Elixir: mix 1.9 breaks paths outside of app

[karlseguin]

We have some shared elixir files (generated pb schemas, test factories) outside of our projects. I guess the correct way to do this is to turn them into proper projects + dependencies. But I thought I'd document the issue in case anyone else runs into it.

To reproduce, create the following structure:

proj/
  lib/
  lib/valid.ex
  mix.exs
bad/
  hello.ex

hello.ex can be simple:

defmodule Hello do
   def world(), do: "hello world"
end

In mix.exs reference both paths: elixirc_paths: ["./lib", "../bad"] and from valid.ex try calling Hello.world/0.

In 1.8.2 this compiles and runs. In 1.9, https://github.com/elixir-lang/elixir/blob/v1.9/lib/mix/lib/mix/compilers/elixir.ex#L267 fails because the List.keyfind/3 returns nil.

Fundamentally, this happens because:

Path.relative_to("/home/karl/code/bad/hello.ex", "/home/karl/code/test")

returns "/home/karl/code/bad/hello.ex" instead of "../bad/hello.ex"

This appears to have been broken by https://github.com/elixir-lang/elixir/commit/a3c5820d125a472c0fb43eee935d1fb4bddfc72d

[josevalim]

Thanks for the report! Indeed using a dep would be the best way to go. I think this can also be fixed in your project by doing elixirc_paths: ["./lib", Path.expand("../bad")].

But I am considering documenting that elixirc_paths have to be relative paths to the current directory. Other assumptions may break by having the paths out.

[josevalim]

We are raising cleaner errors with 7ce98af5a8fabb48265896ff526fbbffa6d06221. Thank you!