Erlang: OTP 20
Elixir 1.20.0-rc.3 (9b80ab5) (compiled with Erlang/OTP 28)

I am wondering whether 1.20 of Elixir is supposed to support type checking macro’s that use defoverridable. In rc.3 I am getting dozens of type check warnings for a construction that may be a bit offbeat but serves my purpose. Before I go and try to construct the approach differently, I wanted to see if it is a gap in type checking, or whether the warnings take into account overridden functions.

The warnings are all of this style:

warning: the following clause will never match:

 _ ->

because it attempts to match on the result of:

 ends = scheduled_end(sc)

which has type:

 dynamic(nil)

type warning found at:
│
188 │           _ →
│           ~~~~
│
└─ lib/sct/tasks/services/task_helper.ex:188: Sct.Tasks.AddThemeCloudExclusionsTask.remaining_time/1

Code snippet:

defmodule Sct.Tasks.TaskHelper do

defmacro using(_) do
quote location: :keep do

  @doc """
  Returns the remaining time for the task
  """
  @spec remaining_time(StrategicCollaboration.t() | DateTime.t() | nil) :: String.t()
  def remaining_time(%StrategicCollaboration{active: true} = sc) do
    case ends = scheduled_end(sc) do
      nil ->
        ""
      _ ->
        remaining_time(ends)
    end
  end
      
  def remaining_time(nil), do: ""

  def remaining_time(%DateTime{} = ends) do
        about_when =
          SctWeb.Formatter.relative_time(ends)

        if about_when =~ "ago" do
          " - ended #{about_when}"
        else
          " - ends in #{about_when}"
          |> String.replace(" in in ", " in ")
          |> String.replace("ends in tomorrow", "ends tomorrow")
        end
      end

  @doc """
  Returns the scheduled end time for the task - stub for override
  """
  @spec scheduled_end(StrategicCollaboration.t()) :: DateTime.t() | nil
  def scheduled_end(%StrategicCollaboration{}), do: nil


  defoverridable(
    remaining_time: 1,
    scheduled_end: 1
  )
end

end
end

Example usage:

use Sct.Tasks.TaskHelper


“”"
Returns the scheduled end date for the task. The enddate is
read from the DB.
“”"
def scheduled_end(%StrategicCollaboration{active: true} = sc) do
  Sct.Schedule.Query.outcome_reasoning_schedule(sc).enddate
end

It appears that the type checking is being done on the macro definition which always returns nil for the scheduled_end and therefore sees anything but nil as a type mismatch in the case statement for remaining_time. Of course when the macro is used in a module where schedule_end is overridden it is possible that the overridden return value is a DateTime but sometimes when it has not been established in the DB it returns nil.

The construction I have for the 100+ tasks that use this TaskHelper module work fine, but the type-checking is flagging it as a warning. With 100’s of warnings it is a lot of noise.

I believe I may be able to re-construct this is some other manner, but wanted to bring it to one’s attention in case I am (likely) missing something.