bug#74966: 31.0.50; Crash report (using igc on macOS)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 21 Dec 2024 15:18:43 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: Gerd Möllmann <gerd.moellmann@gmail.com>, Eli Zaretskii <eliz@gnu.org>, acorallo@gnu.org, spd@toadstyle.org, 74966@debbugs.gnu.org
> 
> "Stefan Monnier" <monnier@iro.umontreal.ca> writes:
> 
> >>> `offset` here should be fixnum that gives the position of this docstring
> >>> in the DOC file.  And FUN should be a function for which we found
> >>
> >> Yes, but the nativecomp code assumes ->doc is an index into a
> >> nativecomp'd subr's constant vector.
> >
> > Aha!
> >
> >> So we overwrite it with a docfile
> >> index, access an out-of-bounds index and crash.
> >>
> >> I think the best thing to do is to use separate fields for the "offset"
> >> doc and the "index" doc; or at least, the second best thing, after
> >> removing the entire docfile hack.
> >
> > I think a much simpler change is to use the sign bit to distinguish indices
> > into the constant vector from indices into the DOC file.
> 
> And use one's-complement, I assume, to guard against some future weird
> nativecomp change resulting in the index -0?  :-)
> 
> I really have no strong preference here.

And I still want to hear from Andrea.  It's his code, so the solution
he prefers gets my vote.