TypeErasure.swift

protocol Query {
    associatedtype QueryType
    var codingKey: String { get } // set?
}

private class _AnyQueryBase<QueryType>: Query {
    var codingKey: String {
        get { fatalError("Must override") }
    }

    init() {
        guard type(of: self) != _AnyQueryBase.self else {
            fatalError("Must subclass")
        }
    }
}

private final class _AnyQueryBox<ConcreteQuery: Query>: _AnyQueryBase<ConcreteQuery.QueryType> {
    var concrete: ConcreteQuery

    override var codingKey: String {
        get { return concrete.codingKey }
    }

    init(_ concrete: ConcreteQuery) {
        self.concrete = concrete
    }
}

final class AnyQuery<QueryType>: Query {
    private let box: _AnyQueryBase<QueryType>

    var codingKey: String {
        get { return box.codingKey }
    }

    init<Concrete: Query>(_ concrete: Concrete) where Concrete.QueryType == QueryType {
        box = _AnyQueryBox(concrete)
    }
}