title: fontspec (xetex/luatex) and fontconfig
body: <p>LuaTeX and XeTeX are passing on a warning on from the fontspec package, and I'm not sure whether I should worry.  The following minimal code will generate the warning:</p>
  
  <pre><code>\documentclass{report}
  \usepackage{fontspec}
  \setmainfont[Script=Cyrillic]{Arial}
  \begin{document}
  \end{document}
  </code></pre>
  
  <p>The warning is</p>
  
  <pre><code>fontspec warning: "script-not-exist"
  Font 'Arial' does not contain script 'Cyrillic'.
  </code></pre>
  
  <p>Replacing 'Arial' with, for example, 'Charis SIL', results in no warning.</p>
  
  <p>I believe I've traced the problem back to fc-list/ fontconfig.  In the output of fc-list for the Arial font, I get the following 'capabilities' line:</p>
  
  <pre><code> capability: "otlayout:arab otlayout:hebr otlayout:latn"(s)
  </code></pre>
  
  <p>This line does not mention Cyrillic (which would be the code 'cyrl', according to the <code>\newfontscript{Cyrillic}{cyrl}</code> line in fontspec-xetex.sty).  Fonts for which I do not get this warning, such as Charis SIL, explicitly mention Cyrillic in their capabilities lines:</p>
  
  <pre><code>capability: "ttable:Silf  otlayout:DFLT otlayout:cyrl otlayout:latn"(s)
  </code></pre>
  
  <p>Unfortunately, documentation of this "capability" line in the output of fc-list is limited to the line</p>
  
  <pre><code>capability  String  List of layout capabilities in the font
  </code></pre>
  
  <p>(that from <a href="https://www.freedesktop.org/software/fontconfig/fontconfig-user.html" rel="nofollow noreferrer">https://www.freedesktop.org/software/fontconfig/fontconfig-user.html</a> and a couple other places on the web)</p>
  
  <p>My question is basically, should I worry about this warning message?  Arial (and Charis) list 'ru', the ISO 639-1 code for Russian, in the 'lang' field.  Just what is this "layout" capability for the Cyrillic script that Charis SIL supports, but Arial apparently does not, and why does it matter?</p>
  
  <p>BTW, this is with the TeX Live 2017 distro.</p>