Hopefully this should be one of our last static typing problems!

In annex-6

declare function eg:index-of-node($sequence as node()*, $srch as node()) as xs:i
nteger*
{
  for $n at $i in $sequence where ($n is $srch) return $i
};

let $arg1 := (<element1>some data 1</element1>,<element2>some data 2</element2>)
let $arg2 := $arg1[2]
return
 eg:index-of-node($arg1,$arg2)


The second arguement of index-of-node has type node(), which it must do for $n is $srch to type check.

But $arg2 has quantifier zero-or-one (section 7.2.13 of the FS) hence the function call fails to static type check.