Changing compile-time config of a Hex dependency leaves it stale and permanently fails compile-env validation

[zachdaniel]

Existing issue

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

Elixir and Erlang/OTP versions

Erlang/OTP 29 [erts-17.0] [source] [64-bit] [smp:14:14] [ds:14:14:10] [async-threads:1] [jit]

Operating system

MacOS

Current behavior

When a fetchable (Hex) dependency reads app config via Application.compile_env/3, and
that key is added or changed in the parent project's config/config.exs after the
dependency was compiled, mix compile fails — and keeps failing on every subsequent
run until the dep is manually rebuilt with mix deps.compile <dep> --force or mix deps.clean <dep> --build.

Reproduction

mix new repro
cd repro
mkdir config
echo 'import Config' > config/config.exs
echo 'defmodule Repro.MixProject do use Mix.Project; def project do [app: :repro,
version: "0.1.0", elixir: "~> 1.17", deps: [{:plug, "1.18.1"}]] end; def application do
[extra_applications: [:logger]] end end' > mix.exs
mix deps.get
mix compile
echo 'config :plug, :statuses, %{418 => "Totally A Teapot"}' >> config/config.exs
mix compile

Output

could not compile dependency :plug, "mix compile" failed. Errors may have been logged
above. ...
** (Mix) the application :plug has a different value set for key :statuses during
runtime compared to compile time. ...
  * Compile time value was not set
  * Runtime value was set to: %{418 => "Totally A Teapot"}

This breaks real-world installer flows: mix igniter.install ash ... writes
compile-time config entries for already-compiled Hex deps and then runs further mix
tasks, which now fail permanently. Originally reported at
ash-project/ash#2750.

Analysis

I got this from claude, I have not verified it, but I'm leaving it here inc are it helps. I was able to confirm that 1.20.0-rc.4 does not have this issue, so it sounds like it may be on the right track at least 😄

---

The regression was introduced by fb2657a ("Fix compile env change triggering full
recompilation of path dependencies", #15188), first released in 1.20.0-rc.5.

Before that commit, a diverged compile env gave the dep status :compile
(Mix.Dep.Loader.compile_env_status/2), which was compilable?, so
Mix.Tasks.Deps.Compile.compile_single/3 removed the dep's build dir and a clean
rebuild picked up the new env.

Expected behavior

Successful compile with the reproduction above