"reference to free variable" only during initialization

[Yuri D'Elia]

I have an oddball warning that I have an hard time debugging. I upgraded
to emacs 25.1.1 on a debian desktop recently, and started to get:

Warning (bytecomp): reference to free variable ‘isearchp-reg-beg’

during startup. The warning is generated by requiring
modeline-posn.el[1], which is using ``isearchp-reg-beg`` (actually
defvar-ed early on) inside a defadvice form.

Is this expected?

Warning aside, the intriguing part is that the warning /only/ occurs
when (require 'modeline-posn) is evaluated in my ~/.emacs, but is not
emitted instead when doing the "allegedly" equivalent:

  emacs -q -l ~/.emacs

even when ~/.emacs itself only contains:

  (add-to-list 'load-path "path-to-modeline.el")
  (require 'modeline-posn)

or when simply doing an eval in a fresh emacs -q session

The remaining difference is that emacs -q is skipping debian's system
startup. But I cannot think of something that could influence the
generation of a bytecomp warning.

Any pointer before going further?

[1] https://www.emacswiki.org/emacs/modeline-posn.el