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Subject: bug#76470: 31.0.50; `setq-local` makes the var buffer-local too soon
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Package: Emacs
Version: 31.0.50

Po Lu has just pointed me to a bug in `setq-local` which shows up when
you do something like:

    (setq-local my-foo (progn (defvar my-foo 1000) my-foo))

Commit afd088bd608 added a workaround for exactly this problem in one of
the uses of `setq-local`.

The problem is that `setq-local` does, fundamentally

    (make-local-variable 'my-foo)
    (let ((val (progn (defvar my-foo 1000) my-foo)))
      (setq my-foo val)

so that during the computation of `val`, the variable has already been
made buffer-local but has not yet been assigned.  So the `defvar` will
correctly define the default value of `my-foo` but `my-foo` will still
be unbound in the current-buffer.

I suggest we change the ordering to:

    (let ((val (progn (defvar my-foo 1000) my-foo)))
      (make-local-variable 'my-foo)
      (setq my-foo val)

As for how to implement that fix.  The simplest way is to just change
the macro, of course.  But maybe a better way is to do it by introducing
a `set-local` function (sibling to `set` and `set-default`) and then
make the macro expand to a call to this function.

When I discussed this with Po Lu he felt this was a risky subtle change
which would interfere with a lot of code.  I was quite surprised at this
reaction because my own gut feeling is that, while it is undoubtedly
a subtle change, it's just a very straightforward bug-fix, highly
unlikely to introduce problems.

WDYT?

        Stefan

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To: Stefan Monnier <monnier@HIDDEN>
Subject: bug#76470: Acknowledgement (31.0.50; `setq-local` makes the var
 buffer-local too soon)
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Subject: bug#76470: 31.0.50; `setq-local` makes the var buffer-local too soon
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Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of
text editors" <bug-gnu-emacs@HIDDEN> writes:

> Po Lu has just pointed me to a bug in `setq-local` which shows up when
> you do something like:
>
>     (setq-local my-foo (progn (defvar my-foo 1000) my-foo))
>
> Commit afd088bd608 added a workaround for exactly this problem in one of
> the uses of `setq-local`.
>
> The problem is that `setq-local` does, fundamentally
>
>     (make-local-variable 'my-foo)
>     (let ((val (progn (defvar my-foo 1000) my-foo)))
>       (setq my-foo val)
>
> so that during the computation of `val`, the variable has already been
> made buffer-local but has not yet been assigned.  So the `defvar` will
> correctly define the default value of `my-foo` but `my-foo` will still
> be unbound in the current-buffer.
>
> I suggest we change the ordering to:
>
>     (let ((val (progn (defvar my-foo 1000) my-foo)))
>       (make-local-variable 'my-foo)
>       (setq my-foo val)
>
> As for how to implement that fix.  The simplest way is to just change
> the macro, of course.  But maybe a better way is to do it by introducing
> a `set-local` function (sibling to `set` and `set-default`) and then
> make the macro expand to a call to this function.

What are the benefits of adding a new function instead of just changing
the macro?

> When I discussed this with Po Lu he felt this was a risky subtle change
> which would interfere with a lot of code.  I was quite surprised at this
> reaction because my own gut feeling is that, while it is undoubtedly
> a subtle change, it's just a very straightforward bug-fix, highly
> unlikely to introduce problems.
>
> WDYT?

I agree that we should fix the bug.

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