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

c.save( { a:0.0 } );

large = NumberLong('9223372036854775807');

// 1. Two large longs overflow.
printjson( c.aggregate( { $project:{ sum:{ $add:[ large, large ] } } } ) );
// 2. Two large longs are cast to doubles if they are summed with a double, and do not overflow.
printjson( c.aggregate( { $project:{ sum:{ $add:[ large, large, 0.0 ] } } } ) );
// 3. The constant folding implementation sums the first two operands, which overflow, then sums the overflow result with the value of '$a'.
printjson( c.aggregate( { $project:{ sum:{ $add:[ large, large, '$a' ] } } } ) );

// 1
{
	"result" : [
		{
			"_id" : ObjectId("50121f8b8e4c36714e472e38"),
			"sum" : NumberLong(-2)
		}
	],
	"ok" : 1
}
// 2
{
	"result" : [
		{
			"_id" : ObjectId("50121f8b8e4c36714e472e38"),
			"sum" : 18446744073709552000
		}
	],
	"ok" : 1
}
// 3
{
	"result" : [
		{
			"_id" : ObjectId("50121f8b8e4c36714e472e38"),
			"sum" : -2
		}
	],
	"ok" : 1
}