From b72dcebdabfc3b7b28c9542633bd48b43bcc6365 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@protonmail.com>
Date: Sun, 24 May 2026 12:05:50 +0000
Subject: [PATCH] Avoid crash in self-insert-command with non-ASCII auto-fill

* src/cmds.c (internal_self_insert): If the autofill function changed
the newline character we inserted, don't attempt to restore point.
* test/src/cmds-tests.el (self-insert-nonascii-autofill): New.
---
 src/cmds.c             |  2 +-
 test/src/cmds-tests.el | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/cmds.c b/src/cmds.c
index e9dee5ed2e3..9ca9a6d28de 100644
--- a/src/cmds.c
+++ b/src/cmds.c
@@ -489,7 +489,7 @@ internal_self_insert (int c, EMACS_INT n)
 	SET_PT_BOTH (PT - 1, PT_BYTE - 1);
       auto_fill_result = call0 (Qinternal_auto_fill);
       /* Test PT < ZV in case the auto-fill-function is strange.  */
-      if (c == '\n' && PT < ZV)
+      if (c == '\n' && PT < ZV && FETCH_BYTE (PT) == '\n')
 	SET_PT_BOTH (PT + 1, PT_BYTE + 1);
       if (!NILP (auto_fill_result))
 	hairy = 2;
diff --git a/test/src/cmds-tests.el b/test/src/cmds-tests.el
index 8c0e4706e3c..2038c01942f 100644
--- a/test/src/cmds-tests.el
+++ b/test/src/cmds-tests.el
@@ -48,5 +48,22 @@
       (self-insert-command 0 10)
       (should-not (equal pt 0)))))
 
+(ert-deftest self-insert-nonascii-autofill ()
+  "Test `self-insert-command' with a non-ASCII autofill function."
+  (with-temp-buffer
+    (let ((auto-fill-function
+           (lambda ()
+             (delete-char 1)
+             (insert #x2000)
+             (forward-char -1))))
+      (dotimes (_ 10)
+        (self-insert-command 1 10)
+        (goto-char 2)
+        (should (equal (point) 2))
+        (should (equal (length (buffer-string)) 1))
+        (should (equal (format "%S" (buffer-string))
+                       "\"\x2000\""))
+        (delete-char -1)))))
+
 (provide 'cmds-tests)
 ;;; cmds-tests.el ends here