Dependency my_app is specified as a dependency but is not reachable by the system

While using erlang.mk and relx to build a newly-created Erlang application, I got the following error:

Failed to solve release:
 Dependency my_app is specified as a dependency but is not reachable by the system.

It turns out that I was missing the modules term in the src/my_app.app.src file. erlang.mk needs it to be present (but empty), or it won’t write it to the ebin/my_app.app file. relx needs it to be in the ebin/my_app.app file, otherwise it fails with the above error.

I’d used rebar to create the application skeleton, and it leaves this out.

The solution is simply to add it:

{application, my_app,
 [
  % etc.
  {modules, []},
  % etc.
 ]}.