Saxon

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

<xsl:stylesheet

version="2.0"

xmlns:xsl="http://www.w3.org/1999/XSL/Transform"

xmlns:xs="http://www.w3.org/2001/XMLSchema"

xmlns:ext="http://example.com/extensions"

extension-element-prefixes="ext"

exclude-result-prefixes="xs">

<xsl:template match="/">

<xsl:variable name="test" as="document-node(element(foo))?">

<ext:instruction>

<!-- Remove this and the error clears -->

<xsl:fallback/>

</ext:instruction>

</xsl:variable>

<xsl:if test="$test/foo[@bar]">

<xsl:message>This is works fine</xsl:message>

</xsl:if>

<xsl:if test="$test/foo[@bar ne 'baz']">

<xsl:message>This is works fine</xsl:message>

</xsl:if>

<xsl:if test="$test/foo[@bar eq 'baz']">

<xsl:message>This throws "XPTY0020: Leading '/' cannot select the root node of the tree containing the context item: the context item is an atomic value"</xsl:message>

</xsl:if>

<xsl:if test="$test/foo[@bar = 'baz']">

<xsl:message>This also throws "XPTY0020: Leading '/' cannot select the root node of the tree containing the context item: the context item is an atomic value"</xsl:message>

</xsl:if>

<!-- This also works fine -->

<xsl:variable name="works" as="element()?">

<ext:instruction>

<xsl:fallback/>

</ext:instruction>

</xsl:variable>

<xsl:if test="$works[@bar eq 'baz']">

<xsl:message>This is works fine</xsl:message>

</xsl:if>

</xsl:template>

</xsl:stylesheet>