List with spaces before the optional argument error

[u-fischer]

The current list implementation doesn't allow a space before the optional argument and so the following errors in a current dev with \DocumentMetadata (and the enumitem emulation) but compiles without it.

\DocumentMetadata{}
\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{description}  [leftmargin=1cm] 
\item[Hallo] Welt
\end{description}
\end{document}

! LaTeX Error: Something's wrong--perhaps a missing \item.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.123 \begin{description}  [
                            leftmargin=1cm]

[josephwright]

Seems to be deliberate :-)

[u-fischer]

Yes, but I don't quite see why. For center or math environments it makes sense, but in a list the next command is the \item so there is not much danger that the processing of the optional argument eats some bracket.

[FrankMittelbach]

I agree that for lists it makes little sense given that anything other than a bracket will be \par or \item.

But it is still the question of we should differenciate from environment to environment, e.g., disallow a space with center or theorem-like envs but allow them with lists. We could, of course, but I'm not sure what is better.

[u-fischer]

I came across the problem in a real document which used enumitem and the emulation failed because of this. Which means that if that stays the emulation would either have to redefine the lists to stay compatible with enumitem (and then we had two different behaviours) or not stay compatible and error.

Generally I think that users do not really expect that (no) spaces matter around such arguments, and I would restrict the strict check to cases where there is a real danger to eat text bracket (center, theorem, math environments).