bug#23648: [PATCH] `defun-declarations-alist' can be unintentionally modified

[Noam Postavsky <npostavs@users.sourceforge.net>]

On Mon, Jul 18, 2016 at 10:33 AM, Eli Zaretskii <eliz@gnu.org> wrote:
>> From: npostavs@users.sourceforge.net
>> Date: Sun, 17 Jul 2016 23:00:33 -0400
>> Cc: 23648@debbugs.gnu.org
>>
>> I agree with analysis and patch here.  Since this just appends another
>> nil, it should be safe for emacs-25, right?
>
> I'm not sure, actually.  How probable is the situation where this
> problem pops up?

It happens with 100% probability when performing completion inside a
(declare ...) form. Starting from emacs -Q, put into *scratch*

(defun foo ()
  (declare (indent 1))
  nil)

Macroexpanding this gives:

(prog1
    (defalias 'foo
      #'(lambda nil nil))
  (put 'foo 'lisp-indent-function '1))

Now move point to just after "indent" and type C-M-i (this gives
message "Sole completion"), macroexpanding now gives

(prog1
    (defalias 'foo
      #'(lambda nil nil))
  "Warning: Unknown defun property `indent' in foo")

>  And when was the bug introduced?

Code seems to have been that way since it was introduced in 24.4:
dd8791e9 "* lisp/emacs-lisp/lisp.el (lisp-completion-at-point):
Provide specialized
completion tables when completing error conditions and
`declare' arguments...."