Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit]

Elixir 1.20.0-rc.0 (0f9d919) (compiled with Erlang/OTP 28)

Operating system

macOS

Current behavior

Compile the following module:

defmodule Test do
  def default_format(data) do
    case String.Chars.impl_for(data) do
      nil -> inspect(data)
      mod -> mod.to_string(data)
    end
  end
end

The warning that I'm getting is:

    warning: nil.to_string/1 is undefined (module nil is not available or is yet to be defined)
    │
  5 │       mod -> mod.to_string(data)
    │                  ~
    │
    └─ lib/test.ex:5:18: Test.default_format/1

Expected behavior

I was expecting the handling of nil on line 4 to make it so that mod was non-nil on line 5. Adding a guard on line 5 removes the warning as you'd expect.