Can anyone explain this to me? (using latest libxslt):
<a><xsl:copy-of select="(@attrib|exsl:node-set(0))"/></a>
<b><xsl:copy-of select="(@attrib|exsl:node-set(0))[position()=1]"/></b>
<x><xsl:copy-of select="(@attrib|exsl:node-set(0))[1]"/></x>
<xsl:variable name="value" select="@attrib"/>
<y><xsl:copy-of select="($value|exsl:node-set(0))[1]"/></y>
Result (for @attrib = 1 at current node):
<a attrib="1">0</a>
<b attrib="1"/>
<x>0</x>
<y attrib="1"/>
<a> and <b> show expected behavior.
<x> is IMHO incorrect.
But why does putting @attrib into a variable "fix" it for <y>?
BTW: Everything is correct when @attrib is not present. The copy-of used here is for debugging; original usage converts the XPath result to a number, and a missing attribute shall not lead to NaN, but a certain default value.
This version of exsl:node-set() presumably creates a node that is in a different tree from from the node @attrib. When two nodes are in different trees, it is implementation-dependent which of them comes first in document order. The construct (X|Y)[position()=1] (or equivalently (X|Y)[1]) selects whichever of X and Y comes first in document order. Therefore it's essentially unpredictable whether it selects X or Y.