bug#37400: Patch for bug in cl-reduce: function called with no arguments when list is empty

["Adrià Garriga" <adria.garriga@gmail.com>]

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Hello Emacs developers,

I found a bug in cl-reduce, which should be reproducible by running the test in the patch. I'm running 26.2 but the bug is also present in master. Please let me know if there are any problems with the patch (including that it is useless :) or if I need to do something else.

Thank you for your hard work maintaining Emacs!

Adrià Garriga-Alonso


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From 3c5df36904c8a5e9137a90309795b3eb9f90dd73 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adri=C3=A0=20Garriga-Alonso?= <adria.garriga@gmail.com>
Date: Wed, 4 Sep 2019 11:36:48 +0100
Subject: [PATCH] cl-reduce avoids calling the function when list is nil

---
 lisp/emacs-lisp/cl-seq.el            | 31 ++++++++++++++--------------
 test/lisp/emacs-lisp/cl-seq-tests.el |  6 ++++++
 2 files changed, 22 insertions(+), 15 deletions(-)

diff --git a/lisp/emacs-lisp/cl-seq.el b/lisp/emacs-lisp/cl-seq.el
index a15c994bc1..8da6ec5a2a 100644
--- a/lisp/emacs-lisp/cl-seq.el
+++ b/lisp/emacs-lisp/cl-seq.el
@@ -134,21 +134,22 @@ If SEQ is empty, return :INITIAL-VALUE and FUNCTION is not
 called.
 
 \n(fn FUNCTION SEQ [KEYWORD VALUE]...)"
-  (cl--parsing-keywords (:from-end (:start 0) :end :initial-value :key) ()
-    (or (listp cl-seq) (setq cl-seq (append cl-seq nil)))
-    (setq cl-seq (cl-subseq cl-seq cl-start cl-end))
-    (if cl-from-end (setq cl-seq (nreverse cl-seq)))
-    (let ((cl-accum (cond ((memq :initial-value cl-keys) cl-initial-value)
-			  (cl-seq (cl--check-key (pop cl-seq)))
-			  (t (funcall cl-func)))))
-      (if cl-from-end
-	  (while cl-seq
-	    (setq cl-accum (funcall cl-func (cl--check-key (pop cl-seq))
-				    cl-accum)))
-	(while cl-seq
-	  (setq cl-accum (funcall cl-func cl-accum
-				  (cl--check-key (pop cl-seq))))))
-      cl-accum)))
+   (when cl-seq
+    (cl--parsing-keywords (:from-end (:start 0) :end :initial-value :key) ()
+      (or (listp cl-seq) (setq cl-seq (append cl-seq nil)))
+      (setq cl-seq (cl-subseq cl-seq cl-start cl-end))
+      (if cl-from-end (setq cl-seq (nreverse cl-seq)))
+      (let ((cl-accum (cond ((memq :initial-value cl-keys) cl-initial-value)
+                            (cl-seq (cl--check-key (pop cl-seq)))
+                            (t (funcall cl-func)))))
+        (if cl-from-end
+            (while cl-seq
+              (setq cl-accum (funcall cl-func (cl--check-key (pop cl-seq))
+                                      cl-accum)))
+          (while cl-seq
+            (setq cl-accum (funcall cl-func cl-accum
+                                    (cl--check-key (pop cl-seq))))))
+        cl-accum))))
 
 ;;;###autoload
 (defun cl-fill (cl-seq cl-item &rest cl-keys)
diff --git a/test/lisp/emacs-lisp/cl-seq-tests.el b/test/lisp/emacs-lisp/cl-seq-tests.el
index 6515eee9f2..56c1826483 100644
--- a/test/lisp/emacs-lisp/cl-seq-tests.el
+++ b/test/lisp/emacs-lisp/cl-seq-tests.el
@@ -311,5 +311,11 @@ Body are forms defining the test."
       (should (eq (cl-assoc x a) (car a)))
       (should (eq (cl-rassoc x a) (cadr a))))))
 
+(ert-deftest cl-seq-reduce ()
+  (let ((bad-add (lambda (x y) (+ x y))))
+    (should-not (cl-reduce bad-add nil))
+    (should (equal 1 (cl-reduce bad-add '(1))))
+    (should (equal 3 (cl-reduce bad-add '(1 2))))))
+
 (provide 'cl-seq-tests)
 ;;; cl-seq-tests.el ends here
-- 
2.21.0