Taking into an account that we use Saxon for many years, it was strange to run into so simple error like the following:

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:variable name="doc" as="element()+"><a/><b/><c/></xsl:variable>   
    <xsl:sequence select="$doc = 3"/>
  </xsl:template>
</xsl:stylesheet>

This is a simplified case that should produce an dynamic error FORG0001 as per General Comparisions; the real code is more complex, as it uses SFINAE and continues.

This case crushes in Saxon with exception:

Exception in thread "main" java.lang.RuntimeException: 
      Internal error evaluating template  at line 3 in module ICE9.6.xslt
    at net.sf.saxon.expr.instruct.Template.applyLeavingTail()
    at net.sf.saxon.trans.Mode.applyTemplates()
    at net.sf.saxon.Controller.transformDocument()
    at net.sf.saxon.Controller.transform()
    at net.sf.saxon.s9api.XsltTransformer.transform()
    at net.sf.saxon.jaxp.TransformerImpl.transform()
    ...
Caused by: java.lang.NumberFormatException: For input string: "" 
    at java.lang.NumberFormatException.forInputString()
    at java.lang.Long.parseLong()
    at java.lang.Long.parseLong()
    at net.sf.saxon.expr.GeneralComparison.quickCompare()
    at net.sf.saxon.expr.GeneralComparison.compare()
    at net.sf.saxon.expr.GeneralComparison.evaluateManyToOne()
    at net.sf.saxon.expr.GeneralComparison.evaluateItem()
    at net.sf.saxon.expr.GeneralComparison.evaluateItem()
    at net.sf.saxon.expr.Expression.process()
    at net.sf.saxon.expr.instruct.Template.applyLeavingTail()
    ... 8 more

We have reported the problem at Saxon's forum, and as usual the problem was shortly resolved.