Saxon

The documentation at https://www.saxonica.com/documentation11/index.html#!v4extensions suggests it should be enough to EITHER set the configuration option ALLOW_SYNTAX_EXTENSIONS (which can be done from the command line), or to set the XsltVersion property in the XsltCompiler object (which can only be done using the API).

In fact it seems only the latter works.

I've made it so that setting --enableSyntaxExtensions on the command line now works.

This was trickier than it appears; we have to make sure that setting enableSyntaxExtensions on the configuration updates the defaultXsltCompilerInfo to set the language version to 4.0.

Bug fix applied in the Saxon 11.1 release.