Polyglossia {french} in Memoir Class. My preferred font is Open Sans. MWE compiled with XeLaTeX:
\documentclass[a4paper,article]{memoir}
\usepackage[demo]{graphicx}
\usepackage{polyglossia}
\setdefaultlanguage{french}
\setmainfont{Open Sans}
\begin{document}
C'est un test pour "small caps": Fig.~\ref{fig:myfigure}.
\begin{figure}[ht] \centering
\includegraphics{nothing}
\caption{c'est ma figure}\label{fig:myfigure}
\end{figure}
\end{document}
- generates warning:
LaTeX Font Warning: Font shape
EU1/OpenSans(0)/m/sc' undefined (Font) usingEU1/OpenSans(0)/m/n' instead on input line 10.
The output is perfectly readable, but with normal font for "Fig" in the caption:

In an attempt to get rid of the warning, and have French-style "Fig" shape, I tried to have small caps rendered in a font that can handle them (like one of the TeX Gyres), but nothing would work on my Arch Linux Tex-Live setup.
Changing the setmainfont line to use TeX Gyre Heros, like this:
\setmainfont[
Extension=.otf,
UprightFont= *-regular,
BoldFont=*-bold,
ItalicFont=*-italic,
BoldItalicFont=*-bolditalic,
]{texgyreheros}
- does work:

- but for my document, TeX Gyre Heros is a little heavy.
Is there a way to make the caption use a small-caps-compatible font (ideal), or to not use small-caps at all (thus removing the Warning)?
My various attempts, like \[captionnamefont]{\upshape}, achieved nothing.


\textscinternally uses\scshape, it's necessary to redefine only the latter, but it should be better done with\LetLtxMacro(see When to use\LetLtxMacro– egreg Mar 03 '15 at 22:45