Saxon

Just to summarise what's happening, as a result of constant folding and function inlining, we get to

<xsl:when test="matches($length, $vp:percent-regex)">
      <xsl:variable name="perc"
                    select="replace($length, $vp:percent-regex, '$1')"/>
      <xsl:sequence
          select="$nominal-page-width * xs:double($perc) div 100.0"/>

and during type-checking, we are able to establish that xs:double($perc) will never succeed; however this branch of the xsl:when will never be taken, so the warning is spurious.

As it happens, in this particular case, we know statically that this branch of the xsl:choose will never be taken, so we ought to be able to be a bit smarter. Of course, it could also happen that we don't know this, in which case we probably shouldn't try to be so smart.

It's tricky to know how best to handle this. In general, if early evaluation of constant expressions (during the type-checking or optimization phases) throws a dynamic error then we report this warning, and I'm reluctant to abandon this as a general principal. But we should probably be a bit smarter when we're in a branch of a conditional, especially if we already know that the condition will be false.