[Tex/LaTex] How to use fakeslant as slanted but not italic font with fontspec

Question

Using fontspec with Luatex it is possible to create a fake slant and use that as italic should no italics be around:

\setmainfont[ AutoFakeSlant=0.15 ]
            {FontName}

However, also if an italic version exist this faked slant will be used both for \itshape and \slshape. How can I keep using the shipped italic font defined by the ItalicFont= parameter to \setmainfont but use fake slant for \slshape?

Answer 1

An example is Linux Libertine, that has no slanted version:

\documentclass{article}
\usepackage{fontspec}
\setmainfont[
  SlantedFont={Linux Libertine O},
  SlantedFeatures={FakeSlant=0.15},
  BoldSlantedFeatures={FakeSlant=0.15},
]{Linux Libertine O}

\begin{document}
\textit{abc}\textsl{abc}\textbf{\textsl{abc}}
\end{document}