Dimension too large after recent longtable update

Question

After recent update of longtable package (2021.05.07) I am getting the error:

Dimension too large. \end{longtabu}

Log:

! Dimension too large.
<to be read again> 
=
l.10 \end{longtabu}
             
I can't work with sizes bigger than about 19 feet.
Continue and I'll use the largest value I can.

MWE:

\documentclass{article}
\usepackage{longtable}%
\usepackage{tabu}%
\begin{document}
\begin{longtabu}  {llX}
a & b & c\\
\end{longtabu}
\end{document}

Answer 1

tabu is unmaintained but as a short term fix you can force an old version of longtable to load

\documentclass{article}
\usepackage{longtable}[=v4.13]%
\usepackage{tabu}%
\begin{document}

\begin{longtabu}  {llX}
a & b & c\\
\end{longtabu}
\end{document}

The changes in longtable have been very minor and only addressing features that fairly obviously needed addressing, but apparently tabu was relying on an internal definition somewhere.

---

It turns out that specifically tabu is relying on longtable using the \box1 scratch register and fails now longtable allocates a box for its internal use (a change made to avoid conflicts with other packages, you can't win...) there is a patch at the tabu github

https://github.com/tabu-issues-for-future-maintainer/tabu/issues/26

Answer 2

The new LaTeX3 package tabularray is an alternative to the outdated tabu package. You can use tblr and longtblr environments with it.

\documentclass{article}
\usepackage{tabularray}
\begin{document}
\begin{longtblr}[
  caption = {Table Caption},
]{llX[2,c]X[3,r]}
\hline
  a & b & c & d \\
\hline
\end{longtblr}
\end{document}