I'm not using the git version. I'm using what Debian is currently shipping.
I just git blamed the line that added the call to backward-up-list.



On Sat, Sep 2, 2023, 04:59 Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Aleksey Kliger <akliger@gmail.com>
> > Date: Fri, 1 Sep 2023 22:10:18 -0400
> >
> > This seems to be due to
> >
> https://github.com/emacs-mirror/emacs/commit/dfc850ca0223ffd1d2048657ea6d20c971865d51
>
> You mean, you haven't updated your Git since February?
>
> > specifically (backward-up-list 1 t t) seems to emit that error message:
> >
> >                 ;; Assume called interactively; don't signal an error.
> >                 (user-error "At top level")
> >
> > Easy reproduction: edit a csharp file, go to some toplevel line (e.g.
> "using System;" that is pervasive
> > in a lot of C# code) and run c-indent-line-or-region.
>
> Theo, could you please look into this?  TIA.
>
> > By the way: (advice-add 'c-guess-basic-syntax :around
> #'csharp-guess-basic-syntax)
> > means that csharp-guess-basic-syntax runs also in all other c-mode
> derived modes.  So this error
> > also happens if you first edit a .cs file and then a normal .c file and
> you try to, for example, indent a
> > toplevel comment in the C code.  Very frustrating.
>