Re: cperl-mode: Eliminating references to obsolete packages

["Harald Jörg" <haj@posteo.de>]

Stefan Monnier writes:

> For example that "format-prompt" problem currently also affects the
> python.el GNU ELPA package.  Luckily, AFAICT we haven't yet released
> a new python.el package (because the version number has not been
> bumped), but the problem is real.

I noticed that some of them (but not python.el) contain a comment like
this:

  ;; This is an Elpa :core package. Don't use functionality that is not
  ;; compatible with Emacs 24.1.

...but I don't know whether that actually helps.  For me it would be
rather cumbersome to find out which functionality isn't compatible
with Emacs 24.1.  Some recent doc strings (e.g. time-convert) show it,
some (e.g. format-prompt) don't.

> [...]
> Some automated tests that try to build&compile (and treat warnings as
> errors) those packages on the corresponding oldest Emacs version
> supported would come in handy.

Wouldn't this rule out the type of fallback patch you suggested?  When
I byte-compile-file cperl-mode.el in Emacs 26.1 _with_ your fallback
patch applied, I still get the warning:

    In end of data:
    cperl-mode.el:8736:1:Warning: the function ‘format-prompt’ is
    not known to be defined.

Sorta obvious, though: The compilation step doesn't _run_ the fboundp
safeguard.

Anyway: Are there frameworks for such tests in the Emacs repository?
On GitHub, I can apply stuff from
https://github.com/marketplace/actions/emacs-lisp-check, but as of now
this also gives false positives:

    github-actions / test-on-self (26.1, true)
    cperl-mode.el#L5880
    You should depend on (emacs "27.1") if you need ‘cperl-force-face’.

This is fairly idiotic since "my" cperl-mode.el itself _defines_
cperl-force-face.  Somehow the check seems to "know" what's
available in vanilla Emacs 26.1 and 27.1.  I discovered that GitHub
action in Sacha Chua's blog
https://sachachua.com/blog/2020/06/2020-06-22-emacs-news/ and just
gave it a try without actually understanding what I was doing :)
-- 
Cheers,
haj