MapSet.new/0 threaded through recursion triggers dialyzer call_without_opaque

[bobergj]

Existing issue

• I have searched existing issues and could not find a duplicate.

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.4.0.2] [source] [64-bit] [smp:14:14] [ds:14:14:10] [async-threads:1] [jit]

Elixir 1.20.2 (compiled with Erlang/OTP 28)

Operating system

macOS 26.5.2

Current behavior

lib/repro.ex:

defmodule RecursiveMapSetRepro do
  def check(ids), do: check(ids, MapSet.new())

  defp check([], _seen), do: false

  defp check([id | ids], seen) do
    if MapSet.member?(seen, id) do
      true
    else
      check(ids, MapSet.put(seen, id))
    end
  end
end

mix.exs:

defmodule Repro.MixProject do
  use Mix.Project

  def project do
    [
      app: :repro,
      version: "0.1.0",
      elixir: "~> 1.20",
      deps: [
        {:dialyxir, "~> 1.4", only: [:dev], runtime: false}
      ]
    ]
  end
end

mix deps.get
mix dialyzer

lib/repro.ex:8:23:call_without_opaque
Function call without opaqueness type mismatch.

Call does not have expected term of type %MapSet{:map => :sets.set(_)} (with opaque subterms) in the 1st position.

MapSet.member?(
  _seen :: %MapSet{
    :map =>
      {:set, non_neg_integer(), non_neg_integer(), pos_integer(), non_neg_integer(),
       non_neg_integer(), non_neg_integer(), tuple(), tuple()}
      | %{_ => []}
  },
  _id :: any()
)

Expected behavior

Dialyzer does not output any errors.

I've looked at #15399 and the issue discussion though, so I understand if it is not fixable. dialyxir :no_opaque option silences the error.

[sabiwara]

Thank you for the report 💜

I've looked at #15399 and the issue discussion though, so I understand if it is not fixable. dialyxir :no_opaque option silences the error.

Unfortunately, I think @dialyzer :no_opaque is the best option here indeed.

[josevalim]

Closing in favor of PR!