Double federated query does not work in v5

[davidkleiven]

Current Behavior

For illustration purpuses a the following repo was populated with two triples

The following query

select * {
 ?s ?p ?o .
 ?_repo1 <http://repo> ?repo1
 service ?repo1 {
   ?s ?p2 ?o2
  }
  
  optional {
    ?_repo2 <http://repo> ?repo2
    service ?repo2 {
      ?s ?p3 ?o3
    }
  }  
}

Fails with

Query evaluation error: SERVICE variables must be bound at evaluation time

But if I remove the last optional clause such that query becomes

select * {
  ?s ?p ?o .
  ?_repo1 <http://repo> ?repo1
   service ?repo1 {
     ?s ?p2 ?o2
    }
  
   ?_repo2 <http://repo> ?repo2
   service ?repo2 {
     ?s ?p3 ?o3
   } 
}

It returns successfully. Also if I remove the first federated call such that the query becomes

select * {
  ?s ?p ?o .
  optional {
    ?_repo2 <http://repo> ?repo2
    service ?repo2 {
       ?s ?p3 ?o3
      }
  }
}

it also works as expected. It appears like the issue started from v5.

Expected Behavior

Expected that the query with where one of the federations are inside an optional clause also works.

Steps To Reproduce

No response

Version

5.1.2

Are you interested in contributing a solution yourself?

Yes

Anything else?

No response