ERL-1002: Dialyzer doesn't error when required atom keys were removed from a map

[OTP-Maintainer]

Original reporter: rodrigues
Affected versions: OTP-22.0, OTP-21.3
Fixed in version: OTP-23.0
Component: dialyzer
Migrated from: https://bugs.erlang.org/browse/ERL-1002

---

I believe the following behaviour is a bug, as I specify that a function should return a map with a certain atom key (required), but it could never return it, and dialyzer still passes.

{code:erlang}
-module(test).
-export([foo/0]).

-type t() :: #{a := non_neg_integer()}.

-spec foo() -> t().
%% this passes in dialyzer, but I think it's a bug,
%% as it will always return a map without the key a
foo() -> maps:remove(a, #{a => 2}).

%% this passes in dialyzer, as expected
%% foo() -> #{a => 2}.

%% only for these the required key is checked
%% foo() -> #{}.
%% foo() -> #{another => thing}.
{code}

{code:shell}
$ dialyzer test.erl
  Checking whether the PLT /Users/rodrigues/.dialyzer_plt is up-to-date... yes
  Proceeding with analysis... done in 0m0.12s
done (passed successfully)
{code}

FYI, if it helps, the context where this was discovered: https://elixirforum.com/t/elixir-allows-map-functions-to-manipulate-structs/23826/14?u=rodrigues

[OTP-Maintainer]

hasse said:

Does https://github.com/erlang/otp/pull/2392 work for you?

[OTP-Maintainer]

rodrigues said:

Hi Hans, yes, it works fine for me, thanks! 👍

In that example, now I get:

{{test.erl:6: Invalid type specification for function test:foo/0. The success typing is}}
{{() -> #{}}}

[OTP-Maintainer]

hasse said:

Thanks for testing. I've merged commit
a62aed81c56c724f7dd7040adecaa28a78e5d37f to erlang:master.