bug#65017: 29.1; Byte compiler interaction with cl-lib function objects, removes symbol-function

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 4 Aug 2023 14:49:56 +0000
> Cc: monnier@iro.umontreal.ca, mattias.engdegard@gmail.com,
>   65017@debbugs.gnu.org, eric.marsden@risk-engineering.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > > symbols-with-pos-enabled gets erroneously
> > > bound to t in internal-macroexpand-for-load (emacs-lisp/macroexp.el).
> > > This is the cause of the bug; in cl--labels-convert it causes the first
> > > eq to return non-nil when comparing 'equal to #<symbol equal at 194>.
> 
> > Why "erroneously"? what are the rules for binding that variable to a
> > non-nil value?
> 
> internal-macroexpand-for-load isn't being called in the context of a
> byte compilation.  It might create a symbol with position which wrongly
> matches, or fails to match, another symbol.  This is what has happened
> in this bug.

If internal-macroexpand-for-load is "verboten" from being called by
the byte-compiler, I'd expect an assertion in it to that effect.
Because someone, some day, might easily forget and call that function
in the byte-compiler.

Btw, why was this binding added there to begin with?

> > I don't see any of that documented in the "Symbols with Position" node
> > of the ELisp manual.
> 
> Well, there is the sentence: "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.".
> 
> Do you think this should be firmed up to something like:  "These objects
> 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."?

Something like that, perhaps even stronger.  And maybe an explanation
what kind of problems could using them outside of the byte compiler
cause.

Thanks.