I disagree with the expected error code for test op-concat-18.

Consider the simpler query

"abc" || fn:concat#3

This is equivalent to

fn:concat("abc", fn:concat#3)

and because the expected type of arguments to fn:concat is xs:anyAtomicType?, section 3.1.5.2 Function Conversion Rules apply, giving

fn:concat(fn:data("abc"), fn:data(fn:concat#3))

fn:data(fn:concat#3) will raise err:FOTY0013.

   <test-case name="op-concat-18">
      <description>Can't have a function item as an argument to concat
</description>
      <created by="Michael Kay" on="2011-06-24"/>
      <dependency type="spec" value="XP30+ XQ30+"/>
      <test>("abc" || "abc" ||  fn:concat#3)</test>
      <result>
         <error code="XPTY0004"/>
      </result>
   </test-case>