Why does an index entry generated from a listings' token preceeding a comma get rejected?

Question

I'm trying to index the tokens in a listing, but they always get rejected if they are followed by a comma.

Here's mwe.tex:

\documentclass{article}
\usepackage{listings}

\lstset{index={foobar}}

\makeindex

\begin{document}
  \begin{lstlisting}
    foobar, baz
  \end{lstlisting}
\end{document}

pdflatex mwe produces mwe.idx:

\indexentry{{\ttfamily  foobar\unhbox \voidb@x \kern \z@ }}{1}

makeindex mwe however rejects the entry, as seen in mwe.ilg:

This is makeindex, version 2.15 [TeX Live 2018] (kpathsea + Thai support).
Scanning input file mwe.idx...
!! Input index error (file = mwe.idx, line = 1):
   -- Extra `@' at position 44 of first argument.
done (0 entries accepted, 1 rejected).
Nothing written in mwe.ind.
Transcript written in mwe.ilg.

I feel like \unhbox \voidb@x \kern \z@ shouldn't be there. I figured out that's the expansion of \lst@nolig.

I located \lst@nolig's definition in listings.sty but its purpose is not clear to me and I'm not sure why this happens only when a token preceeds a comma.

\let\lst@nolig\@empty is a solution, but of course that's not satisfactory.

What's the proper solution to the problem?

Also, what's going on with the \lst@nolig?

Answer 1

The problem is indeed the comma, because in this case \index is passed foobar\lst@nolig. Redefine, locally, \lst@nolig to avoid the issue.

\documentclass{article}
\usepackage{listings}

\makeatletter
\renewcommand{\lstindexmacro}[1]{%
  \begingroup
  \def\lst@nolig{}%
  \index{#1@{\ttfamily#1}}%
  \endgroup
}
\makeatother

\lstset{index={foobar,baz}}

\makeindex

\begin{document}

\begin{lstlisting}
foobar, baz
\end{lstlisting}

\end{document}

The standard definition of \lstindexmacro is just \index{#1}, but this will give sorting problems. Here is the produced .idx file:

\indexentry{foobar@{\ttfamily  foobar}}{1}
\indexentry{baz@{\ttfamily  baz}}{1}