FYI to the DFDL schema development community--we recently discovered
that Xerces-C has a bug where it does not correctly validate choices
with an empty sequence as a branch.
For example, say we have this schema:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";>
<xs:element name="root">
<xs:complexType>
<xs:choice>
<xs:element name="foo" type="xs:int" />
<xs:sequence />
</xs:choice>
</xs:complexType>
</xs:element>
</xs:schema>
This pattern is not uncommon in DFDL schemas, usually used with an
initiator or hidden group ref placed on the empty xs:sequence.
When using Xerces-C, this XML validates as expected:
<root>
<foo>1</foo>
</root>
But XML without the foo element fails to validate even though it is
valid XML, such as:
<root />
or
<root></root>
A bug has been opened with Xerces-C [1], but the project is fairly
inactive so it's not clear when/if it will be fixed. So when developing
DFDL schemas where users might want to use Xerces-C to validate the
resulting infoset, you may want to consider alternatives. For example,
maybe an element can be used instead, or a hidden group ref can be
replace with a normal group ref.
[1] https://issues.apache.org/jira/browse/XERCESC-2243
