Why does loading `newenviron` cause compilation to fail?

Question

To avoid unnecessary effort, please note that I'm asking specifically about the error's cause, as opposed to how to avoid it. Furthermore, I'm aware that \NewDocumentEnvironment now offers the b option.

Consider the following MNWE,

\documentclass[]{article}
\usepackage{newenviron}
\begin{document}
hylo
\end{document}

This fails to compile, claiming there's an extra \endgroup, so that \document ends with \endgroup.

This is pdfTeX, Version 3.141592653-2.6-1.40.25 (TeX Live 2023) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./prawf.tex
LaTeX2e <2023-06-01> patch level 1
L3 programming layer <2023-10-10>
(/usr/local/texlive/2023/texmf-dist/tex/latex/base/article.cls
Document Class: article 2023/05/17 v1.4n Standard LaTeX document class
(/usr/local/texlive/2023/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2023/texmf-dist/tex/latex/newenviron/newenviron.sty
(/usr/local/texlive/2023/texmf-dist/tex/latex/catoptions/catoptions.sty))
! Extra \endgroup.
\document ->\endgroup 
                      \let \BeforeStartOfDocument \@firstofone \cpt@beforest...
l.199 \begin{document}
                      
? 

What causes this error exactly?

Answer 1

While adding the hook code a couple of years back, some details of the \document command changed, in order to add the hooks, which means that packages that patched \document may have made assumptions that are no longer valid. catoptions as used here was always intentionally changing latex internals in unsupported ways, and finally it broke and has not so far been updated.

In practice most uses of newenviron (and environ) packages are better done these days with the \NewDocumentEnvironment b argument type which does not require any package to be loaded.