bug#72328: [PATCH] Nested backquote in pcase

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> > Would be good to hear from Stefan
>> The first step is to declare nested backquotes unsupported and add
>> a warning when we encounter them.
> Where and how would you suggest to do that?

Somethink like...


        Stefan


diff --git a/lisp/emacs-lisp/pcase.el b/lisp/emacs-lisp/pcase.el
index fd6b0c8db5c..fe62820f0cb 100644
--- a/lisp/emacs-lisp/pcase.el
+++ b/lisp/emacs-lisp/pcase.el
@@ -1172,7 +1172,10 @@ pcase--expand-\`
           (upatd (pcase--expand-\` (cdr qpat))))
       (if (and (eq (car-safe upata) 'quote) (eq (car-safe upatd) 'quote))
           `'(,(cadr upata) . ,(cadr upatd))
-        `(and (pred consp)
+        `(and ,@(when (eq (car qpat) '\`)
+                  `((guard ,(macroexp-warn-and-return
+                             "Nested ` are not supported" t nil nil qpat))))
+              (pred consp)
               (app car-safe ,upata)
               (app cdr-safe ,upatd)))))
    ((or (stringp qpat) (numberp qpat) (symbolp qpat)) `',qpat)