When a partial filter expression uses $in we sometimes don't recognize that it can satisfy a query using $in. For example:

> db.foo.createIndex({a: 1, b: 1}, {partialFilterExpression: {b: {$in: [1, 2, 3]}}})
> db.foo.explain().aggregate({$match: {a: 1, b: {$in: [1, 2]}}})
{
    …
    “winningPlan" : {
        "stage" : "COLLSCAN”,
    …
}

This query should be eligible to use this index.

The problem is in our expression_algo::isSubsetOf() logic for $in partial filter expressions  here. In particular, the loop over the elements of the RHS in-list is overly conservative. Instead, we should check if one in-list is a subset of another.

This issue is similar to SERVER-73792 but proposes to target this singular, specific improvement.