arguments: ('Elixir.Timex.Convertable')

I'd use @dialyzer to suppress this warning. I'd also add a comment in the code about the reason, and then make a PR to timex.

You can see the list of possible options here, and a little bit about it here (search for @dialyzer).

In your case, it would be something like:

@dialyzer {:nowarn_function, timestamp_iso8601: 1}

Or maybe even this might work:

@dialyzer {[:no_match], timestamp_iso8601: 1}

I didn't try them out (since I have no full code), but something along those lines should suppress the warning.

Btw, if making a PR, you can try to fix timex locally to verify that your fix is correct. Basically, you edit the code in deps/timex, run mix compile timex, and then verify that dialyzer passes.