conflict between siunitx and unicode-math (greek letter unit)

Question

In optics, the "dioptre" unit's symbol is δ (lowercase delta). As long as I don't use unicode-math, the following code gives the expected result, but with it, the unit is missing in the output:

\documentclass[french]{article}
\usepackage{polyglossia}
\setdefaultlanguage{french}
\usepackage{unicode-math}
\setmathfont{STIX Two Math}

\usepackage[output-decimal-marker={,}, exponent-product=.,, group-digits=true]{siunitx}

\begin{document}
\qty{4}{\delta}
\end{document}

Is there a way to make siunitx and unicode-math play well together in this case?

Answer 1

You should define the appropriate unit name and the corresponding symbol.

\documentclass{article}
\usepackage{unicode-math}
\usepackage{polyglossia}

\setdefaultlanguage{french}
\setmathfont{STIX Two Math}

\usepackage{siunitx}

\sisetup{
  output-decimal-marker={,},
  exponent-product=.,
  group-digits=true
}
\DeclareSIUnit{\dioptre}{\ensuremath{\symup{\delta}}}

\begin{document}

\qty{4}{\dioptre}

\end{document}