bug#57693: 29.0.50; Is there a more reliable version of `char-displayable-p'?

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@gmail.com>
> Cc: Stefan Kangas <stefankangas@gmail.com>,  57693@debbugs.gnu.org
> Date: Sat, 10 Sep 2022 14:06:16 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > The problem which AFAIU Ihor raised was that char-displayable-p
> > returns non-nil, when the character actually cannot be displayed.
> > This is what I'm asking: how come char-displayable-p thought it could
> > find a font for the character, when Emacs failed to find the font when
> > it was time to actually display that character?
> >
> > Ihor says that the reason could be some buffer-specific settings that
> > char-displayable-p didn't take into consideration.  If that's the
> > reason, could someone tell what those settings were in this case?
> 
> The author of the linked bug report solved the problem by installing
> some extra font. So, it was likely not a question of buffer-specific
> settings in that particular case.

Does it mean we don't understand why there was a failure in that case?
It is imperative that we do understand it, to see whether
char-displayable-p could be improved not to fail in those cases.