bug#33890: 27.0.50; Incorrect behaviour of cl-loop macro

[Quek Yu Han]

It appears that a change to the `cl-loop` macro in Emacs 27 introduced a
bug where certain clauses would be evaluated more than once in a loop
with a "when" clause.

See the following minimal example which finds the index of an element in
a list:


(cl-loop
 for x in '(a b c d e)
 for index = 0 then (1+ index)
 when (eq x 'e)
 return index)


In Emacs 26.1 this returns `4` as expected, but in Emacs 27.0.50 it
returns `7`, due to the index being incremented twice per loop. The same
occurs after a `emacs -Q` restart.

For reference here is the macroexpanded form of the above expression,
with the difference between 26 and 27 outlined (everything else is
identical):


(cl-block nil
  (let* ((--cl-var-- '(a b c d e))
         (x nil)
         (index nil)
         (--cl-var-- t)
         (--cl-var-- t)
         --cl-var--)
    (while (and (consp --cl-var--)
                (progn
                  (setq x (car --cl-var--))
                  (setq index
                        (if --cl-var-- 0 (1+ index)))
                  (if (eq x 'e)
                      (setq --cl-var-- index
                            --cl-var-- nil)
                    t)))
      (setq --cl-var-- (cdr --cl-var--))

;;;============== ADDED IN EMACS 27 =================
      (and (and (consp --cl-var--)
                (progn
                  (setq x (car --cl-var--))
                  (setq index
                        (if --cl-var-- 0 (1+ index)))
                  (if (eq x 'e)
                      (setq --cl-var-- index
                            --cl-var-- nil)
                    t))))
;;;==================================================

      (setq --cl-var-- nil))
    --cl-var--))



Thanks!





In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin17.6.0, NS
appkit-1561.40 Version 10.13.5 (Build 17F77))
 of 2018-07-08 built on QYH.local
Repository revision: 98463bed07ecc9057094928ffdc2321eaaf3ab52
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.2

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

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 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/usr/local/share/emacs/site-lisp
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 --prefix=/usr/local/Cellar/emacs-plus/HEAD-98463be --with-xml2
 --without-dbus --with-gnutls --with-imagemagick --with-modules
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Configured features:
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Important settings:
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  value of $LC_CTYPE: UTF-8
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Load-path shadows:
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Features:
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Memory information:
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