How to enable ligatures for emdash/endash in LuaTeX ?

Question

I cannot switch on ligatures for emdash (---) and endash (--) in LuaLaTeX. Here is my test file:

% !Mode:: "TeX:UTF-8"
\documentclass{article}
\usepackage{fontspec}
    \defaultfontfeatures{Ligatures=TeX}
    \setmainfont{Cambria}
    \setsansfont[Mapping=tex-text]{Calibri}
\usepackage{ifluatex,ifxetex}
\begin{document}

\ifxetex
    This is XeTeX
\else\ifluatex
    This is LuaTeX
\fi\fi

\section{rmfamily}
\rmfamily
\begin{enumerate}
\item <<double quote>> by ligatures \verb|<<ligatures>>|
\item «double quotes» by symbols \verb|«double quotes»|
\item en--dash by ligatures \verb|en--dash|
\item em---dash by ligatures \verb|em---dash|
\item en–dash by symbol \verb|en–dash|
\item em—dash by symbol \verb|em—dash|
\end{enumerate}

\section{sffamily}
\sffamily
\begin{enumerate}
\item <<double quote>> by ligatures \verb|<<ligatures>>|
\item «double quotes» by symbols \verb|«double quotes»|
\item en--dash by ligatures \verb|en--dash|
\item em---dash by ligatures \verb|em---dash|
\item en–dash by symbol \verb|en–dash|
\item em—dash by symbol \verb|em—dash|
\end{enumerate}

\end{document} 

When it is compiled with XeLaTeX, \sffamily loaded with the obsolete option Mapping=tex-text

    \setsansfont[Mapping=tex-text]{Calibri}

activates the ligatures --- and --, but in LuaTeX this options seemed to be equivalent to Ligatures=TeX, so that I don't know how to restore default (in LaTeX) behavior of these two ligatures.

Is it bug or a feature, introduced in a recent version of the fontspec package?

I've checked that the ligatures still work as expected under LuaTeX if I don't set OTF fonts by the commands

    \setmainfont{Cambria}
    \setsansfont[Mapping=tex-text]{Calibri}

Answer 1

The TeX-ligatures in luatex are not a font property, they are "faked" features implemented in the font loader code. The problem could be the bug in the luaotfload/context code reported some month ago:

http://archive.contextgarden.net/message/20110310.181448.288f2a38.en.html

I think it has been corrected in context but I doubt that the new code has already found its way in luaotfload.

You can test the font directly (to exclude a fontspec problem and to test the both modes)

\documentclass{article}
\usepackage{luaotfload}
\begin{document}
 \font\test="Calibri:mode=node;+tlig" 

 \font\testb="Calibri:mode=base;+tlig" 
 \test -- --- abc 

 \testb -- --- abc 
\end{document}

Answer 2

This is partially a property of the font, and not a problem with fontspec. XeLaTeX and LuaTeX implement the ligature replacement with different mechanisms. XeLaTeX uses a teckit mapping file, whereas LuaTeX uses luaotfload to do the conversion. The latter is somehow dependent on how the font actually implements certain characters. Changing the font to Linux Libertine O, for example, causes the ligatures to appear correctly.

Update

As noted by Ulrike Fischer, you can change this on a font-by-font basis for fonts that don't work by specifying a value for the Renderer key when you load the font. There are two possible (relevant) values for this key in LuaLaTeX: Full (default) and Basic. Loading certain fonts with the Basic renderer will allow the ligatures. This will not work for all fonts, however. For more discussion, see the following thread on comp.text.tex: en-dash and em-dash in luatex.

The following modified version of your document shows how the problem doesn't arise with Linux Libertine O, and how it can be fixed for Cambria. (Calibri) works the same way.

As Ulrike notes in her answer, a more robust fix will propagate to luaotfload at some point in the future.

% !Mode:: "TeX:UTF-8"
\documentclass{article}
\usepackage{fontspec}
    \defaultfontfeatures{Ligatures={TeX}}
    \setmainfont{Cambria}
    \setsansfont{Calibri}
\usepackage{ifluatex,ifxetex}
\usepackage{expl3}
\begin{document}

\ifxetex
    This is XeTeX
\else\ifluatex
    This is LuaTeX

\fi\fi

\section{Cambria}

\begin{enumerate}
\item <<double quote>> by ligatures \verb|<<ligatures>>|
\item «double quotes» by symbols \verb|«double quotes»|
\item en--dash by ligatures \verb|en--dash|
\item em---dash by ligatures \verb|em---dash|
\item en–dash by symbol \verb|en–dash|
\item em—dash by symbol \verb|em—dash|
\end{enumerate}

\section{Linux Libertine O}
\fontspec{Linux Libertine O}\selectfont
\begin{enumerate}
\item <<double quote>> by ligatures \verb|<<ligatures>>|
\item «double quotes» by symbols \verb|«double quotes»|
\item en--dash by ligatures \verb|en--dash|
\item em---dash by ligatures \verb|em---dash|
\item en–dash by symbol \verb|en–dash|
\item em—dash by symbol \verb|em—dash|
\end{enumerate}

\section{Cambria + Basic Renderer}
\fontspec[Renderer=Basic]{Cambria}\selectfont
\begin{enumerate}
\item <<double quote>> by ligatures \verb|<<ligatures>>|
\item «double quotes» by symbols \verb|«double quotes»|
\item en--dash by ligatures \verb|en--dash|
\item em---dash by ligatures \verb|em---dash|
\item en–dash by symbol \verb|en–dash|
\item em—dash by symbol \verb|em—dash|
\end{enumerate}

\end{document}

Answer 3

I can't compete with the level of discussion between Ulrike and Alan. But thanks to the link provided by Alan, I found out how to get an en-dash with another of those Win-fonts: Corbel. I ran into the same problem two weeks ago and thanks to your help I know now how to handle it:

\documentclass{article}
\usepackage{fontspec}
\setmainfont[Renderer=Basic,Ligatures=TeX]{Corbel}
\begin{document}
-- --- abc 
\end{document}

I get the en-dash and the em-dash this way. Or did I miss something and this way to get dashes is obsolete?