When running this query:

db.results.aggregate([
    {$lookup: {from: "base_other", localField: "key", foreignField: "key", let: {f: "$cor.key.foo"}, pipeline: [{$match: {$expr: {$eq: ["$key", "$$f"]}}}], as: "lf"}}, 
    {$unwind: "$lf"}
]) 

I saw a tassert during edge inference because we try to construct a self-edge.

Using loose terminology, I image we see that (local) key == base_other.key AND (local) cor.key.foo $== base_other.key, so then we infer that (local) key == (local) cor.key.foo. That’s correct! Even though it doesn’t help us with our join plan, it’s a valid (single table) predicate to infer.

This ticket is related to SERVER-117816, and may in fact be addressed at the same time as that ticket. But, I’m tracking it separately because it represents a bug in our existing implementation.