Apply node transforms to triple terms in expressions

[jitsedesmet]

ApplyTransformVisitor.visit(ExprTripleTerm) discarded the ExprTransform instead of applying it, so no algebra transform could reach the variables inside a triple term in an expression.

The visible effect is on sub-SELECT scope renaming. Variables of a sub-SELECT that are not projected out are renamed ?x -> ?/x by TransformScopeRename/Rename.renameVars so they cannot clash with usage outside the sub-SELECT. That renaming did not reach inside a triple term, leaving inconsistent algebra:

(project (?g ?tt)
(extend ((?tt (tripleterm ?x urn:p urn:o))) <- not renamed
(bgp (triple ?g urn:hasX ?/x)))) <- renamed

?x is unbound at evaluation time, so the BIND raises VariableNotBoundException and extend leaves ?tt unbound. The query returns the right number of rows with the triple term column silently missing and no warning or error. For example:

SELECT * {
{ SELECT ?g ?tt { ?g :hasX ?x . BIND( <<( ?x :p :o )>> AS ?tt ) } }
?g :hasB ?b .
}

The renaming only happens when the sub-SELECT is joined with something at the enclosing level, so the same BIND in a plain group, or a sub-SELECT run on its own, was unaffected.

• ApplyTransformVisitor: apply the transform.
• ExprTransform: new default transform(ExprTripleTerm) returning the term unchanged, so existing implementations are unaffected.
• ExprTripleTerm: apply(ExprTransform), as for ExprVar and NodeValue; applyNodeTransform returns this when the node is unchanged.
• NodeTransformExpr, ExprTransformNodeElement: the NodeTransform bridges pass the whole triple term node to the NodeTransform, which is the same contract as triples in a BGP (NodeTransformLib.transform).

GitHub issue resolved #

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