bug#57072: [BUG] update-glyphless-char-display and variation selectors

[Eli Zaretskii <eliz@gnu.org>]

> From: Axel Svensson <svenssonaxel@gmail.com>
> Date: Fri, 12 Aug 2022 08:50:18 +0200
> Cc: 57072@debbugs.gnu.org
> 
> > What does Unicode say about the functionality of the variation
> > selectors beyond VS-16?
> 
> The code charts divide them into three groups:
> - VS1 through VS14 are "Variation selectors" [1]
> - VS15 through VS16 are "Emoji-specific variation selectors" [1]
> - VS17 through VS256 are "Ideographic-specific variation selectors" [2]
> 
> The standard itself in chapter 23.4 [3] makes no distinction between
> them but say that the only sanctioned uses that should have any effect,
> are the ones defined in:
> - StandardizedVariants.txt [4] in the Unicode Character Database, which
>   currently uses only VS1 through VS3. Confusingly though, some of them
>   seem to be used for ideographic purposes.
> - Unicode Technical Standard #51 for emojis [5], which says that VS15 is
>   "used to request a text presentation for an emoji character" while
>   VS16 is "used to request an emoji presentation for an emoji
>   character".
> - Unicode Technical Standard #37 for ideographic variation [6], which
>   confirms that it only uses VS17 through VS256.
> 
> In any case, it seems that admin/unidata/blocks.awk needs fixing, since
> it currently handles only VS1 through VS16 and does so as if they were
> all for emoji use.

AFAIR, blocks.awk does what it does only because VS16 has a special
function of requesting the Emoji presentation of characters that are
otherwise not Emoji, and our character-composition code needs to
realize that.  Unless the selectors beyond VS16 have similar
functions, I don't see any reason why we'd need to modify blocks.awk.

Or what am I missing?  IOW, to which part(s) of blocks.awk did you
allude when you wrote "it currently handles only VS1 through VS16 and
does so as if they were all for emoji use"?