Bad type violation detection in 1.19

[Sgiath]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.1.1] [source] [64-bit] [smp:32:32] [ds:32:32:10] [async-threads:1] [jit:ns]

Elixir 1.19.1 (compiled with Erlang/OTP 28)

Operating system

x86_64-linux, Linux 6.17.4-xanmod1, NixOS, 25.11 (Xantusia), 25.11.20251031.2fb006b

Current behavior

This is basically my code:

case error do
  %{message: message} when is_binary(message) ->
    Logger.error("Error happened: #{message}")

  _otherwise ->
    Logger.error("Weird error happened: #{inspect(error)}")
end

And compiler finds type violation on line 3 - message has type term() but expected a type that implements the String.Chars protocol. But there is a guard is_binary/1 on this case branch so the type is clear. Not sure if it is a bug or the type system is not able to work with guard in this place but it probably should not be reported as type violation.

Expected behavior

No type violation is reported.

[josevalim]

We don't look at guards yet, we are working on it right now, as part of #14558, so you can follow that issue. In any case, can you provide a full reproduction?

Meanwhile, rewriting to this should work:

%{message: <<message::binary>>} ->

Sorry for the false positive.

[Sgiath]

The rewrite worked, thanks!

When I try to compile this file inside a project I get the type violation:

defmodule Test do
  def test do
    try do
      nil
    rescue
      error ->
        case error do
          %{message: message} when is_binary(message) ->
            "Error: #{message}"

          _otherwise ->
            "Weird error: #{inspect(error)}"
        end
    end
  end
end

when I compile it with elixirc I won't get the type violation.

The full output looks like this:

$ mix compile
Compiling 1 file (.ex)
    warning: incompatible value given to string interpolation:

        message

    it has type:

        term()

    but expected a type that implements the String.Chars protocol.
    You either passed the wrong value or you must:

    1. convert the given value to a string explicitly
       (use inspect/1 if you want to convert any data structure to a string)
    2. implement the String.Chars protocol

    where "message" was given the type:

        # type: term()
        # from: lib/test.ex:8:11
        %{message: message}

    hint: the String.Chars protocol is implemented for the following types:

        dynamic(
          %Date{} or %DateTime{} or %NaiveDateTime{} or %Time{} or %URI{} or %Version{} or
            %Version.Requirement{}
        ) or atom() or binary() or empty_list() or float() or integer() or non_empty_list(term(), term())

    typing violation found at:
    │
  9 │             "Error: #{message}"
    │             ~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/test.ex:9: Test.test/0

Generated test app

[josevalim]

Perfect, thanks!