[XeTeX] Erroneous \lastkern

[Ross Moore]

Hi Ulrike,

On 28 Jun 2019, at 6:27 am, Ulrike Fischer <news3 at nililand.de<mailto:news3 at nililand.de>> wrote:

Am Thu, 27 Jun 2019 21:13:05 +0200 schrieb Christian Tellechea:

> there is probably something wrong with XeLaTeX, but I cannot find what.

The difference between xetex and xelatex is the font:

I’m sorry but I don’t understand this as an answer.
I would expect the test of \lastkern  to always give  n
no matter what the driver, font, with LaTeX or not.

The result from  \lastkern  (according to the TeXBook)
is that it is only non-zero when the last item on the current list is a kern.
(It is *not* a place-holder for the last kern that has been used.)


Thus the appearance of the letter A should have reset  \lastkern  to 0,
*except* if there is a kern provided by A itself (font dependent, perhaps yes);
but in that case it is unlikely to have the value of  2019sp .

Of course, maybe this value has been chosen, *because* that font does end with such a kern.
This would be the only valid reason that you could ever get a  y  from this code.


Put another way, the result of the test should not depend upon the initial use of  \kern2019sp  at all.
However, if it is commented-out, then  XeTeX does give  n , so the  y  is *not* coming from the letter A .

To me this *does* indicate a bug;
or at least a difference in how XeTeX behaves, compared with the original TeX.



\documentclass{article}

\begin{document}

Should be An: \kern2019sp A\ifnum\lastkern=2019 y\else n\fi\par

\fontencoding{OT1}\selectfont
Should be An: \kern2019sp A\ifnum\lastkern=2019 y\else n\fi\par

\end{document}

or in plain xetex:

Should be An: \kern2019sp A\ifnum\lastkern=2019 y\else n\fi\par

\font\test="[lmroman10-regular.otf]"\test
Should be An: \kern2019sp A\ifnum\lastkern=2019 y\else n\fi\par

\bye


--
Ulrike Fischer
https://www.troubleshooting-tex.de/<https://protect-au.mimecast.com/s/EjjEC4QO8xSlXW8wCxnhXi?domain=troubleshooting-tex.de>


Cheers,

Ross


Dr Ross Moore
Department of Mathematics and Statistics
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Macquarie University, NSW 2109, Australia
T: +61 2 9850 8955  |  F: +61 2 9850 8114
M:+61 407 288 255  |  E: ross.moore at mq.edu.au<mailto:ross.moore at mq.edu.au>
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