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

[Alan Mackenzie <acm@muc.de>]

On Sat, Aug 05, 2023 at 16:13:13 +0300, Eli Zaretskii wrote:
> > Date: Sat, 5 Aug 2023 13:04:07 +0000
> > Cc: 65051@debbugs.gnu.org, monnier@iro.umontreal.ca, acm@muc.de
> > From: Alan Mackenzie <acm@muc.de>

> > > > No, only the bare symbol is in the obarray.  The symbol with position
> > > > itself is a pseudovector, with contents (i) a bare symbol (a Lisp_Object
> > > > "pointing at" the obarray) and (ii) the unsigned integer position.

> > > Please document this factoid in the ELisp manual, I think it's very
> > > important, and having it undocumented is a Bad Thing.

> > DONE.  The change is near the top of the following patch, amended from
> > the previous version.

> LGTM, thanks.

Here's the latest, probably final, version of the patch.  It includes
new tests in fns-tests.el, adds paragraphs to the descriptions of `eq'
and `equal' in objects.texi, and removes the bit about symbols with
position being "stored somewhat like vectors" (at the suggestion of
Mattias E.).



diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index ad079e0d63a..784d59720ed 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -2206,6 +2206,10 @@ Equality Predicates
 the same object, and @code{eq} returns @code{t} or @code{nil}
 depending on whether the Lisp interpreter created one object or two.
 
+If @var{object1} or @var{object2} is a symbol with position, @code{eq}
+regards it as its bare symbol when @code{symbols-with-pos-enabled} is
+non-@code{nil} (@pxref{Symbols with Position}).
+
 @example
 @group
 (eq 'foo 'foo)
@@ -2363,6 +2367,13 @@ Equality Predicates
 The @code{equal} function recursively compares the contents of objects
 if they are integers, strings, markers, vectors, bool-vectors,
 byte-code function objects, char-tables, records, or font objects.
+
+If @var{object1} or @var{object2} is a symbol with position,
+@code{equal} regards it as its bare symbol when
+@code{symbols-with-pos-enabled} is non-@code{nil}.  Otherwise
+@code{equal} compares two symbols with position by recursively
+comparing their components.  @xref{Symbols with Position}.
+
 Other objects are considered @code{equal} only if they are @code{eq}.
 For example, two distinct buffers are never considered @code{equal},
 even if their textual contents are the same.
diff --git a/doc/lispref/symbols.texi b/doc/lispref/symbols.texi
index 34db0caf3a8..1f3b677d7fb 100644
--- a/doc/lispref/symbols.texi
+++ b/doc/lispref/symbols.texi
@@ -783,10 +783,15 @@ Symbols 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.
+together with an unsigned integer called the @dfn{position}.  Symbols
+with position don't themselves have entries in the obarray (though
+their bare symbols do; @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 +803,20 @@ 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.
+positions 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 behave as symbols, or have the
+other useful properties outlined in the previous paragraph.  @code{eq}
+returns @code{t} when given identical arguments, and @code{equal}
+returns @code{t} when given arguments with @code{equal} components.
+
+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}
@@ -810,17 +824,17 @@ Symbols with Position
 @code{position-symbol}.
 
 @defvar symbols-with-pos-enabled
-When this variable is non-@code{nil}, symbols with position behave
+When this variable is non-@code{nil}, a symbol with position behaves
 like the contained bare symbol.  Emacs runs a little more slowly in
 this case.
 @end defvar
 
 @defvar print-symbols-bare
-When bound to non-@code{nil}, the Lisp printer prints only the bare symbol of
-a symbol with position, ignoring the position.
+When bound to non-@code{nil}, the Lisp printer prints only the bare
+symbol of a symbol with position, ignoring the position.
 @end defvar
 
-@defun symbol-with-pos-p symbol.
+@defun symbol-with-pos-p symbol
 This function returns @code{t} if @var{symbol} is a symbol with
 position, @code{nil} otherwise.
 @end defun
diff --git a/src/fns.c b/src/fns.c
index d7b2e7908b6..9b300d503ff 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;
@@ -2824,8 +2827,8 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
 	if (ASIZE (o2) != size)
 	  return false;
 
-	/* Compare bignums, overlays, markers, and boolvectors
-	   specially, by comparing their values.  */
+	/* Compare bignums, overlays, markers, boolvectors, and
+	   symbols with position specially, by comparing their values.  */
 	if (BIGNUMP (o1))
 	  return mpz_cmp (*xbignum_val (o1), *xbignum_val (o2)) == 0;
 	if (OVERLAYP (o1))
@@ -2857,6 +2860,11 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
 	if (TS_NODEP (o1))
 	  return treesit_node_eq (o1, o2);
 #endif
+	if (SYMBOL_WITH_POS_P(o1)) /* symbols_with_pos_enabled is false.  */
+	  return (BASE_EQ (XSYMBOL_WITH_POS (o1)->sym,
+			   XSYMBOL_WITH_POS (o2)->sym)
+		  && BASE_EQ (XSYMBOL_WITH_POS (o1)->pos,
+			      XSYMBOL_WITH_POS (o2)->pos));
 
 	/* Aside from them, only true vectors, char-tables, compiled
 	   functions, and fonts (font-spec, font-entity, font-object)
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 79ae4393f40..9c09e4f0c33 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -98,6 +98,26 @@
   (should-not (equal-including-properties #("a" 0 1 (k "v"))
                                           #("b" 0 1 (k "v")))))
 
+(ert-deftest fns-tests-equal-symbols-with-position ()
+  "Test `eq' and `equal' on symbols with position."
+  (let ((foo1 (position-symbol 'foo 42))
+        (foo2 (position-symbol 'foo 666))
+        (foo3 (position-symbol 'foo 42)))
+    (let (symbols-with-pos-enabled)
+      (should (eq foo1 foo1))
+      (should (equal foo1 foo1))
+      (should-not (eq foo1 foo2))
+      (should-not (equal foo1 foo2))
+      (should-not (eq foo1 foo3))
+      (should (equal foo1 foo3)))
+    (let ((symbols-with-pos-enabled t))
+      (should (eq foo1 foo1))
+      (should (equal foo1 foo1))
+      (should (eq foo1 foo2))
+      (should (equal foo1 foo2))
+      (should (eq foo1 foo3))
+      (should (equal foo1 foo3)))))
+
 (ert-deftest fns-tests-reverse ()
   (should-error (reverse))
   (should-error (reverse 1))


-- 
Alan Mackenzie (Nuremberg, Germany).