ExpressionCompare::optimize() generates an ExpressionFieldRange for a $ne comparison against a constant. However ExpressionCompare does not properly encapsulate the semantics of not equals matching.

Test:

c = db.c;
c.drop();

c.save( { a:1, b:1 } );
c.save( { a:2, b:1 } );
printjson( c.aggregate( { $project:{ z:{ $ne:[ '$a', 1 ] } } } ) );
printjson( c.aggregate( { $project:{ z:{ $ne:[ '$a', '$b' ] } } } ) );

Observed behavior

is always false
Actual behavior

should be true when the 'a' field value != 1.