Circular dependencies during compilation

It is not, and the compiler will provide a handy error message if you do this by accident:

== Compilation error in file lib/a.ex ==
** (CompileError)  deadlocked waiting on module B
    lib/a.ex:2: (module)
    (stdlib 3.11.2) erl_eval.erl:680: :erl_eval.do_apply/6

== Compilation error in file lib/b.ex ==
** (CompileError)  deadlocked waiting on module A
    lib/b.ex:2: (module)
    (stdlib 3.11.2) erl_eval.erl:680: :erl_eval.do_apply/6

Compilation failed because of a deadlock between files.
The following files depended on the following modules:

  lib/a.ex => B
  lib/b.ex => A

Ensure there are no compile-time dependencies between those files and that the modules they reference exist and are correctly named

Where:

# a.ex
defmodule A do
  B.foo()
end

# b.ex
defmodule B do
  A.foo()
end