MapSet Dialyzer Repro

Minimal reproduction for MapSet Dialyzer opacity warnings introduced by Elixir 1.20. OTP 29 is not required to reproduce it.

Reproduction

This repository defaults to Elixir 1.20.0-rc.5 on OTP 29:

elixir --version

Expected relevant version shape:

Elixir 1.20.0-rc.5 ... Erlang/OTP 29

Run:

mix deps.get
mix dialyzer

Version Matrix

Verified results:

Elixir	Erlang/OTP	Result
1.20.0-rc.5	29	Reproduces
1.20.0-rc.5	28.5	Reproduces
1.19.5	28.5	Does not reproduce

The reproducing code is in lib/demo.ex:

def contains?(ids, id) do
  id_set = MapSet.new(ids)
  MapSet.member?(id_set, id)
end

Dialyzer reports id_set as %MapSet{:map => map()} and rejects it as the first argument to MapSet.member?/2, whose implementation expects a MapSet.t() with opaque :sets.set(_) internals.