bug#40702: 28.0.50; (what-cursor-position) barfs on non-ASCII char

[Eli Zaretskii <eliz@gnu.org>]

> From: Štěpán Němec
>  <stepnem@gmail.com>
> Date: Sun, 19 Apr 2020 15:02:24 +0200
> Cc: 40702@debbugs.gnu.org
> 
> Looking at `what-cursor-position', apparently due to your
> `buffer-file-coding-system' being nil (which seems a bit strange to me:
> is even your (default-value 'buffer-file-coding-system) nil?)

buffer-file-coding-system being nil means 'no-conversion'.  You can
easily simulate that yourself, by an explicit setq, and you will then
get the error described in the report.

> the multibyte string isn't properly encoded and instead passed
> directly to `encoded-string-description', leading to the error.

Emacs 26.3 doesn't signal an error in this case, so I think this is a
regression we should fix.

> That said, there haven't been any relevant recent changes to
> `what-cursor-position'.
> 
> In any case, I think more info is needed: backtrace, system/environment.

Here's a backtrace:

  Debugger entered--Lisp error: (cl-assertion-failed ((not (multibyte-string-p str)) nil))
    cl--assertion-failed((not (multibyte-string-p str)))
    encoded-string-description(#("é" 0 1 (charset unicode)) nil)
    describe-char(146)
    what-cursor-position((4))
    funcall-interactively(what-cursor-position (4))
    call-interactively(what-cursor-position nil nil)
    command-execute(what-cursor-position)