Apache Xerces-J team has made enhancements, to the way XML Schema 1.1, Conditional Type Alternatives (CTA) are evaluated.
Following are summary of the recent XML Schema 1.1 CTA enhancements, being done in Xerces-J:
1) The XML Schema 1.1 spec, allows the implementations to use a smaller XPath 2.0 subset for CTA, and can also provide the full XPath 2.0 language, for CTA evaluations.
Xerces-J had the smaller XPath 2.0 susbset already implemented, for CTA since quite some time (which was contributed by Hiranya Jayathilaka), and that was the only XPath 2.0 support which, Xerces-J CTA implementation had earlier. Xerces-J team recently added full XPath 2.0 support as well, for XML Schema 1.1 CTA, using the PsychoPath XPath 2.0 engine (which is used, by Xerces-J assertions facility as well).
If the user, writes XPath 2.0 expressions adhering to the XPath 2.0 subset for CTA, then the native XPath 2.0 implementation in Xerces-J shall process those XPath expressions. But if the XPath expression parsing fails by the native Xerces-J XPath 2.0 processor, Xerces-J falls back to the PsychoPath processor for XPath evaluation, allowing users to use the full XPath 2.0 language, for CTA.
2) Some time ago, Xerces-J implemented the XSD 1.1 data type, xs:error as well, which is useful in XML Schema 1.1 CTA.
It's been quite a pleasure, working on some of these patches.