> AFAIU, the problem in this case wasn't that the backend wasn't
> supported, it was that it was changed in the middle of a session.

Yes, but only at emacs starting (~/.emacs evaluation), after starting there is no problem to set a font-backend.

On Tue, Mar 10, 2020 at 11:52 AM Eli Zaretskii <eliz@gnu.org> wrote:

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Vinicius José Latorre <viniciusjl@gmail.com>,
> 39865@debbugs.gnu.org
> Date: Tue, 10 Mar 2020 15:29:44 +0100
>
> >> I think that emacs should be more robust about this and do not crash for any bad value.
>
> Eli> I agree. It is not a bad value, we just couldn't yet find a way of
> Eli> switching the font-backend.
>
> Eli, would you accept a patch for emacs-27 that signals a user-error
> if trying to set a font-backend thatʼs not supported by the running
> emacs? Or that ignores it (with a message)?

In general, yes; but I'd need to see the patch.

AFAIU, the problem in this case wasn't that the backend wasn't
supported, it was that it was changed in the middle of a session.