LuaLaTeX empty math symbols

Question

Consider the following minimal example:

\documentclass{minimal}

\usepackage{fontspec}
\usepackage{unicode-math}

\defaultfontfeatures{Ligatures=TeX}
\setmainfont{TeX Gyre Pagella}
\setsansfont[Scale=MatchLowercase]{Tex Gyre Heros}
\setmonofont{Tex Gyre Cursor}
\setmathfont{TeX Gyre Pagella Math}

\begin{document}
$a \boxdiag b$

$a \boxbslash b$
\end{document}

If compiled with lualatex minimal.tex, the output ends up in something like

a b
a b

That means, the symbols \boxdiag and \boxbslash are not printed. Why do I even not get an error message? Are the symbols missing and I need to load a package for them?

Answer 1

As the warning in the log states, the font doesn't have those characters. Since they are just boxes, you could steal them from any font that does without too much typographic pain, xits math works:

\documentclass{minimal}

\usepackage{fontspec}
\usepackage{unicode-math}

\defaultfontfeatures{Ligatures=TeX}
\setmainfont{TeX Gyre Pagella}
\setsansfont[Scale=MatchLowercase]{Tex Gyre Heros}
\setmonofont{Tex Gyre Cursor}
\setmathfont{TeX Gyre Pagella Math}
\setmathfont[range={\boxdiag,\boxbslash}]{XITS Math}

\begin{document}
$a \boxdiag b$

$a \boxbslash b$
\end{document}