Compilation differences between Elixir versions

I tried to compile some deps today using Elixir 1.8.1 and I got an undefined function error. This is very strange because up until Elixir 1.7.3 everything compiles well without any warnings relevant to the problem.

If you look at this line in ueberauth_instagram you can see where the error is (query_params(conn, :profile)):

defp user_query(conn) do
    |> Map.merge(query_params(conn, :profile))
    |> URI.encode_query

I opened up the lib and all its dependencies and could not find any function called query_params with arity /2.

So why would this compile normally in 1.7.3 but not in 1.8.1? Is this a hidden bug that has always been there, and 1.8.1 is more fussy about compiling?

It looks like the function is not being used anywhere, so that might explain it.

Here is the error:

== Compilation error in file lib/ueberauth/strategy/instagram.ex ==
** (CompileError) lib/ueberauth/strategy/instagram.ex:131: undefined function query_params/2
    (elixir) src/elixir_locals.erl:107: :elixir_locals."-ensure_no_undefined_local/3-lc$^0/1-0-"/2
    (elixir) src/elixir_locals.erl:108: anonymous fn/3 in :elixir_locals.ensure_no_undefined_local/3
    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
could not compile dependency :ueberauth_instagram, "mix compile" failed. You can recompile this dependency with "mix deps.compile ueberauth_instagram", update it with "mix deps.update ueberauth_instagram" or clean it with "mix

use Ueberauth.Strategy imports a few modules.

I looked at the imports and found nothing. Plug.Conn and Ueberauth.Strategy.Helpers. In neither version present in mix.lock could I find a reference to query_params/2.

Are you sing the same version uberauth_instagram? Is the query_params/1 function documented? If internal it might have disappeared along the way.

I am using the same version of the lib and its deps. query_params/2 does not exist any where in the lib or its deps. And the function that calls it is not being used in the lib.

It seems like this is a hidden bug. But why did the lib compile on previous versions of Elixir and not 1.8.1? Maybe the compiler became more strict?