LuaTeX and em dashes

Update 3/2019

The bug will be resolved with the next luaotfload update

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To expand a bit on Alan's answer:

It is imho clearly a bug in the fontloader imported from context (you see the same in context if you set \automatichyphenmode=0). It only happens if the fonts are rendered with the mode=node:

\documentclass{article}

\begin{document}
\font\test={file:lmroman10-regular.otf:mode=node;+tlig}
\test

A---B

\font\test={file:lmroman10-regular.otf:mode=base;+tlig}
\test

A---B
\end{document}

The source of the problem is imho that with \automatichyphenmode=0, luatex has at first to convert the last hyphen to a discretionary to allow a linebreak:

A---B  ---> A--\discretionary{-}{}{-}B

and after the line has been set this has to be converted back again to ---, and this step seems to fail.

The problem has been reported, but it is unclear if it will be fixed.

\automatichyphenmode=1 avoids the problem by not converting the hyphen to a discretionary in a number of cases. So you should be aware of the fact that this suppress line breaking in a number of cases:

\documentclass[parskip=half-]{scrartcl}
\begin{document}


\parbox[t]{1pt}{%
\textbf{0}

\automatichyphenmode=0
A-B

A--B

A---B

-begin

A!-B}
\hspace{2cm}
\parbox[t]{1pt}{\automatichyphenmode=1
\textbf{1}

A-B

A--B

A---B

-begin

A!-B}

\end{document}

You can add \automatichyphenmode=1 to your preamble:

\documentclass{article}
\automatichyphenmode=1
\begin{document}

\begin{enumerate}
  \item en--dash
  \item em---dash
  \item em --- dash space
\end{enumerate}
\begin{enumerate}
  \item en–dash unicode
  \item em—dash unicode
  \item em — dash space unicode
\end{enumerate}
\begin{enumerate}
  \item en\textendash{}dash macro
  \item em\textemdash{}dash macro
  \item em \textemdash{} dash space macro
\end{enumerate}

\end{document}