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, 0 deletions

diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 9ade47331df..222065c2e4e 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -766,6 +766,16 @@ inner loops respectively."
                         ((eq x 2) (setq y 'c)))
                   (list x y)))))
       (mapcar fn (bytecomp-test-identity '(0 1 2 3 10 11))))
+
+    ;; `nconc' nil arg elimination
+    (nconc (list 1 2 3 4) nil)
+    (nconc (list 1 2 3 4) nil nil)
+    (let ((x (cons 1 (cons 2 (cons 3 4)))))
+      (nconc x nil))
+    (let ((x (cons 1 (cons 2 (cons 3 4)))))
+      (nconc x nil nil))
+    (let ((x (cons 1 (cons 2 (cons 3 4)))))
+      (nconc nil x nil (list 5 6) nil))
     )
   "List of expressions for cross-testing interpreted and compiled code.")