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

[Alan Mackenzie]

Hello, Eli.

On Sat, Aug 05, 2023 at 15:13:22 +0300, Eli Zaretskii wrote:
> > Date: Sat, 5 Aug 2023 11:52:52 +0000
> > Cc: 65051@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>,
> >   acm@muc.de
> > From: Alan Mackenzie <acm@muc.de>

> > Hello, Eli.

> > On Sat, Aug 05, 2023 at 13:57:50 +0300, Eli Zaretskii wrote:
> > > > Date: Sat, 5 Aug 2023 10:45:37 +0000
> > > > Cc: 65051@debbugs.gnu.org, acm@muc.de
> > > > From: Alan Mackenzie <acm@muc.de>

> > [ .... ]

> > > > > So we can have two different copies of #<symbol foo at 42>, such that
> > > > > their hex values are different?  Isn't that a bug? why don't we
> > > > > conflate such identical symbols?

> > > > No, it's not a bug, anymore than having two `equal' copies of '(a b c)
> > > > would be a bug.

> > > But with symbols we store them in obarray, and obarray ought to find
> > > the existing slot for #<symbol foo at 42> and reuse it, rather than
> > > create a new slot?

> > 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.



diff --git a/doc/lispref/symbols.texi b/doc/lispref/symbols.texi
index 34db0caf3a8..a828d303c04 100644
--- a/doc/lispref/symbols.texi
+++ b/doc/lispref/symbols.texi
@@ -784,9 +784,15 @@ 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.
+objects are stored internally much like vectors, and 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 +804,20 @@ 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}
@@ -820,7 +835,7 @@ Symbols with Position
 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


-- 
Alan Mackenzie (Nuremberg, Germany).