Why does \nobreakdash inside a figure caption break unless previously used?

Question

Simply put, the following does not compile:

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{figure}
    \caption{
        poly\nobreakdash-Si
    }
\end{figure}

poly\nobreakdash-Si % move this to before figure caption and it works

\end{document}

but, as noted in the code comment, if the command has been used in the body of the document before its use inside the figure caption it does compile (and produce the desired result, in a more complicated example).

Why does this happen?

The error message is ! Argument of \@tempb has an extra }. I have tested this with pdflatex and lualatex in TeX Live 2017 and 2020.

Answer 1

latex includes a better version of the amsmath command \nobreakdash, \nobreakdashes. It works with -- and --- and importantly here, it is robust so works in the section heading.

\documentclass{article}
%\usepackage{amsmath}
\begin{document}
\begin{figure}
    \caption{
        poly\nobreakdashes-Si
    }
\end{figure}
poly\nobreakdashes-Si % move this to before figure caption and it works
\end{document}