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

[julesbertholet at quoi dot xyz]

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

--- Comment #3 from Jules Bertholet <julesbertholet at quoi dot xyz> ---
> 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>:

> 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.

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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