In case someone hits this wall, here is what caused the error in my case:
I have an operation:
<wsdl:operation name="FormatReport">
<wsdl:documentation>Runs a report, which is returned as the response</wsdl:documentation>
<wsdl:input message="FormatReportRequest" />
<wsdl:output message="FormatReportResponse" />
</wsdl:operation>
which takes an input:
<wsdl:message name="FormatReportRequest">
<wsdl:part name="parameters" element="reporting:FormatReportInput" />
</wsdl:message>
and another operation:
<wsdl:operation name="FormatReportAsync">
<wsdl:documentation>Creates and submits an Async Report Job to be executed asynchronously by the Async Report Windows Service.</wsdl:documentation>
<wsdl:input message="FormatReportAsyncRequest" />
<wsdl:output message="FormatReportAsyncResponse" />
</wsdl:operation>
taking an input:
<wsdl:message name="FormatReportAsyncRequest">
<wsdl:part name="parameters" element="reporting:FormatReportInputAsync" />
</wsdl:message>
And the input elements are instances of two types:
<xsd:element name="FormatReportInput" type="reporting:FormatReportInputType"/>
<xsd:element name="FormatReportInputAsync" type="reporting:FormatReportAsyncInputType"/>
Here is the catch - the reporting:FormatReportAsyncInputType
type extends (derives from) the reporting:FormatReportInputType
type. That's what seems to confuse the tool and cause the "These members may not be derived." error. You can go around that following teh suggestion in the accepted answer.