It seems that computed contents of a fo:basic-link element does not create
a link in the rendered output. This happens at least with fo:leader and
fo:page-number-citation.

I want to generate an index, linking the page numbers with the occurences
of the keywords like this:
<fo:basic-link internal-destination="{generate-id()}">
  <fo:page-number-citation ref-id="{generate-id()}" />
</fo:basic-link>
No success.

If static text occurs within the link, only this text acts as the link anchor,
for examples in a table of contents (simplified):
<fo:basic-link internal-destination="{generate-id()}">
  <xsl:value-of select="." />   <!-- ok -->
  <fo:leader ... />             <!-- not ok, i.e. not linked -->
  <fo:page-number-citation ref-id="{generate-id()}" />  <!-- dito -->
</fo:basic-link>