bug#71120: 29.3; buglet in cl-loop

[Andrea Corallo <acorallo@gnu.org>]

Andrea Corallo <acorallo@gnu.org> writes:

> Philip Kaludercic <philipk@posteo.net> writes:
>
>> Philippe Schnoebelen <phs@lmf.cnrs.fr> writes:
>>
>>> When I need a list of 100 random dice throws I write
>>>
>>> 	(cl-loop for i from 1 to 100 collect (random 6))
>>>
>>> It compiles just fine.
>>>
>>> If instead I use
>>>
>>> 	(cl-loop for _i from 1 to 100 collect (random 6))
>>>
>>> then I get a compilation warning:
>>>
>>> 	foo.el:1:18: Warning: variable ‘_i’ not left unused
>>>
>>> It should be the other way around.
>>
>> The issue here is that the warning describes an issue in the output, in
>> the latter case
>>
>>   (let* ((_i 1) (--cl-var-- nil))
>>     (while (<= _i 100)
>>       (setq --cl-var-- (cons (random 6) --cl-var--))
>>       (setq _i (+ _i 1)))
>>     (nreverse --cl-var--))
>>
>> As you see, _i is both evaluated in (+ _i 1) and updated.
>>
>> Here you have a minimal working example of the warning:
>>
>>   (byte-compile (lambda () (let ((_x 3)) _x)))
>>
>>   ;; Warning: variable ‘_x’ not left unused
>>
>> My guess is that fixing this would require cl-loop to notice that the
>> counter is prefixed with a "_" and then use some other variable, but
>> that might lead to issues with existing code.  Perhaps this
>> transformation might be safe in that case:
>>
>>   (let* ((<fresh-variable> 1) (--cl-var-- nil))
>>     (while (<= <fresh-variable> 100)
>>       (let ((_i <fresh-variable>))
>> 	(setq --cl-var-- (cons (random 6) --cl-var--))
>> 	(setq <fresh-variable> (+ <fresh-variable> 1))))
>>     (nreverse --cl-var--))
>>
>> I have added Mattias and Stefan to the CCs, as they'll probably have
>> more qualified comments to add.
>
> This is the same transformation that came to my mind reading the orginal
> report, I think it should be safe.

As a datapoint, SBCL does expands this with inside a

(declare (ignore i))

Unfortuantelly we don't support it :/ and our 'ignore' doesn't work here
as goes in the opposite direction :(

  Andrea