[XeTeX] Xetex, Latex3, unicode-math and Math font not found

[David Carlisle]

 > \exp_args:Nxx \setmathfont
>   { \tl_use:N \c__test_MathFont_tl }
>   { [ \tl_use:N \c__test_MathFontSpec_tl ] }
>
> Thanks for the explanations but this form does not work

the x expansion leaves in the braces so this ends up as

\setmainfont{somefontname}{[expansionof \c_...]}

but the braces prevent the optional argument from being seen.

it's not really an expl3 (or xetex) question, you would see the same with

\edef\foo{\noexpand\setmainfont{\zzz}{[\qqq]}}
\foo

(which is more or less what x-expansion is)
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