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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: how to detect that current font does not have particular unicode
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Date: Sat, 19 Dec 2020 11:18:27 +0200
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To: yyoncho <yyoncho@gmail.com>
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 (message from yyoncho on Sat, 19 Dec 2020 10:54:35 +0200)
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> From: yyoncho <yyoncho@gmail.com>
> Date: Sat, 19 Dec 2020 10:54:35 +0200
> 
> I am trying to implement an automatic fallback if the user has configured a font that does not have a
> particular unicode char (e. g. “💡” (U+1F4A1)).

What kind of fallback?  IOW, what did you intend to do if you find out
that the font configured by the user does NOT have a glyph for the
character in question?

I'm asking because some fallbacks don't need to know whether a
particular font can display a particular character.  But I'm not sure
those fallbacks are appropriate for your use case.

> I tried to use char-displayable-p but it seems like it is not
> what I am looking for.

I believe you want font-get-glyphs.