On 5/16/2018 10:16 AM, David Carlisle wrote:
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

\bye

produces


! This can't happen (mathspacing).
l.10 $\Uleft( A \Umiddle class 2 | \Uright)$
                                            %oops
!  ==> 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?
it's a side effect of checking math spacing for noad pairs ... as we default to zero for (traditionally unknown) combinations anyway, we can make that check less stringent (so luatex 1.09+ will not abort)

Hans

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