bug#69290: declare-function doesn't work when combined with --eval and -batch

[Eli Zaretskii]

> From: Konstantin Kharlamov <Hi-Angel@yandex.ru>
> Cc: 69290@debbugs.gnu.org
> Date: Wed, 21 Feb 2024 00:28:46 +0300
> 
> On Tue, 2024-02-20 at 23:31 +0300, Konstantin Kharlamov wrote:
> > On Tue, 2024-02-20 at 22:20 +0200, Eli Zaretskii wrote:
> > > > From: Konstantin Kharlamov <Hi-Angel@yandex.ru>
> > > > Cc: 69290@debbugs.gnu.org
> > > > Date: Tue, 20 Feb 2024 23:13:27 +0300
> > > > 
> > > > > Does --eval '(declare-function hello nil)' tell anything to the
> > > > > byte-compiler?
> > > > 
> > > > Well, I can guess by the way you're asking that the answer is
> > > > "no",
> > > > but
> > > > I have no idea why so. It should.
> > > 
> > > How can it?  The declare-function form is evaluated by the startup
> > > code, and only after that the byte-compiler is invoked to compile
> > > test.el.  At least this is my analysis of what happens here.
> > > 
> > > > It is the same as if you pop up Emacs, evaluate a (defun hello())
> > > > and then call `byte-compile-file` over the `test.el`. There won't
> > > > be
> > > > a warning, despite that `(defun hello ())` was never byte-
> > > > compiled
> > > > (AFAIK Emacs does note byte-compile evaluated code).
> > > 
> > > For the declare-function form to take effect, the byte-compiler
> > > needs
> > > to evaluate the form.  By contrast, defun is evaluated by the Lisp
> > > interpreter and the result is stored in the global state.
> > 
> > Oh, thank you for explanation, I see. It's doesn't seem to be obvious
> > to a bystander, because from the side it seems like in Emacs byte-
> > compiler and interpreter should work in a tandem, as in the example
> > with evaluating (defun hello()). In Emacs context the doc-string that
> > says `Tell the byte-compiler that function FN is defined` would read
> > to
> > me as "modify global state, which later will be read by byte-compiler
> > to deem FN as defined". IOW, to me as a bystander the documentation
> > string does not explain the difference, which is why we just had this
> > somewhat long discussion before I understood why `declare-function`
> > works this way.
> 
> Btw, I just figured out how to show you why this doc-string doesn't say
> anything on the matter. Imagine for a second that `declare-function` is
> getting through from "eval" to the byte-compiler, i.e. the problem
> we're discussing is just not present. Would you change the string `Tell
> the byte-compiler that function FN is defined` to something else in
> this case? I would not, because it's still byte-compiler that does all
> the checking, so the point that "declare-function" is purposed for
> `byte-compiler` stands.

Stefan, any comments or suggestion for how best to document this (if
I'm right)?