[luatex] this can't happen error with Umiddle (luatex 1.07/texlive 2018)

David Carlisle d.p.carlisle at gmail.com
Wed May 16 10:16:10 CEST 2018

The following plain tex example

$\left( A \middle|  \right)$ %OK

$\Uleft( A \Umiddle class 2 | B \Uright)$ %OK

$\Uleft( A \Umiddle class 2 | \Uright)$ %oops



! This can't happen (mathspacing).
l.10 $\Uleft( A \Umiddle class 2 | \Uright)$
!  ==> Fatal error occurred, no output PDF file produced!

I'd actually meant to have the A|B but the error message for my typo
seemed a bit harsh!

Having an empty math list to the right of a class 2  \Umiddle might be
odd but shouldn't be an error I assume?


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