Accents package not working inside captions

Question

I'm writing a document involving phasors and I'd like to use egreg's notation from this post: \bar below symbol, e.g. $\underaccent{\bar}{U}$.

It works well except if I put the phasor inside a caption. This is a MWE:

\documentclass[10pt]{article}
\usepackage{accents}
\usepackage{caption}
\usepackage{tikz}

\begin{document}
\begin{itemize}
\item An ugly phasor: $\underline{U}$.
\item Another, better good-looking phasor: $\underaccent{\bar}{U}$.
\end{itemize}

\begin{figure}[h]\centering
\begin{tikzpicture}
\draw[-latex] (0,0) -- (3,0) node [right] {$\underaccent{\bar}{U}$};
\end{tikzpicture}
\caption{One phasor.}
%\caption{One phasor $\underaccent{\bar}{U}$.} % <-- NOT WORKING
\end{figure}

\begin{center}
\begin{tikzpicture}
\draw[-latex] (0,0) -- (3,0) node [right] {$\underaccent{\bar}{V}$};
\end{tikzpicture}
\captionof{figure}{Another phasor.}
%\captionof{figure}{Another phasor $\underaccent{\bar}{V}$.} % <-- NOT WORKING EITHER
\end{center}
\end{document}

As you can see I tried a workaround with the caption package, but to no avail. I think the relevant lines in my log file are probably the following, but I don't understand them:

! Undefined control sequence.
\cc@@accent #1#2->\let \cc@style 
                                 =#1\cc@fetch {#2}\mathaccent \cc@code {\ifc...

! Illegal parameter number in definition of \reserved@a.

I suppose that there is some kind of incompatibility with the accents package and the captions but I haven't seen anything about it in the documentation. Is there a better way to do this?

I'm working with TeX Live 2020, by the way.

Answer 1

It works if you use \DeclareRobustCommand

\documentclass[10pt]{article}
\usepackage{accents}
\usepackage{caption}
\usepackage{tikz}

\DeclareRobustCommand{\ubar}[1]{\underaccent{\bar}{#1}}

\begin{document}

\listoffigures

\begin{itemize}
\item An ugly phasor: $\underline{U}$.
\item Another, better good-looking phasor: $\underaccent{\bar}{U}$.
\end{itemize}

\begin{figure}[h]\centering
\begin{tikzpicture}
\draw[-latex] (0,0) -- (3,0) node [right] {$\underaccent{\bar}{U}$};
\end{tikzpicture}
\caption{One phasor $\ubar{U}$.} 
\end{figure}

\begin{center}
\begin{tikzpicture}
\draw[-latex] (0,0) -- (3,0) node [right] {$\underaccent{\bar}{V}$};
\end{tikzpicture}
\captionof{figure}{Another phasor $\ubar{V}$.}
\end{center}
\end{document}

yields