I’m having a hard time to fix the following dialyzer warnings:

lib/foo.ex:36:pattern_match
The pattern
{:ok, _pid, _ref, _opts}

can never match the type
{:error, _}

issued about a case/2 expression:

case init_daemon(opts) do
  {:error, reason} ->
    {:error, reason}

  {:ok, pid, ref, opts} ->
    {:ok, %{options: opts, deamons: [%{pid: pid, ref: ref, options: opts}]}}
end

Where init_daemon/1 does roughly this and issues a warning for itself because of an opaque type:

lib/foo.ex:161:call_with_opaque
The call :erlang.monitor('process',_pid@1::ssh:daemon_ref()) contains an opaque term in 2nd argument when terms of different types are expected in these positions}.

def init_daemon(_) do # simplified
  daemon = :ssh.daemon(10022, …irrelevant set of options…)

  case daemon do
    {:ok, pid} ->
      ref = Process.monitor(pid)
      {:ok, pid, ref, %{}}

    {:error, reason} ->
      {:error, reason}
  end
end

My current assumption is, that dialyzer assumes, that in init_daemon/1 the :ok branch will never used, as it were violating opaque types, and therefore considers the :ok clause as superfluous in the first case/2 above. As if I temporarily remove the call to Process.monitor/1 and replace it with a static value, dialyzer does not complain anymore.

But how could I fix that? I need a monitor for the daemon process, or at least some monitorlike behaviour.