These days we're not active xslt developers, though we still consider xslt and xquery are important part of our personal experience and self education.

Besides, we have a pretty large xslt code base that is and will be in use. We think xslt/xquery is in use in many other big and small projects thus they have a strong position as a niche languages.

Thus we think it's important to help to those who support xslt/xquery engines. That's what we're regularly doing (thanks to our code base).

Now, to the problem we just have found. Please consider the code:

<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"
  exclude-result-prefixes="xs t">

  <xsl:template match="/">
    <xsl:param name="new-line-text" as="xs:string" select="' '"/>
    <xsl:variable name="items" as="item()*" select="'select', $new-line-text"/>

    <xsl:message select="t:string-join($items)"/>
  </xsl:template>

  <!--
    Joins adjacent string items.
      $items - items to join.
      Returns joined items.
  -->
  <xsl:function name="t:string-join" as="item()*">
    <xsl:param name="items" as="item()*"/>

    <xsl:variable name="indices" as="xs:integer*" select="
      0,
      index-of
      (
        (
          for $item in $items return
            $item instance of xs:string
        ),
        false()
      ),
      count($items) + 1"/>

    <xsl:sequence select="
      for $i in 1 to count($indices) - 1 return
      (
        $items[$indices[$i]],
        string-join
        (
          subsequence
          (
            $items,
            $indices[$i] + 1,
            $indices[$i + 1] - $indices[$i] - 1
          ),
          ''
        )
      )"/>
  </xsl:function>

</xsl:stylesheet>

The output is:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
  at net.sf.saxon.om.Chain.itemAt(Chain.java:161)
  at net.sf.saxon.om.SequenceTool.itemAt(SequenceTool.java:130)
  at net.sf.saxon.expr.FilterExpression.iterate(FilterExpression.java:1143)
  at net.sf.saxon.expr.LetExpression.iterate(LetExpression.java:365)
  at net.sf.saxon.expr.instruct.BlockIterator.next(BlockIterator.java:49)
  at net.sf.saxon.expr.MappingIterator.next(MappingIterator.java:70)
  at net.sf.saxon.expr.instruct.Message.processLeavingTail(Message.java:264)
  at net.sf.saxon.expr.instruct.Block.processLeavingTail(Block.java:660)
  at net.sf.saxon.expr.instruct.Template.applyLeavingTail(Template.java:239)
  at net.sf.saxon.trans.Mode.applyTemplates(Mode.java:1057)
  at net.sf.saxon.Controller.transformDocument(Controller.java:2088)
  at net.sf.saxon.Controller.transform(Controller.java:1911)
  ...

The problem is reported at: Exception during execution in Saxon-HE-9.5.1-6 and also tracked at https://saxonica.plan.io/issues/2104.

Update: according to Michael Kay the issue is fixed. See note #4:

I have committed a patch to Chain.itemAt() on the 9.5 and 9.6 branches to check for index<0