2 files changed, 33 insertions, 7 deletions

diff --git a/lisp/emacs-lisp/cl-macs.el b/lisp/emacs-lisp/cl-macs.el
index 9af014cf8e9..43eb4261162 100644
--- a/lisp/emacs-lisp/cl-macs.el
+++ b/lisp/emacs-lisp/cl-macs.el
@@ -892,7 +892,7 @@ This is compatible with Common Lisp, but note that `defun' and
 (defvar cl--loop-name)
 (defvar cl--loop-result) (defvar cl--loop-result-explicit)
 (defvar cl--loop-result-var) (defvar cl--loop-steps)
-(defvar cl--loop-symbol-macs)
+(defvar cl--loop-symbol-macs) (defvar cl--loop-guard-cond)
 
 (defun cl--loop-set-iterator-function (kind iterator)
   (if cl--loop-iterator-function
@@ -961,7 +961,7 @@ For more details, see Info node `(cl)Loop Facility'.
           (cl--loop-accum-var nil)      (cl--loop-accum-vars nil)
           (cl--loop-initially nil)      (cl--loop-finally nil)
           (cl--loop-iterator-function nil) (cl--loop-first-flag nil)
-          (cl--loop-symbol-macs nil))
+          (cl--loop-symbol-macs nil) (cl--loop-guard-cond nil))
       ;; Here is more or less how those dynbind vars are used after looping
       ;; over cl--parse-loop-clause:
       ;;
@@ -996,7 +996,24 @@ For more details, see Info node `(cl)Loop Facility'.
                               (list (or cl--loop-result-explicit
                                         cl--loop-result))))
              (ands (cl--loop-build-ands (nreverse cl--loop-body)))
-             (while-body (nconc (cadr ands) (nreverse cl--loop-steps)))
+             (while-body
+              (nconc
+               (cadr ands)
+               (if (or (not cl--loop-guard-cond) (not cl--loop-first-flag))
+                   (nreverse cl--loop-steps)
+                 ;; Right after update the loop variable ensure that the loop
+                 ;; condition, i.e. (car ands), is still satisfied; otherwise,
+                 ;; set `cl--loop-first-flag' nil and skip the remaining
+                 ;; body forms (#Bug#29799).
+                 ;;
+                 ;; (last cl--loop-steps) updates the loop var
+                 ;; (car (butlast cl--loop-steps)) sets `cl--loop-first-flag' nil
+                 ;; (nreverse (cdr (butlast cl--loop-steps))) are the
+                 ;; remaining body forms.
+                 (append (last cl--loop-steps)
+                         `((and ,(car ands)
+                                ,@(nreverse (cdr (butlast cl--loop-steps)))))
+                         `(,(car (butlast cl--loop-steps)))))))
              (body (append
                     (nreverse cl--loop-initially)
                     (list (if cl--loop-iterator-function
@@ -1506,10 +1523,11 @@ For more details, see Info node `(cl)Loop Facility'.
                      ,(cl--loop-let (nreverse loop-for-sets) 'setq ands)
                      t)
                   cl--loop-body))
-        (if loop-for-steps
-            (push (cons (if ands 'cl-psetq 'setq)
-                        (apply 'append (nreverse loop-for-steps)))
-                  cl--loop-steps))))
+        (when loop-for-steps
+          (setq cl--loop-guard-cond t)
+          (push (cons (if ands 'cl-psetq 'setq)
+                      (apply 'append (nreverse loop-for-steps)))
+                cl--loop-steps))))
 
      ((eq word 'repeat)
       (let ((temp (make-symbol "--cl-var--")))
diff --git a/test/lisp/emacs-lisp/cl-macs-tests.el b/test/lisp/emacs-lisp/cl-macs-tests.el
index f0bde7af397..edb1530cad5 100644
--- a/test/lisp/emacs-lisp/cl-macs-tests.el
+++ b/test/lisp/emacs-lisp/cl-macs-tests.el
@@ -497,4 +497,12 @@ collection clause."
                           vconcat (vector (1+ x)))
                  [2 3 4 5 6])))
 
+
+(ert-deftest cl-macs-loop-for-as-equals-and ()
+  "Test for https://debbugs.gnu.org/29799 ."
+  (let ((arr (make-vector 3 0)))
+    (should (equal '((0 0) (1 1) (2 2))
+                   (cl-loop for k below 3 for x = k and z = (elt arr k)
+                            collect (list k x))))))
+
 ;;; cl-macs-tests.el ends here