Why is it important to define object equality in methods that are required to be implemented in the Hashable protocol?

In the Hashable protocol, defining object equality (via the == method) is critical because the object's hash value is used for fast lookup in collections such as sets or dictionaries.

If two objects are considered equal (by ==), their hash values must match. This ensures the correct operation of hash tables: objects with the same hash and equality are considered the same element.

Without a proper equality definition, errors can occur, such as duplicates in a Set or inability to find an element in a Dictionary, even if it exists.

Example:

struct User: Hashable {
    let id: Int
    let name: String

    static func == (lhs: User, rhs: User) -> Bool {
        return lhs.id == rhs.id
    }

    func hash(into hasher: inout Hasher) {
        hasher.combine(id)
    }
}

Here, equality is defined by id, and the hash is built on the same field, which guarantees correctness.