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From: Eli Zaretskii <eliz@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: how to detect that current font does not have particular unicode
 char
Date: Sat, 19 Dec 2020 14:00:07 +0200
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 (message from yyoncho on Sat, 19 Dec 2020 13:33:05 +0200)
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> From: yyoncho <yyoncho@gmail.com>
> Date: Sat, 19 Dec 2020 13:33:05 +0200
> Cc: emacs-devel <emacs-devel@gnu.org>
> 
>  What kind of fallback?  
> 
> I want to detect if char is visible and use a different one. E. g. bulb can be replaced with "!". 

But that's misleading, isn't it?  The user could think the character
really was "!".

You could instead set up the glyphless-char-display char-table to have
"!" in the slot for U+1F4A1, then Emacs will display the "!" character
inside a box, which at least cannot dupe the user.

>  > 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.
> 
> I will try that out. I am not sure what FONT-OBJECT parameter stands for?

The return value of one of functions that return such objects.  I
think in your case font-at will be useful in that role.