If you have a schema with the setup as follows:
<xsd:extension base="baseType"> ...
Then the baseType referenced must (by rule 1 http://www.w3.org/TR/xmlschema-1/#src-ct) be a complex type. However the spec does not specify that the baseType must itself have complexContent. It appears that the spec allows simpleContent. However XmlBeans as it stands will fail specifying that a complex type with complex content must be provided. Example error message is below:
bmecat_2005.xsd:3681:5: error: src-ct.1: The specified base type 'typeMLSTRING00050@http://www.bmecat.org/bmecat/2005fd' must be a complex type with complex content.
If we remove the check at StscComplexTypeResolver line 529 that checks for simpleContent (without removing the check for simpleType) the checkintest appears to work and we get files generated which compile correctly. I will attach a patch which does that and updates the error message.