Type checker crash in unwrap_domain_tuple/2 with Map.update!/3, remote capture, and empty-map guard

[Gustavo-Ziaugra-Arvo]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]
Elixir 1.20.0-rc.3 (2db87eb) (compiled with Erlang/OTP 28)

Operating system

macOS (Darwin 25.3.0, arm64)

Current behavior

The type checker crashes with a FunctionClauseError in Module.Types.Descr.unwrap_domain_tuple/2 when all three of the following conditions are met:

1. A guard narrows a map parameter by excluding the empty map (e.g. when properties == %{} or when map_size(properties) == 0)
2. A subsequent clause calls Map.update!/3 (not Map.update/4)
3. The third argument is a remote function capture (&Module.fun/1)

Using a local capture (&fun/1), an anonymous function (fn v -> Module.fun(v) end), or Map.update/4 does not trigger the crash.

Minimal reproduction:

  defmodule Helpers do
    def transform(value), do: String.trim(value)
  end

  defmodule Repro do
    def update(properties) when properties == %{}, do: properties

    def update(properties) do
      Map.update!(properties, "name", &Helpers.transform/1)
    end
  end

$ elixir repro.exs

  ** (RuntimeError) found error while checking types for Repro.update/1:

  ** (FunctionClauseError) no function clause matching in Module.Types.Descr.unwrap_domain_tuple/2

      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:285: Module.Types.Descr.unwrap_domain_tuple(:term, ...)
      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:270: Module.Types.Descr.domain_to_args/1
      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:278: Module.Types.Descr.domain_to_flat_args/2
      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:1421: Module.Types.Descr.fun_apply_with_strategy/3
      (elixir 1.20.0-rc.3) lib/module/types/apply.ex:1457: anonymous fn/3 in Module.Types.Apply.map_update_or_replace_lazy/4
      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:3518: anonymous fn/4 in Module.Types.Descr.map_update_fun/5
      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:3865: Module.Types.Descr.map_update_put_domain/3
      (elixir 1.20.0-rc.3) lib/module/types/descr.ex:3853: anonymous fn/3 in Module.Types.Descr.map_update_put_domains/3

Expected behavior

Compilation should succeed (possibly with type warnings). The guard narrowing the map to not empty_map() combined with a remote function capture in Map.update!/3 should not cause the type checker to crash.

[josevalim]

Minimal case in the type system suite:

fun_apply(dynamic(fun([integer()], integer())), [none()])

It is a known limitation (applying none crashes). :)

[gldubc]

Fix incoming! This was on my todo list already :)