Inconsistent local/global assignment of hooks

[cgnieder]

Brief outline of the bug

Debugging expl3 code shows errors in the hook code (using LaTeX 2020-10-01 PL3).
This is not a problem in itself (I guess) but is at least annoying when debugging own code.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}
\ExplSyntaxOn
\sys_load_debug:
\debug_on:n {all}
\ExplSyntaxOff
\begin{document}
\end{document}

This example shows several errors of the form

./test.tex:7: LaTeX3 Error: Inconsistent local/global assignment

For immediate help type H <return>.
 ...                                              
                                                  
l.7 \begin{document}
                    

This is a coding error.

Global assignment to a constant variable '\__hook begindocument'.

Log file (required) and possibly PDF file

test.log

[FrankMittelbach]

bugger :-(

[FrankMittelbach]

part of PL4