Saxon

Oops, let's try that again. I actually lost it when I submitted it with the wrong formatting and had to reconstruct it from memory. That will teach me to use Preview.

Stylesheet:

<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:template match="/">
    <xsl:variable name="node" select="//whatever"/>
    <xsl:sequence select="$node/(/foo/string())"/>
  </xsl:template>

</xsl:stylesheet>

Error:

java.lang.UnsupportedOperationException: Cannot copy a variable reference whose binding is unknown
        at net.sf.saxon.expr.LocalVariableReference.copy(LocalVariableReference.java:57)
        at net.sf.saxon.expr.SlashExpression.rebuildSteps(SlashExpression.java:1039)
        at net.sf.saxon.expr.SlashExpression.getLeadingSteps(SlashExpression.java:1068)
        at net.sf.saxon.expr.HomogeneityChecker.typeCheck(HomogeneityChecker.java:58)
        at net.sf.saxon.style.StyleElement.typeCheck(StyleElement.java:1555)
        at net.sf.saxon.style.XSLSequence.validate(XSLSequence.java:95)
        at net.sf.saxon.style.StyleElement.validateSubtree(StyleElement.java:1744)
        at net.sf.saxon.style.StyleElement.validateChildren(StyleElement.java:1779)
        at net.sf.saxon.style.StyleElement.validateSubtree(StyleElement.java:1748)
        at net.sf.saxon.style.XSLTemplate.validateSubtree(XSLTemplate.java:595)
        at net.sf.saxon.style.PrincipalStylesheetModule.preprocess(PrincipalStylesheetModule.java:404)
        at net.sf.saxon.style.Compilation.compilePackage(Compilation.java:286)
        at net.sf.saxon.style.StylesheetModule.loadStylesheet(StylesheetModule.java:259)
        at net.sf.saxon.style.Compilation.compileSingletonPackage(Compilation.java:107)
        at net.sf.saxon.s9api.XsltCompiler.compile(XsltCompiler.java:785)
        at net.sf.saxon.Transform.doTransform(Transform.java:741)
        at net.sf.saxon.Transform.main(Transform.java:80)
Fatal error during transformation: java.lang.UnsupportedOperationException: Cannot copy a variable reference whose binding is unknown

Workaround: root($node)/foo/string()

Ha! You already fixed the formatting for me, thanks. :-)

Thanks for speculating on the cause too. Interesting stuff.

Being a good citizen and reporting the problem led to my workaround, so now we can immediately go back to using the latest version. Win/win.

My initial guess was a bit off-target, though not totally. There's some early reorganisation of the path expression going on, during the type-checking phase (and thus well before any variable inlining), when it is realised that the trailing "/string()" means the expression will always return atomic values and therefore no "homogeneity check" is needed (this is the check that the expression doesn't return a mixture of atomic values and nodes). The involves a rearrangement of the expression from A/(B/C) form to (A/B)/C, which involves copying sub-expressions; and copying subexpressions that include variable references is always requires a certain amount of care, and in this case is failing.

It's rather remarkable that this failure doesn't happen a lot; it's hard to see what the conditions are that make it so rare. For the same reason, it's not immediately obvious what we need to change to fix it.