In the spec, under 3.6.3.3 Overriding Components from a Used Package, we say:

<quote>
Within an overriding stylesheet function, xsl:original() may be used to call the overridden stylesheet function.
</quote>

And we say, about its scope:

<quote>
If the overridden component C(P) has visibility public then the using package Q will also contain a component C′(Q) whose declaration is the same as the declaration of C(P) and whose visibility is hidden. This component is used as the target of a binding for the symbolic reference xsl:original described below.
</quote>

A programmer could do the following, assuming function f:test exists in a used package:

<xsl:use-package name="urn:test-cases">
  <xsl:override>
    <xsl:function name="f:test">
      <xsl:apply-templates select="xsl:original#0" />
    </xsl:function>
  </xsl:override>
</xsl:use-package>

<xsl:template name="xsl:initial-template">
   <xsl:value-of select="f:test()" />
</xsl:template>

<xsl:template match=".">
   <!-- calls the original function??? -->
   {.()}
</xsl:template>

This would mean that the scope of the original function, which has visibility HIDDEN, can be overridden by making it available as a reference.

The text says nothing about this, I think, and only mentions *calling* the function through xsl:original. Breaking the inheritance chain this way, can make a HIDDEN component available from within an area where HIDDEN components should not be available.

This can be considered "by design", or not, in which case it is an error, but either way, I think we should be explicit about it and clarify it, with a Note or otherwise.

The issue does not rise with other components, because you cannot create a referencing item from other components the same way you can do it with functions and function items.