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

["julesbertholet at quoi dot xyz" <sourceware-bugzilla@sourceware.org>]

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

            Bug ID: 31370
           Summary: wcwidth() does not treat DEFAULT_IGNORABLE_CODE_POINTs
                    as zero-width
           Product: glibc
           Version: 2.40
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: localedata
          Assignee: unassigned at sourceware dot org
          Reporter: julesbertholet at quoi dot xyz
                CC: libc-locales at sourceware dot org
  Target Milestone: ---

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:

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

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

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