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

[Zdenek Wagner]

You have to use $e_{\textnormal{m}}$ because the _ character expect a
token and wothiut braces it is \textnormal. When it is expanded, it
finds that an argument is missing, it does not see {m}. That's why you
have to use the outer braces because _ then takse a balanced text,
i.e. \textnormal{m} will be used as the underscore.

Zdeněk Wagner
http://ttsm.icpf.cas.cz/team/wagner.shtml

čt 17. 2. 2022 v 17:37 odesílatel Vincent Lefevre <vincent at vinc17.net> napsal:
>
> 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).
>
> --
> Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)