自从引入了主要关联类型，是否还存在类型抹消的用例？

虽然主要关联类型在Swift中已广泛应用，但类型抹消仍然有其用途。比如我们可以使用类型抹消来实现一个通用的容器类型，该容器类型可以存储任何继承自Equatable协议的类型。

代码示例：

protocol AnyEquatable {
    func equalTo(_ other: AnyEquatable) -> Bool
}

extension Equatable where Self: AnyObject {
    func equalTo(_ other: AnyEquatable) -> Bool {
        guard let other = other as? Self else { return false }
        return self == other
    }
}

struct AnyEquatableValue: AnyEquatable {
    private let _equalTo: (AnyEquatable) -> Bool
    init<T: Equatable>(_ value: T) {
        _equalTo = { other in
            guard let other = other as? T else { return false }
            return value == other
        }
    }
    
    func equalTo(_ other: AnyEquatable) -> Bool {
        return _equalTo(other)
    }
}

struct AnyEquatableArray: AnyEquatable {
    private let _equalTo: (AnyEquatable) -> Bool
    init<T: Equatable>(_ values: [T]) {
        _equalTo = { other in
            guard let other = other as? AnyEquatableArray else { return false }
            guard values.count == other.values.count else { return false }
            for i in 0..<values.count {
                if !values[i].equalTo(other.values[i]) {
                    return false
                }
            }
            return true
        }
    }
    private let values: [AnyEquatableValue]
    
    subscript(index: Int) -> AnyEquatableValue {
        return values[index]
    }
    
    var count: Int { return values.count }
    
    func equalTo(_ other: AnyEquatable) -> Bool {
        return _equalTo(other)
    }
}


let intArray: [Int] = [1, 2, 3]
let anyEquatableArray = AnyEquatableArray(intArray.map { AnyEquatableValue($0) })

let strArray: [String] = ["Hello", "Swift", "!"]
let anyEquatableArray2 = AnyEquatableArray(strArray.map { AnyEquatableValue($0) })

print(anyEquatableArray.equalTo(anyEquatableArray2)) // false

let anyEquatableArray3 = AnyEquatableArray(intArray.map { AnyEquatableValue($0) })
print(anyEquatableArray.equalTo(anyEquatableArray3)) // true