Saxon

https://www.saxonica.com/documentation12/index.html#!v4extensions says that either setting xquery version "4.0"; in the query, or using -allowSyntaxExtensions:on should be enough to enable the use of the 4.0 syntax extensions. Either it is a bug, or the documentation is wrong, if these do not actually work.

I think the reason -qversion is undocumented is that it was originally used to discriminate between 1.0/3.0/3.1, and then dropped out of use when we only supported 3.1. So it was always there, but was dropped from the documentation and not reinstated when it became meaningful again. (Search for -qversion, and there are a couple of very old references in the change history).

It looks like the XQuery declaration "xquery version '4.0'" is enough to enable 4.0 in the query parser, but not in the tokenizer, and =!> is being rejected by the tokenizer.

Most of the places where the tokenizer needs to know the language level are cases where the parser ends up not seeing the difference, for example underscores in numeric literals. I will change the logic for =!> so it is rejected at the parser level, which gives better diagnostics.

But I'll also fix it so that 4.0 is allowed by the tokenizer if it is allowed by the parser.

Specifying xquery version '4.0'; in the XQuery version declaration is now enabling 4.0 features in the parser and the tokenizer, but not in the function library. This is because the function library for a query module is initialized before parsing starts. It's complicated by the fact that we need to consider not only the top-level module, but also library modules; in principle a library module can use a different version number from the main module.