bug#67116: byte-compile-let: reversing the order of evaluation of the clauses CAN make a difference.

[Eli Zaretskii]

> Cc: 67116@debbugs.gnu.org
> Date: Sat, 11 Nov 2023 23:52:38 -0500
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> > In lisp/emacs-lisp/bytecomp.el (byte-compile-let), when the following
> > form (from jit-lock--debug-fontify):
> >
> >                           (let
> >                               ((beg pos)
> >                                 (end (setq pos
> >                                                (next-single-property-change
> >                                                 pos 'fontified
> >                                                 nil (point-max)))))
> >                             (put-text-property beg end 'fontified nil)
> >                             (jit-lock-fontify-now beg end))
> >
> > gets byte compiled, the order of evaluating BEG and END gets reversed so
> > that END gets evaluated first.
> 
> Sounds like a bug.

It does?  I always thought that the order of evaluation in a let form
is unspecified, and in practice I had several bugs of exactly this
nature, which I fixed by using let*, as expected.

Why on Earth should we require any particular order of evaluation in a
let form??