Mixed atom/domain map update incorrectly makes the atom field mandatory

[lukaszsamson]

Existing issue

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

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.4.0.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Interactive Elixir (1.21.0-dev)

Operating system

any

Current behavior

The following code demonstrates that map update incorrectly infers input domain with mixed keys

Repro:

    defmodule MixedKey do
      def replace(m, flag) do
        key = if flag, do: :a, else: 1
        %{m | key => :new}
      end

      def run, do: replace(%{1 => :old}, false)
    end

    IO.inspect(MixedKey.run()) # %{1 => :new}

Result: the code works fine but reports a false positive warning:

warning: incompatible types given to replace/2:

    replace(%{1 => :old}, false)

given types:

    %{integer() => :old}, false

but expected one of:

    %{..., a: term()}, term()

└─ iex:25: MixedKey.run/0

Note this issue is distinct from #15625

Expected behavior

No false positive warning. atom keys should not be inferred as required unless the whole kay type is constrained to atoms

[josevalim]

If the keys are dynamic, which is the case in this example, we should probably mark the expected type as dynamic, therefore not expecting all keys to be there.

[lukaszsamson]

I tried marking the expected map as dynamic, but it doesn't fix the problem. The argument is already dynamic, so refining it with dynamic(%{..., a: term()}) still leaves a required in the upper bound, and the call with %{1 => :old} warns. The problem is that atom_fetch/1 returns only the atom subset of a gradual type, which is not right for deriving a required key.