LuaLaTeX: Replace character "ß" in CMU fonts

Question

The glyph for the character "ß" differs considerably between Latin Modern and the CMU fonts (CMU Serif, CMU Sans Serif etc., which use the same glyphs for ß as cm-super; see this answer):

I much prefer Latin Modern's version of ß, but I would like to use the CMU fonts because of their Unicode capabilities (they provide a lot more characters than Latin Modern). I would like to avoid editing the font file (I don't want to maintain my own versions of fonts!).

Instead, I was wondering how one would replace a single character with another font in LuaLaTeX. I've seen XeTeX solutions (e.g. this question), but nothing that works with LuaTeX. So far, I've tried the following:

\documentclass{scrartcl}
\usepackage{fontspec}
\setmainfont{CMU Serif}
\setsansfont{CMU Sans Serif}
\setmonofont{CMU Typewriter Text}

\usepackage{newunicodechar}
\newunicodechar{ß}{{\fontspec{Latin Modern Roman}ß}}

\begin{document}
    ß
\end{document}

However, I have two problems with this approach:

• In the \newunicodechar definition of ß, I would need some way to detect if the font should be set in serif, sans-serif or typewriter (variants such as bold, italic etc. seem to work).
• I'm having doubts whether making ß an active character is a good idea. I've heard that hyphenation/kerning/other details in TeX can have problems under these circumstances.

What would be the best way to implement this replacement?

Update: I have extended my first attempt to detect changes in the font family. Still, there is the question whether using active characters can cause any unwanted problems.

\documentclass{scrartcl}
\usepackage{fontspec}
\setmainfont{CMU Serif}
\setsansfont{CMU Sans Serif}
\setmonofont{CMU Typewriter Text}
\usepackage{newunicodechar}

\newfontfamily{\lmodernrm}{Latin Modern Roman}
\newfontfamily{\lmodernsf}{Latin Modern Sans}
\newfontfamily{\lmoderntt}{Latin Modern Mono}
\DeclareTextFontCommand{\textlmodernrm}{\lmodernrm}
\DeclareTextFontCommand{\textlmodernsf}{\lmodernsf}
\DeclareTextFontCommand{\textlmoderntt}{\lmoderntt}

\makeatletter
\newunicodechar{ß}{%
    \ifx\f@family\rmdefault\textlmodernrm{ß}%
    \else\ifx\f@family\sfdefault\textlmodernsf{ß}%
    \else\ifx\f@family\ttdefault\textlmoderntt{ß}%
    \fi\fi\fi}
\makeatother

\begin{document}
    ß \textsl{ß} \textit{ß} \textbf{ß}

    \sffamily
    ß \textsl{ß} \textit{ß} \textbf{ß}

    \ttfamily
    ß \textsl{ß} \textit{ß}
\end{document}

Answer 1

You could set Latin Modern as the main font if you like it for the Latin range, and then use CMU Serif as a fallback font for higher Unicode ranges. Depends on how much higher Unicode you need. I still hope someone can devise a mechanism for an automatic font-fallback system like in CSS. (Define fallback font for specific Unicode characters in LuaLaTeX)

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Latin Modern Roman}
\newfontfamily{\unicodefont}{CMU Serif}
\DeclareTextFontCommand{\textunicode}{\unicodefont}
\begin{document}
Straße \textunicode{μετανοεῖτε}
\end{document}

Answer 2

The Latin Modern font family is available in OpenType format as well; see The Latin Modern (LM) Family of Fonts site. Even better, it's distributed with both TeXLive and MikTeX. (Am I maybe missing something about capabilities of CMU that are not available in LM?) Thus, you could write

% !TEX TS-program = lualatex
\documentclass{scrartcl}
\usepackage{fontspec}
\setmainfont{Latin Modern Roman}
\setsansfont{Latin Modern Sans}
\setmonofont{Latin Modern Mono}
\begin{document}
ß \textsf{ß} \texttt{ß}
\end{document}