GNU bug report logs - #57397
29.0.50; cl-letf blindly macroexpands places

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From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: 57397 <at> debbugs.gnu.org
Cc: Lars Ingebrigtsen <larsi <at> gnus.org>, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: bug#57397: 29.0.50; cl-letf blindly macroexpands places
Date: Thu, 25 Aug 2022 06:42:17 +0200

Hello,

background: to get rid of warnings about the recently obsoleted
generalized variables in my init file... because I'm lazy and wanted to
try (and there were only two) I blindly replaced them with synthetic
places - like this:

#+begin_src emacs-lisp
(defun test1 (b query)
  (cl-letf (((buffer-local-value 'buffer-save-without-query b)
             (or buffer-save-without-query query)))
    (with-current-buffer b (save-buffer))))
;; ~~>
(defun test2 (b query)
  (cl-letf (((gv-synthetic-place
              (buffer-local-value 'buffer-save-without-query b)
              (lambda (v) `(setq-local buffer-save-without-query ,v)))
             (or buffer-save-without-query query)))
    (with-current-buffer b (save-buffer))))
#+end_src

[ I know it is an nonsense example wrt what it does and how it does it,
that doesn't matter. ]

This should get rid of the warning - but in this case it doesn't (in
the other it did).  The reason is that Stefan made cl-letf (the helper
cl--letf more precisely), macroexpand each place:

| 91a7f934ac * lisp/emacs-lisp/cl-macs.el: Fix bug#26073.

to support symbol macros better:

#+begin_src emacs-lisp
(defun cl--letf (bindings simplebinds binds body)
  ;; It's not quite clear what the semantics of cl-letf should be...10..
  (if (null bindings)
      (if (and (null binds) (null simplebinds)) (macroexp-progn body)..20..)
    (let* ((binding (car bindings))
           (place (macroexpand (car binding) macroexpand-all-environment)));<--!!
      (gv-letplace (getter setter) place..12..))))
#+end_src

I think this is an error in the general case.  Maybe it's enough to
expand only symbol macros?  It's at least always wrong when (car PLACE)
is a macro name with a gv spec defined.

TIA,

Michael.


In GNU Emacs 29.0.50 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw scroll bars) of 2022-08-25 built on drachen
Repository revision: bd5b704447ac5ec0559a824209bb01b271d29959
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --with-x-toolkit=lucid'

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