[Bug localedata/31370] wcwidth() does not treat DEFAULT_IGNORABLE_CODE_POINTs as zero-width

["carlos at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31370

Carlos O'Donell <carlos at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-02-12
                 CC|                            |carlos at redhat dot com

--- Comment #2 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Jules Bertholet from comment #0)
> Unicode specifies (https://www.unicode.org/faq/unsup_char.html#3) that
> characters with the `Default_Ignorable_Code_Point` property
> 
> > should be rendered as completely invisible (and non advancing, i.e. “zero width”), if not explicitly supported in rendering.
> 
> Hence, `wcwidth()` should give them all a width of 0, with two exceptions:

Please provide a patch to libc-alpha@sourceware.org following:
https://sourceware.org/glibc/wiki/Contribution%20checklist

Please also provide justification for the zero width by quoting another
implementation that also provides zero width e.g. CLDR.

The goal is for glibc to harmonize closer to CLDR.

It seems sensible to me that they would be zero width if they are
non-advancing, but that isn't always what an end user needs (as seen below).

> - the soft hyphen (U+00AD SOFT HYPHEN) is assigned width 1 by longstanding
> precedent

We use 1 in UTF-8 (default width). So this matches. The expectation is that the
system is trying to determine a width where the hyphen is chosen during the
display process.

> - U+115F HANGUL CHOSEONG FILLER combines with jungseong and jongseong jamo
> to form a width-2 syllable block, and should therefore keep its width 2

We use 2 in UTF-8. So this matches.
<U1100>...<U115F>       2

> However, `wcwidth()` currently also incorrectly assigns non-zero width to
> U+3164 HANGUL FILLER and U+FFA0 HALFWIDTH HANGUL FILLER.

This needs justification by highlighting that we are harmonizing the
implementation with CLDR.

Currently we have:
<U3131>...<U318E>       2

While U+FFA0 is default 1.

Thanks for filling this issue.

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