Saxon

This reproduces the problem:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:t="this">

  <xsl:template match="/">
    <xsl:variable name="s" as="element()*">
      <h>
        <f/>
      </h>
    </xsl:variable>
     
    <xsl:apply-templates mode="t:a" select="$s"/>
  </xsl:template>

  <xsl:template mode="t:a" match="h">
    <xsl:sequence select="f/t:f2(@a, @b, @c)"/>
  </xsl:template>

  <xsl:template mode="t:a" priority="2" match="h">
    <xsl:choose>
      <xsl:when test="t:f1(*) = 'b'">
        <s/>
      </xsl:when>
      <xsl:otherwise>
        <xsl:next-match/>
      </xsl:otherwise>
    </xsl:choose>
  </xsl:template>
  
  <xsl:function name="t:f1" as="item()*">
    <xsl:param name="e" as="element()?"/>
    
    <xsl:sequence select="'d'"/>
  </xsl:function>
    
  <xsl:function name="t:f2" as="item()*">
    <xsl:param name="a" as="xs:string?"/>
    <xsl:param name="b" as="xs:string?"/>
    <xsl:param name="c" as="xs:string?"/>

    <xsl:sequence select="'d'"/>
  </xsl:function>

</xsl:stylesheet>

With some modifications like this:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:t="this">

  <xsl:template match="/">
    <xsl:variable name="s" as="element()*">
      <h>
        <f/>
      </h>
    </xsl:variable>
     
    <xsl:apply-templates mode="t:a" select="$s"/>
  </xsl:template>

  <xsl:template mode="t:a" match="h">
    <xsl:sequence select="f/t:f2(@a, @b, @c)"/>
  </xsl:template>

  <xsl:template mode="t:a" priority="2" match="h">
    <xsl:next-match/>
  </xsl:template>
  
  <xsl:function name="t:f1" as="item()*">
    <xsl:param name="e" as="element()?"/>
    
    <xsl:sequence select="'d'"/>
  </xsl:function>
    
  <xsl:function name="t:f2" as="item()*">
    <xsl:param name="a" as="xs:string?"/>
    <xsl:param name="b" as="xs:string?"/>
    <xsl:param name="c" as="xs:string?"/>

    <xsl:sequence select="'d'"/>
  </xsl:function>

</xsl:stylesheet>

I get different error:

     [java] Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
     [java] 	at java.util.ArrayList.rangeCheck(ArrayList.java:635)
     [java] 	at java.util.ArrayList.get(ArrayList.java:411)
     [java] 	at net.sf.saxon.expr.XPathContextMajor.getTargetComponent(XPathContextMajor.java:778)
     [java] 	at net.sf.saxon.expr.XPathContextMinor.getTargetComponent(XPathContextMinor.java:554)
     [java] 	at net.sf.saxon.expr.UserFunctionCall.callFunction(UserFunctionCall.java:507)
     [java] 	at net.sf.saxon.expr.UserFunctionCall.iterate(UserFunctionCall.java:473)
     [java] 	at net.sf.saxon.expr.SlashExpression.map(SlashExpression.java:852)
     [java] 	at net.sf.saxon.expr.ContextMappingIterator.next(ContextMappingIterator.java:61)
     [java] 	at net.sf.saxon.tree.iter.HomogeneityCheckerIterator.next(HomogeneityCheckerIterator.java:65)
     [java] 	at net.sf.saxon.expr.Expression.process(Expression.java:555)
     [java] 	at net.sf.saxon.expr.instruct.Template.applyLeavingTail(Template.java:338)
     [java] 	... 8 more

Sorry for the delay on this, caused by the need to use a diagnostic build with HE-only code.

Now resolved; the problem occurs with a tail-call on xsl:next-match because it is not saving the "current component" correctly; the current component is needed for the new dynamic binding mechanism in 9.6 to work. This has been introduced to support calls across separately compiled packages.

The reason the failure doesn't occur on Saxon-EE is that the function calls are all inlined, so no dynamic binding occurs. (Function inlining is possible because the functions are implicitly private to the package, so they cannot be overridden from a different package. This should also mean that the calls can be statically bound even when not inlined. This little optimization has not been implemented in 9.6.

I've committed a patch (9.6 and 9.7 branches) and before marking as resolved I will now look at the "different error" that occurs with the modified stylesheet.