bug#49797: 28.0.50; Setting face to custom fontset doesn't work

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Sat, 31 Jul 2021 17:37:05 -0400
> 
> If I evaluate the following:
> 
> (progn
>   (create-fontset-from-fontset-spec
>    (font-xlfd-name
>     (font-spec :registry "fontset-lunamono")))
>   (set-fontset-font "fontset-lunamono" 'latin (font-spec :family "SF Mono"))
>   (set-face-attribute 'default nil :fontset "fontset-lunamono”))
> 
> I expect Latin characters to be shown in SF Mono font, but that’s not what 
> happened. If I instead set font for the default fontset, the characters in 
> the buffer are correctly displayed in SF Mono font. I tried this code snippet 
> with Emacs 26, 27 and 28, and all of them have this problem.
> 
> I can see that the fondest is correctly modified by looking at the result of 
> describe-fontset, so I guess it’s the face that isn’t finding the correct 
> font to display the character.

CC'ing Handa-san, in the hope that he could help us with this issue.

Personally, I lack a clear understanding of how user-defined fontsets
are supposed to be used instead of the standard fontsets.  Is that
only by using the :font attribute of a face?