Extension operation disappears for some queries in rdf4j v4

[abrokenjester]

Discussed in #3991

^{Originally posted by apano-on June 15, 2022}
It seems like the operation with the extension has disappeared in at least one scenario for rdf4j v4.0.0 (as well as v4.0.2). Is there a rationale for this change?

Steps to reproduce

The following query is translated differently in the new version of rdf4j

SELECT (?l AS ?v)
WHERE { ?s rdfs:label ?l . }
ORDER BY ?v

Current Behavior (rdf4j v4.0.2 translation/algebra tree)

QueryRoot
   Projection
      ProjectionElemList
         ProjectionElem "l" AS "v"
      Order
         OrderElem (ASC)
            Var (name=v)
         StatementPattern
            Var (name=s)
            Var (name=_const_9285ccfc_uri, value=http://www.w3.org/2000/01/rdf-schema#label, anonymous)
            Var (name=l)

Expected Behavior (rdf4j v3 translation/algebra tree)

Projection
   ProjectionElemList
      ProjectionElem "l" AS "v"
   Order
      OrderElem (ASC)
         Var (name=v)
      Extension
         ExtensionElem (v)
            Var (name=l)
         StatementPattern
            Var (name=s)
            Var (name=_const_9285ccfc_uri, value=http://www.w3.org/2000/01/rdf-schema#label, anonymous)
            Var (name=l)
```</div>

[abrokenjester]

I'm going to partially revert some of the changes made for GH-2990, and consistently use an extension again. This seems to be more in line with the intent of the SPARQL spec (see https://www.w3.org/TR/sparql11-query/#sparqlSelectExpressions in particular).