babel and crossreftools incompatibility?

Question

The following MWE is giving error:

./test.tex:10: Missing \endcsname inserted.
<to be read again> 
               :   
l.10 \section{A section}\label{sec:one`}

The source:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[french,main=english]{babel}

\usepackage{crossreftools}

\begin{document}

\section{A section}\label{sec:one}

Some math $1+1=2$.

\end{document}

There is no error if I:

• delete \usepackage{crossreftools}; OR
• omit french from the options to `babel; OR
• replace french by german or russian.

The Incompatibilities section of the docs for crossreftools say "...any unexpandable content written to the aux-file by redefinitions of \label or @currentlabel etc will cause problems.

Is that what the french option to babel is doing — writing such unexpandable content to the .aux?

If so, is there some work-around?

Answer 1

crossreftools creates a list of labels and tries to write the label key to it. That is a bit fragile ... You could add \detokenize:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[french,main=english]{babel}

\usepackage{crossreftools}
\makeatletter
\renewcommand{\crtaddlabeltotoc}[1]{%
  \ifcrtfinal
  \else
  \addcontentsline{\crt@listoflabelsfileextension}{\crt@listoflabelsstructurelevel}{%
   \protect\numberline{\expandafter\crtrefnumber\expandafter{\detokenize{#1}}}#1}%
  \fi
}
\makeatother

\begin{document}
\crtlistoflabels
\tableofcontents
\section{A section:abc }  \label{sec:onea} \label{seconea}
Some math $1+1=2$.

\end{document}

Answer 2

The French option has made : active to produce French punctuation spacing and this interferes with its use in \label the simplest solution is simply to avoid punctuation characters in \label, \label{sec-one} works for example.

Normally babel ensures : still works in \label but apparently crossreftools disables that.

Answer 3

There is another place where a \detokenize is needed:

\renewcommand{\@@crtextr@ct@ref}[2]{%
  \expandafter\@@@crtextr@ct@ref\expandafter{\detokenize{#2}}{#1}%
}
\newcommand{\@@@crtextr@ct@ref}[2]{%
  \expandafter\ifx\csname r@#1\endcsname\relax
  \crt@refundefined%
  \else
  \expandafter\expandafter\csname crt@ref@splitter@#2\endcsname\csname r@#1\endcsname%
  \fi
}

My preference is to make the changes in my own copy of crossreftools.sty. And then just replace the original definitions with the new ones. That means in each case the \renewcommand should be replaced by \newcommand.