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

[Konstantin Kharlamov <Hi-Angel@yandex.ru>]

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.