Index duplication error caused by tokenizing an optional argument in a macro

Question

I'm trying to create a command \term for printing a definition and then indexing it. It has one optional argument and one mandatory argument; if the optional argument is absent, the mandatory argument is used in its place. Here's the code I have:

% .tex file
\documentclass{article}
\usepackage{makeidx}
\usepackage{xparse}

\makeindex
\NewDocumentCommand{\term}{o m}{
    {\bfseries #2}
    \IfNoValueTF{#1}{
        \index{#2}
    }{
        \index{#1}
    }
 }

\begin{document}
A \term[pistar-boojum@$\pi_*$-boojum]{$\pi_*$-boojum} is\dots

Now I'm going to index $\pi_*$-boojum again.\index{pistar-boojum@$\pi_*$-boojum}

\printindex
\end{document}

The problem is, these two entries are displayed separately in the index:

Looking at the .ind file pinpoints the problem:

% .ind file
\begin{theindex}

  \item $\pi _*$-boojum, 1
  \item $\pi_*$-boojum, 1

\end{theindex}

I think the source of this issue is that \IfNoValueTF tokenizes the optional argument, inserting the space between \pi and _*. Then, makeindex treats \pi _* and \pi_* as different things.

Hence, my question is: is there a way to use the mandatory argument as the default value of an optional argument without tokenizing the optional argument? A priori it should be possible to check whether a string is empty without processing it, but I'd also be content to know that it's impossible in LaTeX.

Here are some things I tried:

• Using \IfNoValueTF as above
• Using \@dblarg as in this answer
• Using the ifmtarg package.
• Wrapping in \mbox as in this answer
• Using \detokenize as in this answer

Answer 1

Your problem here isn't xparse, it's \index, or rather trying to read already-tokenize material. The latter quite deliberately reads arguments more-or-less verbatim. As with all verbatim things, this only works if the input has not previously been tokenized. To do that and wrap \index into another command, you will have to do a verbatim read. We don't have 'verbatim optional arguments' in xparse (at least at the moment!), so one might do something like

\NewDocumentCommand{\term}{t[}{%
  \IfBooleanTF#1{\termauxi]}{\termauxii}%
}
\NewDocumentCommand\termauxi{vm}{%
  \index{#1}%
  {\bfseries #2}%
}
\NewDocumentCommand\termauxii{v}{%
  \index{#1}%
  {\bfseries\scantokens{#1}}%
}

The above yields .idx file

\indexentry{pistar-boojum@$\pi_*$-boojum}{1}
\indexentry{pistar-boojum@$\pi_*$-boojum}{1}
\indexentry{$\pi_*$-boojum}{1}

from the input in the question plus a third test line:

A \term[pistar-boojum@$\pi_*$-boojum]{$\pi_*$-boojum} is\dots

Now I'm going to index $\pi_*$-boojum again.\index{pistar-boojum@$\pi_*$-boojum}

A \term{$\pi_*$-boojum} is \dots

Of course, \scantokens can be a bit risky but it does allow us to have a more-or-less verbatim mandatory argument ...