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

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

> I'd need to know more details.  Why did this test fail for that user?

You can find the details here:
https://lists.gnu.org/r/emacs-orgmode/2022-07/msg00402.html

Alternative link to the same thread:
https://list.orgmode.org/27918f90-4a82-9f70-611e-7fc5475e2e60@oracle.com/

> Also, what exactly does the test above intend to test?  Are you
> interested in whether this character can be displayed at all,
> regardless of which font is to be used, or do you want it to be
> displayed with the default face's font?

The purpose of the test is to figure out if we should use the pretty
UTF-8 characters or if we should use the less pretty ASCII ones.

The result is then used in the org agenda buffer.  So it had better
display, basically.  The problem is that some users saw that they got
the UTF-8 characters with the above code, but they displayed as the
"hex-squares" (I forget the official name).

> To answer your question: the most accurate way is to actually try
> displaying the character and see if that works.  Not sure if it helps
> you, though especially since that code is in a defcustom, AFAIU.

That doesn't help here, unfortunately.  :-(

We would need a way to figure this out programmatically.  (It's in a
defcustom, yes.)