Downcasting (relabelling) in the coercion rules

[michaelhkay]

The proposed coercion rules (aka function conversion rules) permitting down-casting (aka "relabelling") introduce a backwards incompatibility.

For example in XSLT3 test case as-1711 we have

  <xslt:variable name="var1" select="/doc-schemaas/elem-NMTOKEN" as="xs:token"/>

   <xslt:template match="/doc-schemaas">
            <xslt:value-of select="$var1 instance of xs:NMTOKEN"/>
   </xslt:template> 

where the result of atomising elem-NMTOKEN is of type xs:NMTOKEN.

Under the new rules this is "relabelled" as xs:token, causing the "instance of" test to return false, where in XSLT 2.0/3.0 it returned true.

I think the relabelling rules should probably be amended so that if the supplied value is already an instance of the required type, no relabelling takes place - it retains its existing type.

[michaelhkay]

Fixed as suggested