Don't rewrite (nconc X nil) -> X for any X (bug#63103)

Since the last cdr of a non-terminal argument to `nconc` is
overwritten no matter its value:

  (nconc (cons 1 2) nil) => (1)

a terminating nil arg cannot just be eliminated unconditionally.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-nconc):
Only eliminate a terminal nil arg to `nconc` if preceded by
a nonempty proper list.  Right now we only bother to prove this
for `(list ...)`, so that

  (nconc (list 1 2 3) nil) -> (list 1 2 3)

* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--test-cases): Add test cases.

1 files changed, 10 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index da997212eef..0f7a3cb2665 100644
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -1531,7 +1531,16 @@ See Info node `(elisp) Integer Basics'."
                    (prev (car newargs)))
                (cond
                 ;; Elide null args.
-                ((null arg) (loop (cdr args) newargs))
+                ((and (null arg)
+                      ;; Don't elide a terminal nil unless preceded by
+                      ;; a nonempty proper list, since that will have
+                      ;; its last cdr forced to nil.
+                      (or (cdr args)
+                          ;; FIXME: prove the 'nonempty proper list' property
+                          ;; for more forms than just `list', such as
+                          ;; `append', `mapcar' etc.
+                          (eq 'list (car-safe (car newargs)))))
+                 (loop (cdr args) newargs))
                 ;; Merge consecutive `list' args.
                 ((and (eq (car-safe arg) 'list)
                       (eq (car-safe prev) 'list))