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

[Hans Hagen]

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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