Consider the $lookup below:

{
    "$lookup": {
        "from": "supplier",
        "pipeline": [],
        "as": "supplier",
    }
} 

It looks like a cartesian product because it does not specify any join predicates. However, if later in the query you would connect the supplier collection with collection X, and X itself with the base collection, then no cartesian product is required.

We currently bail when we see an empty pipeline here. If the pipeline would have a single $match without join predicates, we'd bail here instead (because the implementation of SERVER-115666 is a bit too strict). We should support these cases, because they're not actually cartesian products.