From c1eb458d6b9f6e1c4fa9ce0ae0c91f0314ca94ac Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@protonmail.com>
Date: Mon, 25 May 2026 11:28:38 +0000
Subject: [PATCH] Avoid relying on FOR_EACH_TAIL internals in 'Fnthcdr'
 (bug#81115)

The new FOR_EACH_TAIL code detects simple cycles sooner than the old
code did, leading to integer overflows.

* src/fns.c (Fnthcdr): Avoid integer overflow if cycle is detected
early.
---
 src/fns.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/fns.c b/src/fns.c
index d692a92580a..4284d82d6a5 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -1824,7 +1824,10 @@ DEFUN ("nthcdr", Fnthcdr, Snthcdr, 2, 2, 0,
 	  mpz_export (&iz, NULL, -1, sizeof iz, 0, 0, mpz[0]);
 	  num += iz;
 	}
-      num += cycle_length - large_num % cycle_length;
+      if (num < cycle_length)
+	num += cycle_length;
+      num -= large_num % cycle_length;
+      eassume (num >= 0);
     }
   num %= cycle_length;