How to display italics (obliques) with SourceSans Pro?

Question

I'm working in a document with lualatex where I want to use the Adobe Source family: Source Sans Pro, Source Serif Pro and Source Code Pro.

My problem is that Source Serif Pro does not contain italics yet and I use quite \emph to highlight the text, which isn't noticeable when using this family. And I saw on the font site that the italics will be drawn someday, for now and in the near future it seems to Source Serif will continue without them.

So my first attempt to solution was add the line

\renewcommand{\familydefault}{\sfdefault}

in this way I could use SourcesSansProas the main font and in theory be able to use its obliques for \emph instead. Unfortunately my solution isn't working.

I can think of there may be a way to work with obliques that I don't know, or if isn't possible, redefine the command \emph to use boldface instead of italics for highlight the text.

This is my MWE BTW:

\documentclass[fontsize=10pt,paper=letter,headings=small,bibliography=totoc,DIV=9,headsepline=true,titlepage=on]{scrartcl} 
%\usepackage[spanish,mexico]{babel} 
\usepackage{fontspec} 
\defaultfontfeatures{Mapping=tex-text}
\usepackage{xunicode}
\usepackage{xltxtra}
\setmainfont{SourceSerifPro} 
\setsansfont{SourceSansPro} 
\setmonofont{SourceCodePro} 
\renewcommand{\familydefault}{\sfdefault}

\begin{document}

This is a proof paragraph containing \emph{highlighted text} and placeholder text: 
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

\end{document}

Answer 1

It is rather unclear what you actually want. But here two suggestions: You could still use SourceSerifPro as main font and fake a slanted font:

\documentclass[]{scrartcl}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{SourceSerifPro}[
 ItalicFont = SourceSerifPro,
 ItalicFeatures={FakeSlant=0.2}]

\begin{document}

This is a \emph{highlighted text} and placeholder text

\end{document}

Or you can use SourceSansPro as main font which will work out-of-the-box:

\documentclass[]{scrartcl}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{SourceSansPro}

\begin{document}

This is a \emph{highlighted text} and placeholder text

\end{document}

Answer 2

Italics for Source Serif Pro were released in August 2018. You should be able to use them directly now.

https://github.com/adobe-fonts/source-serif-pro/releases/tag/2.007R-ro/1.007R-it

As of September 2018, the sourceserifpro package has Source Serif Pro italics. So, \emph{} will function as desired if the command \usepackage{sourceserifpro} is placed in your document's preamble.