Bug #6093: Bug on predicate with last() - Saxon - Saxonica Developer Community

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Bug #6093

closed

Bug on predicate with last()

Added by Shuxin Li 10 months ago. Updated 8 months ago.

Status:

Closed

Priority:

Normal

Assignee:

Michael Kay

Category:

XPath conformance

Sprint/Milestone:

Start date:

2023-06-23

Due date:

% Done:

100%

Estimated time:

Legacy ID:

Applies to branch:

11, 12, trunk

Fix Committed on Branch:

11, 12, trunk

Fixed in Maintenance Release:

12.3, 11.6

Platforms:

.NET, Java

Description

With xml document:

<A><B/></A>

and xpath query

//*[((.,.)/parent::*/last() ! (. > 1)) = true()]

Saxon-he 12.2 returns node B. From my understanding node B shouldn't be selected, for (., .)/parent:: selects node A, and last() for node A in this case should evaluate to 1, which does not satisfy the requirement. The same query in BaseX returns empty result set.

However I'm not very certain about either what the context actually should be here or what the last() function should really return. Might this be undefined behavior?

This is a reduced case from:

<A><B><C/></B><D/></A>

and

//*[((C,.)/following::*/last() ! (. > 1)) = true()]

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Updated by Michael Kay 10 months ago

Yes, I think you're right. The expression tree (from Saxon-EE with -opt:0) is

<query>
   <globalVariables/>
   <body>
      <docOrder baseUri="file:/Users/mike/GitHub/saxon2020/"
                ns="array=~ err=~ fn=~ local=http://www.w3.org/2005/xquery-local-functions map=~ math=~ saxon=~ xs=~ xsi=~ xml=~"
                line="1"
                intra="1">
         <slash>
            <root/>
            <filter flags="b">
               <axis name="descendant" nodeTest="NE"/>
               <gc op="=" card="M:N" comp="CAVC">
                  <forEach>
                     <slash>
                        <slash>
                           <sequence>
                              <dot type="1NE"/>
                              <dot type="1NE"/>
                           </sequence>
                           <axis name="parent" nodeTest="NE"/>
                        </slash>
                        <fn name="last"/>
                     </slash>
                     <vc op="gt" onEmpty="0">
                        <dot type="1ADI"/>
                        <int val="1"/>
                     </vc>
                  </forEach>
                  <true/>
               </gc>
            </filter>
         </slash>
      </docOrder>
   </body>
</query>

The docOrder node around the inner <slash> has been eliminated indicating that the duplicate parent isn't being eliminated before calling last().

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Updated by Michael Kay 10 months ago

What's happening here is that the GeneralComparison XXX = true() decides that neither of its operands needs to be sorted, so it's calling "unordered()" on its operands and this is propagating the elimination of the docOrder() operation.

If the RHS of a SlashExpression is position-sensitive then a docOrder() on the LHS shouldn't be eliminated in this way.

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