Allow `&rest' or `&optional' without following variable (Bug#29165)

This is sometimes convenient when writing macros, so that the empty
variable case doesn't need to be handled specially.  Older versions of
Emacs accepted this in some cases (especially the interpreter in Emacs
25 and below was very accepting).

                            |   interpreted/compiled   |
| arglist                   | 25 & earlier | 26  | 27  |
|---------------------------+--------------+-----+-----|
| (&rest)                   | y/n          | n/n | y/y |
| (&rest &rest)             | y/n          | n/n | n/n |
| (&rest &rest x)           | y/n          | n/n | n/n |
| (&rest x &rest)           | y/n          | n/n | n/n |
| (&rest x &rest y)         | y/n          | n/n | n/n |
|---------------------------+--------------+-----+-----|
| (&optional)               | y/n          | n/n | y/y |
| (&optional &optional)     | y/n          | n/n | n/n |
| (&optional x &optional)   | y/n          | n/n | n/n |
| (&optional x &optional y) | y/y          | n/n | n/n |
|---------------------------+--------------+-----+-----|
| (&optional &rest)         | y/n          | n/n | y/y |
| (&optional x &rest)       | y/n          | n/n | y/y |
| (&optional &rest y)       | y/y          | n/n | y/y |
|---------------------------+--------------+-----+-----|
| (&rest &optional)         | y/n          | n/n | n/n |
| (&rest &optional y)       | y/n          | n/n | n/n |
| (&rest x &optional y)     | y/n          | n/n | n/n |

The values in the table above can be produced with the following code:

(with-current-buffer (get-buffer-create "*ck-args*")
  (erase-buffer)
  (dolist (arglist '((&rest)
                     (&rest &rest)
                     (&rest &rest x)
                     (&rest x &rest)
                     (&rest x &rest y)
                     (&optional)
                     (&optional &optional)
                     (&optional x &optional)
                     (&optional x &optional y)
                     (&optional &rest)
                     (&optional x &rest)
                     (&optional &rest y)
                     (&rest &optional)
                     (&rest &optional y)
                     (&rest x &optional y)))
    (insert
     (format "%c/%c\n"
             (condition-case err
                 (progn (funcall `(lambda ,arglist 'ok))
                        ?y)
               (error ?n))
             (condition-case err
                 (progn (byte-compile-check-lambda-list arglist)
                        ?y)
               (error ?n))))
    (display-buffer (current-buffer))))

* src/eval.c (funcall_lambda):
* lisp/emacs-lisp/bytecomp.el (byte-compile-check-lambda-list): Don't
check for missing variables after `&rest' and `&optional'.
* test/src/eval-tests.el (eval-tests--bugs-24912-and-24913)
(eval-tests-accept-empty-optional-rest): Update tests accordingly.
* etc/NEWS: Update announcement accordingly.
* doc/lispref/functions.texi (Argument List): Update manual to
indicate that variable names are optional.

1 files changed, 7 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 99f3f27486b..d1db830afa1 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -380,6 +380,13 @@ backslash.  For example:
     (read "‘smart") => (invalid-read-syntax "strange quote" "‘")
     (read "\\‘smart") == (intern "‘smart")
 
++++
+** Omitting variables after '&optional' and '&rest' is now allowed.
+For example (defun foo (&optional)) is no longer an error.  This is
+sometimes convenient when writing macros.  See the ChangeLog entry
+titled "Allow `&rest' or `&optional' without following variable" for a
+full listing of which arglists are accepted across versions.
+
 ** Internal parsing commands now use syntax-ppss and disregard
 open-paren-in-column-0-is-defun-start.  This affects mostly things like
 forward-comment, scan-sexps, and forward-sexp when parsing backward.