The test calls a function that uses an expression that select the root node from a context item that is not a node:

<xsl:function name="f:doc" xmlns:f="http://f.com/">
  <xsl:analyze-string select="'abcd'" regex="efgh">
    <xsl:matching-substring>
      <xsl:value-of select="//a"/>
    </xsl:matching-substring>
  </xsl:analyze-string>
</xsl:function>
</xsl:stylesheet>

However, the function is never executed. The test is marked with:

    "." used when not a node - static error even though never executed. 

I don't think that is true. If the function is never called, the error does not need to be raised.