Saxon

I have committed a patch on the 9.6 and 9.7 branches. The effect of the patch is to suppress optimization of filter expressions in stylesheets that specify version="1.0".

Unfortunately, this issue can be reproduced again by using Saxon 9.6.0.5.

To reproduce it, run from command line:

D:\workspace\eXml>java -Xmx60m -cp lib/saxon9ee.jar net.sf.saxon.Transform -xsl:samples/test.xsl -s:samples/test.xml
java.lang.ClassCastException: net.sf.saxon.expr.GeneralComparison10 cannot be cast to net.sf.saxon.expr.ComparisonExpression
        at com.saxonica.ee.optim.IndexedFilterExpression.replaceOperand(IndexedFilterExpression.java:252)
        at net.sf.saxon.expr.Expression.simplify(Expression.java:139)
        at net.sf.saxon.expr.Expression.simplify(Expression.java:137)
        at net.sf.saxon.expr.Expression.simplify(Expression.java:137)
        at net.sf.saxon.expr.parser.ExpressionVisitor.simplify(ExpressionVisitor.java:158)
        at net.sf.saxon.expr.GeneralComparison.typeCheck(GeneralComparison.java:314)
        at net.sf.saxon.expr.parser.ExpressionVisitor.typeCheck(ExpressionVisitor.java:188)
        at net.sf.saxon.expr.GeneralComparison10.optimize(GeneralComparison10.java:168)
        at net.sf.saxon.expr.parser.ExpressionVisitor.optimize(ExpressionVisitor.java:244)
        at net.sf.saxon.expr.FunctionCall.optimize(FunctionCall.java:168)
        at net.sf.saxon.functions.SystemFunctionCall.optimize(SystemFunctionCall.java:238)
        at net.sf.saxon.functions.NotFn.optimize(NotFn.java:64)
        at net.sf.saxon.expr.parser.ExpressionVisitor.optimize(ExpressionVisitor.java:244)
        at net.sf.saxon.expr.LetExpression.optimize(LetExpression.java:250)
        at net.sf.saxon.expr.parser.ExpressionVisitor.optimize(ExpressionVisitor.java:244)
        at net.sf.saxon.expr.instruct.Choose.optimize(Choose.java:393)
        at net.sf.saxon.expr.parser.ExpressionVisitor.optimize(ExpressionVisitor.java:244)
        at net.sf.saxon.style.XSLTemplate.optimize(XSLTemplate.java:611)
        at net.sf.saxon.style.StylesheetPackage.optimizeTopLevel(StylesheetPackage.java:1216)
        at net.sf.saxon.style.StylesheetPackage.compile(StylesheetPackage.java:1083)
        at net.sf.saxon.style.Compilation.compilePackage(Compilation.java:201)
        at net.sf.saxon.style.Compilation.compileSingletonPackage(Compilation.java:94)
        at net.sf.saxon.s9api.XsltCompiler.compile(XsltCompiler.java:543)
        at net.sf.saxon.Transform.doTransform(Transform.java:601)
        at net.sf.saxon.Transform.main(Transform.java:80)
Fatal error during transformation: java.lang.ClassCastException: net.sf.saxon.expr.GeneralComparison10 cannot be cast to net.sf.saxon.expr.ComparisonExpression

test.xsl:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:sch="http://purl.oclc.org/dsdl/schematron"
    xmlns:loc="http://www.thaiopensource.com/ns/location"
    xmlns:err="http://www.thaiopensource.com/ns/error"
    xmlns:xsltc="http://www.thaiopensource.com/ns/xsltc"
    xmlns:jing-extension-functions="http://www.thaiopensource.com/ns/extension-functions"
    xmlns:osaxon="http://icl.com/saxon"
    xmlns:nsaxon="http://saxon.sf.net/"
    xmlns:xalan-node-info="http://xml.apache.org/xalan/java/org.apache.xalan.lib.NodeInfo"
    version="1.0">
    <xsl:template match="/">
        <result>
            <xsl:apply-templates select="/" mode="all"/>
        </result>
    </xsl:template>
    <xsl:variable name="id-set" select="//*[@id]"/>
    <xsl:template match="*[@id]"
        mode="M1"
        priority="2"
        name="R1.1"
        loc:column-number="29"
        loc:line-number="6"
        loc:system-id="jar:file:/D:/workspace/eXml/lib/zip/epub/epubcheck-3.0.1.jar!/com/adobe/epubcheck/schema/20/sch/id-unique.sch">
        <xsl:if test="not(count($id-set[@id = current()/@id]) = 1)"
            loc:column-number="15"
            loc:line-number="8"
            loc:system-id="jar:file:/D:/workspace/eXml/lib/zip/epub/epubcheck-3.0.1.jar!/com/adobe/epubcheck/schema/20/sch/id-unique.sch">
            <failed-assertion test="count($id-set[@id = current()/@id]) = 1">
                <xsl:call-template name="location"/>
                <statement>Duplicate ID '<xsl:value-of select="current()/@id"
                    loc:column-number="63"
                    loc:line-number="8"
                    loc:system-id="jar:file:/D:/workspace/eXml/lib/zip/epub/epubcheck-3.0.1.jar!/com/adobe/epubcheck/schema/20/sch/id-unique.sch"/>'</statement>
            </failed-assertion>
        </xsl:if>
    </xsl:template>
    <xsl:template match="*[@id]"
        mode="all"
        priority="2"
        loc:column-number="29"
        loc:line-number="6"
        loc:system-id="jar:file:/D:/workspace/eXml/lib/zip/epub/epubcheck-3.0.1.jar!/com/adobe/epubcheck/schema/20/sch/id-unique.sch">
        <xsl:call-template name="R1.1"/>
        <xsl:apply-templates select="*" mode="all"/>
    </xsl:template>
    <xsl:template match="*" mode="M1"/>
    <xsl:template match="*|/" mode="all">
        <xsl:apply-templates select="*" mode="all"/>
    </xsl:template>
    <xsl:template name="location">
        <xsl:attribute name="column-number">
            <xsl:value-of select="nsaxon:column-number()"/>
        </xsl:attribute>
        <xsl:attribute name="line-number">
            <xsl:value-of select="nsaxon:line-number()"/>
        </xsl:attribute>
        <xsl:attribute name="system-id">
            <xsl:value-of select="nsaxon:system-id()"/>
        </xsl:attribute>
    </xsl:template>
</xsl:stylesheet>

test.xml

<?xml version="1.0" encoding="UTF-8"?>
<personnel/>

What's happening in this case is the following sequence of events:

(a) The relevant expression is count($id-set[@id = current()/@id]) = 1

(b) Saxon establishes correctly that the inner expression $id-set[@id = current()/@id] can benefit from indexing, because the comparison is between two untypedAtomic values which is the same in 1.0 and 2.0 mode. So this subexpression gets rewritten as an IndexedFilterExpression

(c) Subsequently, Saxon establishes that the expression count(X) = 1 can be expressed as a simple ValueComparison since the types of both operands are known, and both are singletons.

(d) Having rewritten count(X) = 1 as a ValueComparison, Saxon invokes simplify() on the new ValueComparison. The invokes simplify() recursively on contained subexpressions. This establishes that the GeneralComparison [@id = current()/@id] is executed in 1.0 mode, and attempts therefore to revert it to an instance of GeneralComparison10; but an instance of GeneralComparison10 cannot be used in an IndexedFilterExpression and the attempt to do so causes the ClassCastException.

Removing the call on simplify() solves the problem, but it's difficult to very that it has no adverse side-effects. Generally speaking, simplify() should only be called on an expression immediately after it is constructed by the XPath parser, and should not be called again on expressions constructed during the typeCheck() or optimize() phases unless there is specific reason for it. So I think removing this call (at GeneralComparison.java line 314) is sound, but it needs testing.

No change made on the 9.7 branch, which already omits the call to simplify().