Subject: bug#78608: 31.0.50; Todo mode bug revealed by setq-default change

From: Stefan Monnier <[email-masked]>
Date: Wed, 28 May 2025 12:59:52 -0400

>>  * Incompatible Lisp Changes in Emacs 31.1
>>  
>> +** 'setq-local' computes the value before making the variable local.
>> +The previous code made the variable buffer local before
>> +computing the value to assign to it.
>> +This bug fix is a subtle change to the semantics which should make
>> +no difference to the vast majority of uses but can occasionally affect
>> +code in surprising ways if the computation of the value refers to the
>> +variable.
>
> I think this should describe the case upon which Stephen stumbled:
> code which uses setq-local inside a let-binding (perhaps an implicit
> one, such as the one the various with-SOMETHING macros do), and
> expects the buffer-local value to stay after the let-binding is
> exited.

Maybe you should write it, then because you seem to understand that case
better than I (I did look at the code to try and figure out what was the
specific origin of the problem but .. failed).  But note that it's only
one case that can be affected, there are others.

> I'd remove the last sentence entirely, because it is not useful.

I wrote it as a more generic description: the todo code is but one case
where the behavior difference can show up.  Maybe it's too generic, but
I don't know how to be more specific without being *too* specific
(i.e. fail to include cases which are affected).

        Stefan