$or followed by always false $elemMatch can fail pipeline optimization

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    • Type: Bug
    • Resolution: Unresolved
    • Priority: Major - P3
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
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      const coll = db.coll;
coll.drop();
assert.commandWorked(coll.insert({}));
const query = [
    {$addFields: {t: 0}},
    {$match: {$or: [{t: 0, b: 0}],
              t: {$elemMatch: {$not: {$in: []}}}}}
];
coll.aggregate(query).toArray();
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      const coll = db.coll; coll.drop(); assert .commandWorked(coll.insert({})); const query = [ {$addFields: {t: 0}}, {$match: {$or: [{t: 0, b: 0}], t: {$elemMatch: {$not: {$in: []}}}}} ]; coll.aggregate(query).toArray();
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      The following repro leads to the error "Failed to optimize pipeline :: caused by :: unknown operator: $alwaysFalse", despite being a valid agg pipeline.

      I've noticed a $project/$addFields is required to repro, followed by an $or predicate and an always false $elemMatch predicate

            Assignee:
            Timour Katchaounov
            Reporter:
            Matt Boros
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              Created:
              Updated: