A XeTeX snag

Testing the current LaTeX3 code means worrying about what primitives are available. Recent releases of pdfTeX have included a number of new primitives, and the expl3 system currently uses \pdfstrcmp if it finds it. This has already caused one issue, as things were broken if it was not available. So I’ve been modifying the test system a little, to run the tests with pdfLaTeX and XeLaTeX. The ‘new’ primitives are not available with XeTeX, so this is a good test of our code: it’s supposed to work with both. I’ve found one odd thing going on with the LateX3 code, but also one thing that seems to be a XeTeX snag.

The snag can be tracked down (thanks to Morten Høgholm) to a minimal case:

\message{\ifcat\par\relax T\else F\fi}

If you try this in TeX or pdfTeX, you get T in the log (as The TeXbook says you should). With XeTeX, you get F. Whether this is a bug or a feature, I don’t know. I’m sure Jonathan Kew will elaborate, but it reminds me of the need to test using different set ups. You can imagine some very odd errors appearing with this type of thing!