Mysterious "Undefined control sequence" error under XeLaTeX

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I am using MacTeX-2020, which has this version of pgfmorepages:

\ProvidesPackage{pgfmorepages}[2020/04/16 v1.20 multiple page manipulation]

The following document renders, if I comment out all three pgfmorepages-related lines. But with those lines in place, xelatex stops and complains.

\documentclass[10pt]{article}
\usepackage[
    paper=a5paper,
]{geometry}
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{pgfmorepages}
\usepackage{lipsum}
\pgfmorepagesloadextralayouts
\pgfpagesuselayout{4 on 2, book format}[a4paper,landscape]
\defaultfontfeatures[\rmfamily,\sffamily]{}
\defaultfontfeatures{Scale=MatchLowercase}
\defaultfontfeatures[\rmfamily]{Scale = 1}
\setmainfont{Vollkorn}

\begin{document}

\lipsum[1-20]{}

\end{document}

! Undefined control sequence.
\pgfpages@shipoutshipoutbox ...es@originalshipout 
                                                  \vbox {\hbox {\hskip -1in\...
l.18 \lipsum[1-20]{
                   }

[loopspace]

Hi,

Is it just XeLaTeX? Or are the other engines also affected?

Can you find out what version of expl3 is on your system? There was a moment where they changed how the LaTeX shipout mechanism worked and this might be related.

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I only use XeLaTeX, but if you can tell me exactly what you'd like me to test with other engines I can give it a go.

expl3.sty begins with this line:

\def\ExplFileDate{2020-12-03}%

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And actually the TeX Live Utility just offered an upgrade to l3kernel, so now it has 2020-12-07, but the problem is still happening.

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This seems to be an issue upstream with how the LaTeX3 team are adjusting things. I've filed an issue there https://github.com/latex3/latex3/issues/829

If you need something before it is fixed upstream, putting the following in your preamble before loading pgfmorepages seems to work:

\ExplSyntaxOn
\makeatletter
\RemoveFromHook{file/after/pgfmorepages.sty}[firstaid]
\AddToHook{file/after/pgfmorepages.sty}[firstaid]{%
  \cs_set_nopar:Npn \pgfhookintoshipout {
    \cs_set_eq:NN \pgfpages@originalshipout \tex_shipout:D
    \cs_set_eq:NN \tex_shipout:D \pgfpages@interceptshipout
  }
}
\makeatother
\ExplSyntaxOff

[loopspace]

This appears to be now fixed in https://github.com/latex3/latex3/issues/829 and has been published to CTAN. Updating your distribution should now fix it.

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Thank you!