Optional incorrectly binds values from LHS in the RHS graph pattern

[frensjan]

Current Behavior

RDF4J incorrectly binds values from LHS of an OPTIONAL to variables in the RHS graph pattern.

See also w3c/sparql-dev#100 (comment)

Note that incorrectly may be nuanced. Different engines seem to implement this differently.

Expected Behavior / Steps To Reproduce

PREFIX : <http://example.org/>
                
SELECT * WHERE {
    VALUES ?x { :x }
    OPTIONAL {
        FILTER( BOUND(?x) )
        BIND( :y as ?y )
        BIND( ?x as ?z )
    }
}

should produce

---------------
| x  | y  | z |
===============
| :x | :y |   |
---------------

but produces

----------------
| x  | y  | z  |
===============
| :x | :y | :x |
----------------

Version

4.3.8

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