bug#68931: 30.0.50; Gnus byte-compilation error with (display . [not expire]) with git emacs

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

> For my information: the problem here is that
> `gnus-category-make-function' runs `gnus-byte-compile' on a form like
> this:

BTW, `gnus-category-make-function` is a misnomer since it does not
return a function.

> (lambda nil (not (#f(compiled-function () #<bytecode 0x1980ad44c232>))))
>
> And the extra parens around (#f(compiled-function...)) are not strictly legal?

A Lisp function call has the form (FUN . ARGS) where FUN is not expected
to be a function *value* but rather should be a function name (and we
also tolerate a lambda expression, which is a function's source code).

The patch below should fix the core of the problem, but it probably
requires corresponding adjustments in `gnus-function-implies-unread-1`
and `gnus-category-make-function`.


        Stefan


diff --git a/lisp/gnus/gnus-agent.el b/lisp/gnus/gnus-agent.el
index 604eea4c33d..cfcbea2e233 100644
--- a/lisp/gnus/gnus-agent.el
+++ b/lisp/gnus/gnus-agent.el
@@ -2911,13 +2911,9 @@
 	(car func)
       (gnus-byte-compile `(lambda () ,func)))))
 
-(defun gnus-agent-true ()
-  "Return t."
-  t)
+(defalias 'gnus-agent-true #'always)
 
-(defun gnus-agent-false ()
-  "Return nil."
-  nil)
+(defalias 'gnus-agent-false #'ignore)
 
 (defun gnus-category-make-function-1 (predicate)
   "Make a function from PREDICATE."
@@ -2925,8 +2921,9 @@ gnus-category-make-function-1
    ;; Functions are just returned as is.
    ((or (symbolp predicate)
 	(functionp predicate))
-    `(,(or (cdr (assq predicate gnus-category-predicate-alist))
-	   predicate)))
+    (let ((fun (or (cdr (assq predicate gnus-category-predicate-alist))
+	           predicate)))
+      (if (symbolp fun) `(,fun) `(funcall ',fun))))
    ;; More complex predicate.
    ((consp predicate)
     `(,(cond