dilayzer warning when using :erlang.exit in a with else clause

[albertored]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.10.4 (compiled with Erlang/OTP 21)

Current behavior

I get a strange dialyzer warning when writing a custom mix task.

I don't know if it is really a bug or if is related to elixir so feel free to close it.

I want the task to exit with different exit codes based on the error, this is a minimal example showing my issue

defmodule A do
  def a do
    with :ok <- File.cp("a", "b"),
         {:ok, _} <- File.rm_rf("a") do
      :ok
    else
      {:error, _err} ->
        IO.puts("1")
        exit({:shutdown, 1})

      _ ->
        IO.puts("2")
        exit({:shutdown, 2})
    end
  end
end

and this is the warning I get on the File.cp line

The pattern can never match the type.

Pattern:
{:error, __err}

Type:
{:error, atom(), binary()}

The warning is related to the use of :erlang.exit/1, removing those calls the warning disappears. It seems that after encountering an :erlang.exit/1 dialyzer stops and doesn't see the next clauses of the else.

I can remove the warning by moving the :erlang.exit/1 call in a case statement but I was wondering why this happens and if it can be some sort of bug.

defmodule A do
  def a do
    with :ok <- File.cp("a", "b"),
         {:ok, _} <- File.rm_rf("a") do
      :ok
    else
      {:error, _err} -> {:error, 1}
      _ -> {:error, 2}
    end
    |> case do
      :ok -> :ok
      {:error, exit_code} -> exit({:shutdown, exit_code})
    end
  end
end

Expected behavior

No dialyzer warnings

[josevalim]

Duplicate of #6738. :)