Hyperref conflicting with enumitem

Question

The code

\documentclass[11pt]{article}
\usepackage{hyperref}
\usepackage{enumitem}
\newlist{ilist}{enumerate}{3}
\setlist*[ilist]{leftmargin=*}
\setlist*[ilist,1]{label=\arabic*.}
\setlist*[ilist,2]{label=\arabic{ilisti}.,before=\refstepcounter{ilisti}}
\setlist*[ilist,3]{label=\arabic{ilisti}.,before=\refstepcounter{ilisti}}
\begin{document}
\begin{ilist}
\item A
\begin{ilist}
\item B
\begin{ilist}
\item C
\end{ilist}
\end{ilist}
\end{ilist}
\end{document}

is somehow producing the error

The error resolves if I comment out \usepackage{hyperref}.

Any ideas as to why?

With very few exceptions, hyperref should be loaded after all the other packages. — samcarter_is_at_topanswers.xyz, Feb 13 '24 at 23:54
@samcarter_is_at_topanswers.xyz in this case you need to load it after all the enumitem definitions not just after the package, which looks a bit strange, but.... — David Carlisle, Feb 14 '24 at 00:02
@samcarter_is_at_topanswers.xyz that was true years ago, but now we want to remove these restrictions and if things do not work with a different loading order we try to correct that. — Ulrike Fischer, Feb 14 '24 at 09:43
@UlrikeFischer Do you mean that the rule that hyperref must, with rare exceptions, be loaded last will no longer apply? — Denis Bitouzé, Feb 21 '24 at 11:51
@DenisBitouzé see https://tex.stackexchange.com/a/709547/2388 — Ulrike Fischer, Feb 21 '24 at 12:13

score 9 · Accepted Answer · answered Feb 14 '24 at 09:32

It is an error in hyperref. hyperref tries to provide a \theH<counter> definition for every new counter and uses for this \arabic{#1}. But this means that commands in the #1 are not directly expanded and this falls over here as enumitem uses \@definecounter{\enit@c\romannumeral\count@} to define the new list counters and so e.g. \theHilisti looks like this:

> \theHilisti=macro:
->\arabic {\enit@c \romannumeral \count@ }.

I will change the hyperref definition. A work-around for now is this

\documentclass[11pt]{article}
\usepackage{hyperref}
\makeatletter
\def\@definecounter#1{%
     \H@definecounter{#1}%
     \expandafter
     \gdef\csname theH#1\expandafter\endcsname\expandafter
          {\expandafter\@arabic\csname c@#1\endcsname}}%
\makeatother   
\usepackage{enumitem}
\newlist{ilist}{enumerate}{3}
\setlist*[ilist]{leftmargin=*}
\setlist*[ilist,1]{label=\arabic*.}
\setlist*[ilist,2]{label=\arabic{ilisti}.,before=\refstepcounter{ilisti}}
\setlist*[ilist,3]{label=\arabic{ilisti}.,before=\refstepcounter{ilisti}}
\begin{document}
\begin{ilist}
\item A
\begin{ilist}
\item B
\begin{ilist}
\item C
\end{ilist}
\end{ilist}
\end{ilist}
\end{document}

Hyperref conflicting with enumitem

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