With beamer, "Missing { inserted." (regression)

[Enrico Gregorio]

> On 17 Feb 2022, at 17:37 , Vincent Lefevre <vincent at vinc17.net> wrote:
> 
> Under Debian/unstable with the TeX Live 2021 Debian packages
> and with the old TeX Live 2020 Debian packages, pdflatex on
> 
> \documentclass{beamer}
> \begin{document}
> \begin{frame}
> $e_\textnormal{m}$
> \end{frame}
> \end{document}
> 
> gives the following error:
> 
> ! Missing { inserted.
> <to be read again> 
>                   \def 
> l.5 \end{frame}
> 
> There are no issues with TeX Live 2018 under Debian 10 buster.
> The author of the full .tex file uses a macOS machine, so I assume
> that there are no issues there either (but I don't know which
> TeX distribution it is).

The syntax $e_\textnormal{m}$ has never been supported and
if it works is just by accident. The proper syntax is to
brace _any_ subscript (or superscript), but there’s usually 
no problem when there is just a _single_ token.

However, try $A_\notin$ and you’ll learn that $A_{\notin}$
is much better.

Ciao
Enrico