GNU bug report logs - #78792
[PATCH] Fix compilation of defalias with a constant macro cons pair

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: zach shaftel <zach <at> shaf.tel>
To: bug-gnu-emacs <at> gnu.org
Subject: [PATCH] Fix compilation of defalias with a constant macro cons pair
Date: Sat, 14 Jun 2025 21:40:08 -0400

[Message part 1 (text/plain, inline)]

Tags: patch

byte-compile-file-form-defalias destructures (cons 'macro 'fn) and
'(macro . fn) the same way, so in the latter case fn would be unquoted,
and then eval would later signal (void-variable fn). this patch just
re-quotes fn, and adds a test for this scenario.


In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.49, cairo version 1.18.4) of 2025-06-11 built on bigbox
Repository revision: f90cee6f761b0c2bffcfa64556284884c0f7348f
Repository branch: feature/igc
System Description: Arch Linux

Configured using:
 'configure --with-modules --with-native-compilation --with-tree-sitter
 --without-gsettings --without-gconf --without-gpm --with-pgtk
 --without-compress-install --with-mps 'CFLAGS=-mtune=native
 -march=native -O3 -fuse-ld=mold''

[0001-Fix-compilation-of-defalias-with-a-constant-macro-co.patch (text/patch, attachment)]

Message #8 received at 78792 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: zach shaftel <zach <at> shaf.tel>, Stefan Monnier <monnier <at> iro.umontreal.ca>,
 Mattias Engdegård <mattiase <at> acm.org>,
 Andrea Corallo <acorallo <at> gnu.org>
Cc: 78792 <at> debbugs.gnu.org, shigeru.fukaya <at> gmail.com
Subject: Re: bug#78792: [PATCH] Fix compilation of defalias with a constant
 macro cons pair
Date: Sun, 15 Jun 2025 08:25:11 +0300

> Cc: Shigeru Fukaya <shigeru.fukaya <at> gmail.com>
> Date: Sat, 14 Jun 2025 21:40:08 -0400
> From:  zach shaftel via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> 
> byte-compile-file-form-defalias destructures (cons 'macro 'fn) and
> '(macro . fn) the same way, so in the latter case fn would be unquoted,
> and then eval would later signal (void-variable fn). this patch just
> re-quotes fn, and adds a test for this scenario.

Thanks, I've added the relevant people to this discussion,

> In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
>  3.24.49, cairo version 1.18.4) of 2025-06-11 built on bigbox
> Repository revision: f90cee6f761b0c2bffcfa64556284884c0f7348f
> Repository branch: feature/igc
> System Description: Arch Linux
> 
> Configured using:
>  'configure --with-modules --with-native-compilation --with-tree-sitter
>  --without-gsettings --without-gconf --without-gpm --with-pgtk
>  --without-compress-install --with-mps 'CFLAGS=-mtune=native
>  -march=native -O3 -fuse-ld=mold''
> 
> >From f0b9f227c5711c1cbf7a38c20fd05b184536c12a Mon Sep 17 00:00:00 2001
> From: Zach Shaftel <zach <at> shaf.tel>
> Date: Sat, 14 Jun 2025 20:57:21 -0400
> 
> * lisp/emacs-lisp/bytecomp.el (byte-compile-file-form-defalias): Quote
> the function name in '(macro . function-name), since we eval it later.
> * test/lisp/emacs-lisp/bytecomp-tests.el
> (test-eager-load-macro-expand-defalias): New test.
> ---
>  lisp/emacs-lisp/bytecomp.el            |  3 ++-
>  test/lisp/emacs-lisp/bytecomp-tests.el | 18 ++++++++++++++++++
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
> index 0ec8db214bc..973792c4daa 100644
> --- a/lisp/emacs-lisp/bytecomp.el
> +++ b/lisp/emacs-lisp/bytecomp.el
> @@ -5159,7 +5159,8 @@ byte-compile-file-form-defalias
>         (pcase-let*
>             ;; `macro' is non-nil if it defines a macro.
>             ;; `fun' is the function part of `arg' (defaults to `arg').
> -           (((or (and (or `(cons 'macro ,fun) `'(macro . ,fun)) (let macro t))
> +           (((or (and (or `(cons 'macro ,fun) `'(macro . ,(app (list 'quote) fun)))
> +                      (let macro t))
>                   (and (let fun arg) (let macro nil)))
>               arg)
>              ;; `lam' is the lambda expression in `fun' (or nil if not
> diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
> index 8b0c1dad4c0..b14f10d6eb7 100644
> --- a/test/lisp/emacs-lisp/bytecomp-tests.el
> +++ b/test/lisp/emacs-lisp/bytecomp-tests.el
> @@ -1322,6 +1322,24 @@ test-eager-load-macro-expand-lexical-override
>        (defun def () (m))))
>    (should (equal (funcall 'def) 4)))
>  
> +(ert-deftest test-eager-load-macro-expand-defalias ()
> +  (ert-with-temp-file elfile
> +    :suffix ".el"
> +    (write-region
> +     (concat ";;; -*- lexical-binding: t -*-\n"
> +             (mapconcat #'prin1-to-string
> +                        '((defalias 'nothing '(macro . ignore))
> +                          (defalias 'something (cons 'macro #'identity))
> +                          (defalias 'five (cons 'macro (lambda (&rest _) 5)))
> +                          (eval-when-compile
> +                            (defun def () (or (nothing t) (something (five nil))))))
> +                        "\n"))
> +     nil elfile)
> +    (let* ((byte-compile-debug t)
> +           (byte-compile-dest-file-function #'ignore))
> +      (byte-compile-file elfile)
> +      (should (equal (funcall 'def) 5)))))
> +
>  (defmacro bytecomp-tests--with-temp-file (file-name-var &rest body)
>    (declare (indent 1))
>    (cl-check-type file-name-var symbol)
> -- 
> 2.49.0
> 

Message #11 received at 78792 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattias.engdegard <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: shigeru.fukaya <at> gmail.com, Andrea Corallo <acorallo <at> gnu.org>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>, zach shaftel <zach <at> shaf.tel>,
 78792 <at> debbugs.gnu.org
Subject: Re: bug#78792: [PATCH] Fix compilation of defalias with a constant
 macro cons pair
Date: Wed, 18 Jun 2025 10:11:09 +0200

>> byte-compile-file-form-defalias destructures (cons 'macro 'fn) and
>> '(macro . fn) the same way, so in the latter case fn would be unquoted,
>> and then eval would later signal (void-variable fn). this patch just
>> re-quotes fn, and adds a test for this scenario.

This seems to be a genuine bug indeed, thank you for reporting.

>> @@ -5159,7 +5159,8 @@ byte-compile-file-form-defalias
>>        (pcase-let*
>>            ;; `macro' is non-nil if it defines a macro.
>>            ;; `fun' is the function part of `arg' (defaults to `arg').
>> -           (((or (and (or `(cons 'macro ,fun) `'(macro . ,fun)) (let macro t))
>> +           (((or (and (or `(cons 'macro ,fun) `'(macro . ,(app (list 'quote) fun)))
>> +                      (let macro t))
>>                  (and (let fun arg) (let macro nil)))

Patch looks fine as far as I can tell. There is a test (naturally!) as well.

Message #14 received at 78792 <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> protonmail.com>
To: Mattias Engdegård <mattias.engdegard <at> gmail.com>
Cc: 78792 <at> debbugs.gnu.org, shigeru.fukaya <at> gmail.com,
 zach shaftel <zach <at> shaf.tel>, Stefan Monnier <monnier <at> iro.umontreal.ca>,
 Eli Zaretskii <eliz <at> gnu.org>, Andrea Corallo <acorallo <at> gnu.org>
Subject: Re: bug#78792: [PATCH] Fix compilation of defalias with a constant
 macro cons pair
Date: Thu, 19 Jun 2025 13:41:41 +0000

Mattias Engdegård <mattias.engdegard <at> gmail.com> writes:

>>> byte-compile-file-form-defalias destructures (cons 'macro 'fn) and
>>> '(macro . fn) the same way, so in the latter case fn would be unquoted,
>>> and then eval would later signal (void-variable fn). this patch just
>>> re-quotes fn, and adds a test for this scenario.
>
> This seems to be a genuine bug indeed, thank you for reporting.

Did this occur in the wild or do you have to trigger it deliberately?
I don't think defmacro ever produces a problematic defalias, but maybe
some other macro does?

That's just curiosity, of course.  Both the analysis and the patch seem
correct to me.

>>> @@ -5159,7 +5159,8 @@ byte-compile-file-form-defalias
>>>        (pcase-let*
>>>            ;; `macro' is non-nil if it defines a macro.
>>>            ;; `fun' is the function part of `arg' (defaults to `arg').
>>> -           (((or (and (or `(cons 'macro ,fun) `'(macro . ,fun)) (let macro t))
>>> +           (((or (and (or `(cons 'macro ,fun) `'(macro . ,(app (list 'quote) fun)))
>>> +                      (let macro t))
>>>                  (and (let fun arg) (let macro nil)))
>
> Patch looks fine as far as I can tell. There is a test (naturally!) as well.

TBH, I had to look up the (app F X) pcase pattern.  I think that means
we should use it more, not less, so people will become used to it and
stop emulating it badly, which is what I've done so far...

The rest of this email is relevant only because we should maybe just
delete this code instead of fixing it:

We should have a hard look at whether this function makes sense in its
current form: if we can't parse/compile a defalias form, and it defined
a function, that's okay.  But if we can't compile it and it defined a
macro, shouldn't that be a fatal error?

And since we can't tell whether a Lisp expression evaluates to a macro
definition or a function definition, maybe it's time to handle the
remaining common cases only and make the rest errors (which is okay, not
every conceivable Elisp program can be byte-compiled)?

Here's an example which we miscompile (make sure NOT to evaluate this in
the session which then compiles it):

;; -*- lexical-binding: t; -*-
(defalias 'mybq3 (symbol-function '\`))
(message "%S" (mybq3 (,3)))

Pip

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