Opaque MapSet type causes Dialyzer error

[1player]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.7.4 (compiled with Erlang/OTP 21)
• Operating system: macOS 10.13.6

Current behavior

def public_field?(field, %MapSet{} = public_fields), do: MapSet.member?(public_fields, field)

gives Dialyzer error The call 'Elixir.MapSet':'member?'(_public_fields@1::#{'__struct__':='Elixir.MapSet', _=>_},_field@1::any()) does not have an opaque term of type 'Elixir.MapSet':t(_) as 1st argument

Expected behavior

Dialyzer not to give any error.

Similar to elixir-ecto/ecto#2693, fixed by making the type not opaque.

[michalmuskala]

I would say that this is an issue or deficiency of dialyzer. There are two basic options: make the type completely opaque or make it completely public. We generally want to make it opaque, with just information that it's a struct public. This is not supported by dialyzer and making the type opaque and it's structure private is a much safer solution for us. Otherwise people might start using the internals or relying on implementation details which is explicitly not supported.

[josevalim]

Closing in favor of #8480.