Missing \ExplSyntaxOff in xparse-2020-10-01.sty

[komascript]

xparse-2020-10-01.sty has a \ExplSyntaxOn but no corresponding \ExplSyntaxOff. Because of this, with

\RequirePackage[2020/02/01]{latexrelease}
\documentclass{article}
\typeout{space catcode before loading xsparse=\the\catcode`\ }%
\RequirePackage{xparse}%
\typeout{space catcode after loading xparse=\the\catcode`\ }%
\begin{document}
Test
\end{document}

you get

space catcode before loading xsparse=10
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3kernel/expl3.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def))
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse-2020-10-01.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse-generic.tex)))
space catcode after loading xparse=9

But with

\RequirePackage[2020/02/02]{latexrelease}
\documentclass{article}
\typeout{space catcode before loading xsparse=\the\catcode`\ }%
\RequirePackage{xparse}%
\typeout{space catcode after loading xparse=\the\catcode`\ }%
\begin{document}
Test
\end{document}

The result is as expected:

space catcode before loading xsparse=10
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3kernel/expl3.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def))
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse-2020-10-01.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse-generic.tex)))
space catcode after loading xparse=10

[komascript]

BTW: IMHO same bug in xparse-2018-04-12.sty, but I was not able force loading this version using latexrelease.

[PhelypeOleinik]

@komascript To load xparse-2018-04-12.sty you need real LaTeX 2020/02/02 or earlier (emulated with latexrelease won't be enough). More precisely, xparse.sty loads xparse-2018-04-12.sty if \NewDocumentCommand is undefined, and if \fmtversion<2020-10-01.

[PhelypeOleinik]

Hm, I transferred this issue because the bug is not in xparse: note the same files are loaded in both cases, and you get the same behaviour with any expl3-based package:

\begin{filecontents}{myexpl3pkg.sty}
\ProvidesExplPackage{test}{2020-20-20}{1.0}{Testing}
\end{filecontents}
%
\RequirePackage[2020/02/01]{latexrelease}
\documentclass{article}
\typeout{space catcode before loading myexpl3pkg=\the\catcode`\ }%
\RequirePackage{myexpl3pkg}
\typeout{space catcode after loading myexpl3pkg=\the\catcode`\ }%
\begin{document}
Test
\end{document}

The \ExplSyntaxOn/Off status is managed by \@pushfilename and \@popfilename in the same way as \makeatletter/other. Prior to 2020-02-02 the expl3 syntax management was patched by expl3.sty into \@push/\@popfilename, and after the expl3 integration it is there by default. The problem is that rolling back removes that part of the code and the automatic \ExplSyntaxOff after the package ends is no longer there.

I think this is one scenario where rolling back shouldn't roll back all the way through. We can either do a fake rollback and keep the expl3 syntax management in place (after all rolling back doesn't remove expl3), or do a clever checking whether expl3.sty is loaded and if it isn't, undefine \ProvidesExplPackage, so that a package will need to load expl3.sty explicitly, thus apply the patch... I don't like the looks of the second option :)