正如错误所指出的那样，问题是，您不能将具有“自我”或相关类型要求的协议用作实际类型，因为您会丢失有关这些要求的类型信息。在这种情况下，您将丢失==实现参数的类型信息，因为Equatable它们必须与一致类型（即Self）具有相同的类型。
解决方案几乎总是建立一个类型的橡皮擦。在期望类型相等的情况下，如果它们的id属性相等，则可以简单地id将其存储起来并在==实现中进行比较。
struct AnyVehicle : Equatable {
static func ==(lhs: AnyVehicle, rhs: AnyVehicle) -> Bool {
return lhs.id == rhs.id
}
let id : String
init<T : Vehicle>(_ base: T) {
id = base.id
}
}
（请注意，为了符合Swift命名约定，我将您的ID属性重id命名为）
但是，更通用的解决方案是在类型橡皮擦中存储一个函数，该函数可以在类型转换之后Vehicle根据 其==实现比较两个任意符合的实例，以确保它们与创建类型橡皮擦的具体类型相同。 。
struct AnyVehicle : Equatable {
static func ==(lhs: AnyVehicle, rhs: AnyVehicle) -> Bool {
// forward to both lhs's and rhs's _isEqual in order to determine equality.
// the reason that both must be called is to preserve symmetry for when a
// superclass is being compared with a subclass.
// if you kNow you're always working with value types, you can omit one of them.
return lhs._isEqual(rhs) || rhs._isEqual(lhs)
}
let base: Identifiable
private let _isEqual: (_ to: AnyVehicle) -> Bool
init<T : Vehicle>(_ base: T) {
self.base = base
_isEqual = {
// attempt to cast the passed instance to the concrete type that
// AnyVehicle was initialised with, returning the result of that
// type's == implementation, or false otherwise.
if let other = $0.base as? T {
return base == other
} else {
return false
}
}
}
}
print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Tractor(id: "foo"))) // false
print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Car(id: "bar"))) // false
print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Car(id: "foo"))) // true
var array = [AnyVehicle]()
array.append(AnyVehicle(Car(id: "VW")))
array.append(AnyVehicle(Car(id: "Porsche")))
array.append(AnyVehicle(Tractor(id: "John Deere")))
array.append(AnyVehicle(Tractor(id: "Steyr")))
var op = Operator()
// compiles fine as AnyVehicle conforms to Equatable.
op.operationOnCollectionOfEquatables(array: array)
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