GNU bug report logs - #26624
26.0.50; Generalized variable `buffer-local-value' does't restore local flag

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From: Philipp Stephani <p.stephani2 <at> gmail.com>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 26624 <at> debbugs.gnu.org
Subject: bug#26624: 26.0.50; Generalized variable `buffer-local-value' does't restore local flag
Date: Sun, 24 Sep 2017 15:19:48 +0000

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Philipp Stephani <p.stephani2 <at> gmail.com> schrieb am So., 2. Juli 2017 um
18:53 Uhr:

> Michael Heerdegen <michael_heerdegen <at> web.de> schrieb am So., 18. Juni
> 2017 um 06:17 Uhr:
>
>> Philipp Stephani <p.stephani2 <at> gmail.com> writes:
>>
>> > It's possible to fix this (see attached patch), but at the expense of
>> > breaking other valid use cases such as (cl-incf (buffer-local-value
>> > ...)). Not sure whether the bug can be fixed at all without breaking
>> > other stuff.
>>
>> I have no solution, but some thoughts.
>>
>> The more I think about it, the more I come to the conclusion that
>> `buffer-local-value' does not have a well defined according place.
>>
>> The function `buffer-local-value' is not injective: it maps different
>> states to the same value because it can't express whether the VARIABLE's
>> binding is buffer-local or not.  But we need this information because we
>> need to undo creating a buffer local binding in the setter when closing
>> the `letf'.
>>
>> And the setter, accepting only a value for the binding, isn't
>> surjective, because the argument doesn't hold any information of
>> buffer-localness.  Moreover, we want the setter to always create a
>> buffer-local binding in one situation (setf), but this isn't true for
>> the setter we need to use for `cl-letf'.
>>
>> We could widen the semantics of `cl-letf' to do what we want in this
>> case, but I'm not sure if it's worth the trouble.  Not if there are more
>> cases like this.
>>
>>
> Thanks for this great analysis. Given this, it seems that the place
> definition for `buffer-local-value' should be removed from gv.el.
>

Here's a patch that does just that.

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