cl-print: handle circular objects when `print-circle' is nil (Bug#27117)

* lisp/emacs-lisp/cl-print.el (cl-print--currently-printing): New variable.
(cl-print-object): When `print-circle' is nil, bind it to a list of
objects that are currently printing to avoid printing the same object
endlessly.
* test/lisp/emacs-lisp/cl-print-tests.el (cl-print-circle): New test.

1 files changed, 8 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/cl-print-tests.el b/test/lisp/emacs-lisp/cl-print-tests.el
index 772601fe87d..dfbe18d7844 100644
--- a/test/lisp/emacs-lisp/cl-print-tests.el
+++ b/test/lisp/emacs-lisp/cl-print-tests.el
@@ -47,4 +47,12 @@
                "\\`(#1=#s(foo 1 2 3) #1#)\\'"
                (cl-prin1-to-string (list x x)))))))
 
+(ert-deftest cl-print-circle ()
+  (let ((x '(#1=(a . #1#) #1#)))
+    (let ((print-circle nil))
+      (should (string-match "\\`((a . #[0-9]) (a . #[0-9]))\\'"
+                            (cl-prin1-to-string x))))
+    (let ((print-circle t))
+      (should (equal "(#1=(a . #1#) #1#)" (cl-prin1-to-string x))))))
+
 ;;; cl-print-tests.el ends here.