Subject: Re: bug#65051: internal_equal manipulates symbols with position without checking symbols-with-pos-enabled.

From: Stefan Monnier <[email-masked]>
Date: Sat, 12 Aug 2023 14:46:24 -0400

Thanks,

        Stefan

diff --git a/doc/lispref/symbols.texi b/doc/lispref/symbols.texi
index 34db0caf3a8..d2e397faf80 100644
--- a/doc/lispref/symbols.texi
+++ b/doc/lispref/symbols.texi
@@ -782,11 +782,16 @@ Symbols with Position
 @cindex symbol with position
 
 @cindex bare symbol
-A @dfn{symbol with position} is a symbol, the @dfn{bare symbol},
-together with an unsigned integer called the @dfn{position}.  These
-objects are intended for use by the byte compiler, which records in
-them the position of each symbol occurrence and uses those positions
-in warning and error messages.
+A @dfn{symbol with position} is a pair of a symbol, the @dfn{bare
+symbol}, together with an unsigned integer called the @dfn{position}.
+Symbol with position cannot themselves have entries in obarrays
+(contrary to their bare symbols; @pxref{Creating Symbols}).
+
+Symbols with position are for the use of the byte compiler, which
+records in them the position of each symbol occurrence and uses those
+positions in warning and error messages.  They shouldn't normally be
+used otherwise.  Doing so can cause unexpected results with basic
+Emacs functions such as @code{eq} and @code{equal}.
 
 The printed representation of a symbol with position uses the hash
 notation outlined in @ref{Printed Representation}.  It looks like
@@ -798,11 +803,21 @@ Symbols with Position
 
 For most purposes, when the flag variable
 @code{symbols-with-pos-enabled} is non-@code{nil}, symbols with
-positions behave just as bare symbols do.  For example, @samp{(eq
-#<symbol foo at 12345> foo)} has a value @code{t} when that variable
-is set (but @code{nil} when it isn't set).  Most of the time in Emacs this
-variable is @code{nil}, but the byte compiler binds it to @code{t}
-when it runs.
+position behave just as their bare symbols would.  For example,
+@samp{(eq #<symbol foo at 12345> foo)} has a value @code{t} when the
+variable is set; likewise, @code{equal} will treat a symbol with
+position argument as its bare symbol.
+
+When @code{symbols-with-pos-enabled} is @code{nil}, any symbols with
+position continue to exist, but do not always behave as symbols.
+Most importantly @code{eq} only returns @code{t} when given truly
+identical arguments, for performance reasons.  @code{equal} on the
+other hand continues to treat a symbol with
+position argument as its bare symbol.
+
+Most of the time in Emacs @code{symbols-with-pos-enabled} is
+@code{nil}, but the byte compiler and the native compiler bind it to
+@code{t} when they run.
 
 Typically, symbols with position are created by the byte compiler
 calling the reader function @code{read-positioning-symbols}
diff --git a/src/fns.c b/src/fns.c
index ac30670b3ac..fde4ef6b08b 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -5166,7 +5166,7 @@ sxhash_obj (Lisp_Object obj, int depth)
 	    hash = sxhash_combine (hash, sxhash_obj (XOVERLAY (obj)->plist, depth));
 	    return SXHASH_REDUCE (hash);
 	  }
-	else if (symbols_with_pos_enabled && pvec_type == PVEC_SYMBOL_WITH_POS)
+	else if (pvec_type == PVEC_SYMBOL_WITH_POS)
 	  return sxhash_obj (XSYMBOL_WITH_POS (obj)->sym, depth + 1);
 	else
 	  /* Others are 'equal' if they are 'eq', so take their