[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

--- Comment #4 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Jules Bertholet from comment #3)
> > Please provide a patch to libc-alpha@sourceware.org
> 
> https://sourceware.org/pipermail/libc-alpha/2024-February/154574.html
> 
> > Please also provide justification for the zero width by quoting another implementation that also provides zero width e.g. CLDR.
> 
> CLDR doesn't address width issues at all, this is defined by Unicode itself.
> The Unicode Standard, version 15.0, §5.21 - Characters Ignored for Display
> <https://www.unicode.org/versions/Unicode15.1.0/ch05.pdf#G40095>:

What do the libicu APIs return for these characters?

> > The list of characters which should be ignored for display in fallback rendering is given by a character property: Default_Ignorable_Code_Point (DI). Those characters include almost all format characters, all variation selectors, and a few other exceptional characters, such as Hangul fillers. The exact list is defined in DerivedCoreProperties.txt in the Unicode Character Database.
> 
> U+115F HANGUL CHOSEONG FILLER needs a carveout due to the unique behavior of
> the conjoining Korean jamo characters. One composed Hangul "syllable block"
> like 퓛 is made up of two to three individual component characters, or
> "jamo". These are all assigned an `East_Asian_Width` of `Wide` by Unicode,
> which would normally mean they would all be assigned width 2 by glibc; a
> combination of (leading choseong jamo) + (medial jungseong jamo) + (trailing
> jongseong jamo) would then have width 2 + 2 + 2 = 6. However, glibc (and
> other wcwidth implementations) special-cases jungseong and jongseong,
> assigning them all width 0, to ensure that the complete block has width 2 +
> 0 + 0 = 2 as it should. U+115F is meant for use in syllable blocks that are
> intentionally missing a leading jamo; it must be assigned a width of 2 even
> though it has no visible display to ensure that the complete block has width
> 2.

Justification like this is *great* to have in the commit message e.g. here in a
v2.
https://patchwork.sourceware.org/project/glibc/patch/20240211175840.228824-2-julesbertholet@quoi.xyz/

> You can read more about Unicode jamo in the Unicode spec, sections 3.12
> <https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G24646> and 18.6
> <https://www.unicode.org/versions/Unicode15.0.0/ch18.pdf#G31028>.

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