Type system crashes when callbacks are injected into protocols

[ityonemo]

Existing issue

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

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.3] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Elixir 1.19.5 (compiled with Erlang/OTP 28)

and

Erlang/OTP 29 [erts-17.0.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Elixir 1.20.1 (compiled with Erlang/OTP 29)

Operating system

Linux

Current behavior

The following code crashes with the following stacktrace:

== Compilation error in file test/after_callback_test.exs ==
** (FunctionClauseError) no function clause matching in List.duplicate/2    
    
    The following arguments were given to List.duplicate/2:
    
        # 1
        %{dynamic: :term}
    
        # 2
        -1
    
    Attempted function clauses (showing 1 out of 1):
    
        def duplicate(elem, n) when is_integer(n) and n >= 0
    
    (elixir 1.20.1) lib/list.ex:190: List.duplicate/2
    (elixir 1.20.1) lib/module/types.ex:155: Module.Types.default_domain/4
    (elixir 1.20.1) lib/module/types.ex:301: Module.Types.local_handler/5
    (elixir 1.20.1) lib/module/types.ex:86: anonymous fn/5 in Module.Types.infer/7
    (elixir 1.20.1) lib/enum.ex:2622: Enum."-reduce/3-lists^foldl/2-0-"/3
    (elixir 1.20.1) lib/module/types.ex:78: Module.Types.infer/7

Code:

  defmodule InjectCallback do
    defmacro __before_compile__(_env) do
      quote do
        @callback extra() :: term()
      end
    end
  end

  defprotocol P do
    @before_compile InjectCallback
    def f(x)
  end

  defmodule S do
    defstruct []
  end

  defimpl P, for: S do
    def f(_), do: :ok
    def extra(), do: :extra
  end

The InjectCallback exists to suppress the warning to not create additional non-protocol callbacks. The following code will also crash the type checker (does not crash the type checker in <= 1.18), though it does emit a warning:

  defprotocol P do
    @callback extra() :: term
    def f(x)
  end

  defmodule S do
    defstruct []
  end

  defimpl P, for: S do
    def f(_), do: :ok
    def extra(), do: :extra
  end

Expected behavior

Expected behavior: No failure (as with 1.18.x)

Anyways, this is obviously behavior that "is discouraged", but not obvious that it should be banned (or crash the type checker in any case)

[ityonemo]

resolution is for Module.Types.impl_for/1 should use protocol.protocol(:functions) instead of protocol.behaviour_info(:callbacks).

[josevalim]

Thanks! That's an easy fix indeed but keep in mind we may not guarantee to have this working long term.