eqref in captions with mathtools

Question

I've now tried mathtools for the first time, for the sake of showonlyrefs. It works fine, except if I put an \eqref command into a figure caption, in which case I get:

! Argument of \@caption has an extra }.
! Paragraph ended before \@caption was complete.

Here is the MWE:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\mathtoolsset{showonlyrefs=true,showmanualtags=true} % do not number unreferenced equations
\begin{document}
  \begin{equation}\label{eq:EVE-HEH}
    x
  \end{equation}
  \begin{figure}
    \caption{See eigenvalue equation \eqref{eq:EVE-HEH}.}
  \end{figure}
\end{document}

What do I do wrong?

Update: Found here that \protect before \eqref in caption helps, and indeed it does. However this is not mentioned in the mathtools. Is it the right solution?

Answer 1

Unfortunately \eqref is not a robust command, so it needs \protect in front of it when used in a moving argument, such as the argument of \caption or \section.

If you happen to use many times \eqref in such places, you can consider "robusting" it:

\usepackage{fixltx2e,amsmath}
\MakeRobust{\eqref}

Note that \eqref is provided by amsmath, rather than mathtools (this last package loads amsmath).