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

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

Emacs master branch.

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.  Since the value for END contains (setq
pos ...), BEG gets this updated value of POS rather then the original
intended value.

This particular bug in jit-lock.el can be fixed by using let* rather
than let, but this isn't the point.  I believe (without testing) that
the interpreted code for the form would evaluate BEG before END, hence
testing it interpreted (e.g. under edebug) will give a false sense of
correctness.

The comment in byte-compile-let:

      ;; Bind the variables.
      ;; For `let', do it in reverse order, because it makes no
      ;; semantic difference, but it is a lot more efficient since the
      ;; values are now in reverse order on the stack.

, is not true.  It can make a semantic difference.  So doing the binding
in reverse order is a bug.

-- 
Alan Mackenzie (Nuremberg, Germany).