`\nvrightarrow` causes an error when it is used in the title of a definition and is referenced from another file

Question

The following code is saved in test.tex:

\documentclass{scrartcl}

\usepackage{unicode-math,xr-hyper,hyperref,cleveref}
\newtheorem{definition}{Definition}

\begin{document}

\begin{definition}[$\nvrightarrow$]\label{mylabel}
Hello, world!
\end{definition}

\end{document}

test.tex compiles successfully with LuaLaTeX.

The following code is saved in test2.tex:

\documentclass{scrartcl}

\usepackage{xr-hyper,hyperref,cleveref}
\externaldocument{test}[test.pdf]

\begin{document}

Hello, world!

\end{document}

This code fails to compile with LuaLaTeX. The following error message appears in test2.log:

! Undefined control sequence.
<argument> $\nvrightarrow 
               $
l.5 \externaldocument{test}[test.pdf]
                                   
? 
! Emergency stop.
<argument> $\nvrightarrow 
               $
l.5 \externaldocument{test}[test.pdf]

However, if the body of test.tex is changed as follows (the title and the body of the definition are switched):

\begin{definition}[Hello, world!]\label{mylabel}
$\nvrightarrow$
\end{definition}

or as follows (the label is deleted):

\begin{definition}[$\nvrightarrow$]
Hello, world!
\end{definition}

then both files compile successfully.

What causes the error in the first example, and how can it be fixed?

Answer 1

Your label create in the aux:

\newlabel{mylabel}{{1}{1}{$\nvrightarrow $}{definition.1}{}}

\nvrightarrow is defined by unicode-math, but only at begin document. So you either should provide a definition for the command in your other document or should load unicode-math there too, and then should load the external file after begin document.

\documentclass{scrartcl}
\usepackage{unicode-math}
\usepackage{xr-hyper,hyperref,cleveref}

\begin{document}
\externaldocument{test}[test.pdf]
Hello, world!

\end{document}

Answer 2

Looks like unicode-math is responsible for the trouble? I updated test.tex as follows

\documentclass{scrartcl}

\usepackage{stix}
\usepackage{xr-hyper,hyperref,cleveref}
%\usepackage{unicode-math,xr-hyper,hyperref,cleveref}
\newtheorem{definition}{Definition}

\begin{document}
\begin{definition}[{$\nvrightarrow$}]\label{mylabel}
Hello, world!
\end{definition}
\end{document}

and updated test2.tex as follows

\documentclass{scrartcl}
\usepackage{stix}
\usepackage{xr-hyper,hyperref,cleveref}
\externaldocument{test}[test.pdf]
\begin{document}
Hello, world!
\end{document}

The generated pdf files (using lualatex) look like the following: