smashoperator produces black line with Linux Libertine O

Question

When I use fontspec to set the main font to "Linux Libertine O", the smashoperator command (and also mathclap and friends) produce a black line above the smashed subscript (with luatex).

\documentclass{minimal}
\usepackage{mathtools}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}

\begin{document}

\[
    \smashoperator{\sum_{i \in \mathrm{longidentifier}}} f(i)
\]

\end{document}

The problem does not occur with other fonts like Times New Roman or Linux Biolinum O. Is there a workaround?

Answer 1

I think the problem boils down to the following. But I can't say if the font or the binaries (xetex has the problem too) is at fault as I have no idea what \radical0 (use by mathtools) is supposed to do and why it is affected by a change of the operator font.

Imho the best would be to ask the authors of mathtools.

\documentclass{article}
\usepackage[no-math]{fontspec}
\setmainfont{Linux Libertine O}
\ExplSyntaxOn 
\DeclareSymbolFont{operators}\g_fontspec_encoding_tl\g_fontspec_mathrm_tl\mddefault\updefault
\ExplSyntaxOff
\pagestyle{empty}
\begin{document}
$\radical0 a$
\end{document}