After SERVER-115982, the join optimizer does not support translation of a $lookup that has absorbed a single-table filter. This can occur when the query contains a lookup-unwind-match sequence, where the match is only on the “as” field of the lookup. For example:

        [            
            {
                $lookup: {
                    from: foreignColl1.getName(),
                    localField: "a",
                    foreignField: "a",
                    as: "foreignColl1",
                },
            },
            {$unwind: "$foreignColl1"},
            {$match: {"foreignColl1.c": {$eq: "bar"}}},
        ], 

It should be relatively easy to add support for this. The join graph does not need to be adjusted (as it already supports single-table predicates, just the join graph construction logic. Note that both kinds of lookup syntax (local/foreignField and pipeline) support additional filters.