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, 3 insertions, 7 deletions

diff --git a/src/eval.c b/src/eval.c
index 08a73b1e4a5..a6e1d86c4ab 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3035,7 +3035,6 @@ funcall_lambda (Lisp_Object fun, ptrdiff_t nargs,
     emacs_abort ();
 
   i = optional = rest = 0;
-  bool previous_optional_or_rest = false;
   for (; CONSP (syms_left); syms_left = XCDR (syms_left))
     {
       maybe_quit ();
@@ -3046,17 +3045,15 @@ funcall_lambda (Lisp_Object fun, ptrdiff_t nargs,
 
       if (EQ (next, Qand_rest))
         {
-          if (rest || previous_optional_or_rest)
+          if (rest)
             xsignal1 (Qinvalid_function, fun);
           rest = 1;
-          previous_optional_or_rest = true;
         }
       else if (EQ (next, Qand_optional))
         {
-          if (optional || rest || previous_optional_or_rest)
+          if (optional || rest)
             xsignal1 (Qinvalid_function, fun);
           optional = 1;
-          previous_optional_or_rest = true;
         }
       else
         {
@@ -3080,11 +3077,10 @@ funcall_lambda (Lisp_Object fun, ptrdiff_t nargs,
           else
             /* Dynamically bind NEXT.  */
             specbind (next, arg);
-          previous_optional_or_rest = false;
         }
     }
 
-  if (!NILP (syms_left) || previous_optional_or_rest)
+  if (!NILP (syms_left))
     xsignal1 (Qinvalid_function, fun);
   else if (i < nargs)
     xsignal2 (Qwrong_number_of_arguments, fun, make_number (nargs));