[xsl] Use of XSLT 3 current-merge-group() as argument to user-defined function

["Martin Honnen martin.honnen@xxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

I am trying to use the function current-merge-group inside of an 
xsl:merge-action in a call to a user-defined function

  <xsl:merge-action>
     <xsl:sequence select="mf:my-fun(current-merge-group(), 
$levels-to-reconstruct - 1)"/>
  </xsl:merge-action>

While that works fine with Altova I don't get it to work with Saxon 9.8, 
even with HE or with a stylesheet not using streamable merge sources, 
Saxon indicates "XTDE3480: There is no current merge group".

Looking at the spec 
https://www.w3.org/TR/xslt-30/#dt-current-merge-group it says "The 
current merge group and current merge key are available within the 
sequence constructor contained by an xsl:merge-action element." and "All 
invocation constructs set the current merge group and current merge key 
to absent" where the section 
https://www.w3.org/TR/xslt-30/#dt-invocation-construct then details 
"XPath function calls that bind to stylesheet functions" as one of the 
invocation constructs so Saxon seems to be following the spec to reject 
the use of current-merge-group() as the argument to my function.

I wonder why there is that restriction, at least when no streaming is 
used I can't directly see a reason why a merge group can't be passed on 
to a function.

I can rewrite the code to

  <xsl:merge-action>
    <xsl:sequence select="let $group := current-merge-group() return 
mf:my-fun($group, $levels-to-reconstruct - 1)"/>
  </xsl:merge-action>

to have it executed in Saxon as well.

Why does the spec not allow passing current-merge-group() directly to a 
user defined function?