Subject: internal_equal manipulates symbols with position without checking symbols-with-pos-enabled.

From: Alan Mackenzie <[email-masked]>
Date: Fri, 4 Aug 2023 14:00:29 +0000

Hello, Emacs.

The code in question is in internal_equal (src/fns.c) ~37 lines from its
start.  We have:

  if (SYMBOL_WITH_POS_P (o1))
    o1 = SYMBOL_WITH_POS_SYM (o1);
  if (SYMBOL_WITH_POS_P (o2))
    o2 = SYMBOL_WITH_POS_SYM (o2);

at the top level of the function.  Thus

    (equal 'foo #<symbol foo at 42>)

will return non-nil, regardless of the setting of
symbols-with-pos-enabled.  It should return non-nil only when that flag
variable is non-nil.  This is a bug.

#########################################################################

A simple fix is:

diff --git a/src/fns.c b/src/fns.c
index bfd19e8c8f2..b1f0c4ecdd6 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -2773,10 +2773,13 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
 
   /* A symbol with position compares the contained symbol, and is
      `equal' to the corresponding ordinary symbol.  */
-  if (SYMBOL_WITH_POS_P (o1))
-    o1 = SYMBOL_WITH_POS_SYM (o1);
-  if (SYMBOL_WITH_POS_P (o2))
-    o2 = SYMBOL_WITH_POS_SYM (o2);
+  if (symbols_with_pos_enabled)
+    {
+      if (SYMBOL_WITH_POS_P (o1))
+	o1 = SYMBOL_WITH_POS_SYM (o1);
+      if (SYMBOL_WITH_POS_P (o2))
+	o2 = SYMBOL_WITH_POS_SYM (o2);
+    }
 
   if (BASE_EQ (o1, o2))
     return true;

-- 
Alan Mackenzie (Nuremberg, Germany).