Hi.
Digging a little more, I found that \l__xeCJK_listings_letter_bool is
never defined.
Adding the following line in
/usr/share/texlive/texmf-dist/tex/xelatex/xecjk/xeCJK-listings.sty (eg.
line 160) corrects the bug:
\bool_new:N \l__xeCJK_listings_letter_bool
I also tried the following source file:
\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\bool_if:NTF \l__TEST { TRUE } { FALSE }
\ExplSyntaxOff
\end{document}
It compiles OK with LaTeX3 from 2017.20170629-1 (showing "FALSE"), but
reports an error (I think this is the proper result) with 2017.20170724-1.
Regards,
Alexis Bienvenüe.
