bug#22847: #17062: 24.3 current-fill-column breaks fill-match-adaptive-prefix

[Glenn Morris <rgm@gnu.org>]

Eli Zaretskii wrote:

>> TLDR:
>> Let's remove the test for nil fill-column in current-fill-column.
>
> I don't understand what you propose to do instead.
> current-fill-column does arithmetics on fill-column when it's non-nil,
> so we cannot just remove the test, because the function will then
> signal an error.

Yes, I'm fine with the error.

> I see 3 possible ways to fix these bugs:
>
>   . Fix the code which is not prepared for fill-column being nil to be
>     prepared.  This leaves everyone happy, except, perhaps, the person
>     who would need to fix all those places in Emacs.

I think this would be a waste of time for the Emacs, and third party,
maintainers.

>   . Change current-fill-column to return most-positive-fixnum when
>     fill-column is nil.

I suppose this would be ok, so long as it comes with something like a
once-per session display-warning about this being an obsolete usage that
will be removed soon.

>   . Disallow fill-column being nil and remove the test from
>     current-fill-column without changing anything else, i.e. let it
>     signal an error, perhaps with some text that tells this value is
>     no longer supported.  This will break setups of those who use that
>     value to disable auto-fill, something that was available since
>     forever, so I don't think we can do that.

That's what I would do. I don't have a problem breaking an undocumented
feature that already fails in several places, and has a trivial
workaround (don't want auto-fill - don't turn it on). Other times I can
recall similar breakage happening: byte-compile of nil, setq with odd
number of arguments. People gripe for a bit, then get on with life.