Although parts of the following describe behavior observed in Phoenix, this is not a Phoenix bug (I’ll explain why later). It appears to be an Elixir bug, so I’m posting this in the “Elixir” category.

The possible bug

Recently, I encountered the following warning while developing a Phoenix application:

warning: mtime (modified time) for "lib/demo_web/controllers/page_html.ex" was set to the future, resetting to now

Have any of you encountered this issue? I haven’t seen any similar reports in the community.

To reproduce it:

1. Create an app: mix phx.new demo --no-ecto --no-live --no-tailwind
2. Enter the dev environment: cd demo && iex -S mix phx.server
3. Visit http://localhost:4000 in your browser and keep the tab open (this is necessary, as it triggers the edge case)
4. Modify demo/lib/demo_web/controllers/page_html/home.html.heex and save
5. The warning should appear in your IEx. If not, repeat step 4 a few more times.

The chain of logic leading to the warning:

1. We modify demo/lib/demo_web/controllers/page_html/home.html.heex.
2. Phoenix.LiveReloader detects the change and pushes an event to the browser.
3. The browser requests / to refresh, triggering Phoenix.CodeReloader to reload the code.
4. The refreshed page requests /phoenix/live_reload/socket to re-establish live reloading, triggering Phoenix.CodeReloader to reload the code again.

I suspect the warning is caused by two rapid successive calls to Phoenix.CodeReloader.reload/2, which in turn calls mix compile.elixir. To verify this, I created a minimal reproduction: GitHub - plastic-gun/elixir-compile-warning-1 · GitHub — and indeed, the warning reproduced.

I need help testing whether this is consistently reproducible across platforms and elixir versions. So far I’ve only tested on macOS — it would be great if those on Linux and Windows could test as well.

If this does turn out to be a bug, we’ll have already collected the necessary environment information.

How to help?

Run the project at GitHub - plastic-gun/elixir-compile-warning-1 · GitHub and report your results in the following format:

Reproduced?: Y/N
OS: <like macOS 15.7.5>
Erlang and Elixir:
<output of `elixir --version`>