问题与类型擦除无关，如果您删除 struct AnyAnimal< Food> 定义，则会得到相同的
错误消息。

如果您从 struct Cow 然后
中删除 feed（）方法，编译器无法推断关联的类型食品。因此，无论您
在默认实现中使用了一个具体类型：

  extension动物{
 func feed食物：草） - > Void {
 print（unknown）
} 
} 

 struct Cow：Animal {
} 
 

或者使用
的默认实现为每个类型定义一个类型别名 Food ：

  extension动物{
 func feed（food：Food） - > Void {
 print（unknown）
} 
} 

 struct Cow：Animal {
 typealias Food = Grass 
} 
 

也可以为 Food 定义一个默认类型>在
协议中：

 协议动物{
 associatedtype Food = Grass 
 func feed （食物：食物） - >无效
} 


扩展动物{
 func feed（food：Food） - > Void {
 print（unknown）
} 
} 

 struct Cow：Animal {
} 
 

I have been struggling very hard with Swift Protocols and Associated Types for a long time. I started again with basic to really understand what is going wrong and I followed this article of TypeErasure in Swift Protocols with Associated Type Requirement by Rob Napier but still i have no luck.

Find the code below

// An Animal can eat
protocol Animal {
    associatedtype Food
    func feed(food: Food) -> Void
}

struct AnyAnimal<Food>: Animal {
    private let _feed: (Food) -> Void
    init<Base: Animal where Food == Base.Food>(_ base: Base) {
        _feed = base.feed
    }
    func feed(food: Food) { _feed(food) }
}

// Kinds of Food
struct Grass {}

struct Cow: Animal {
    func feed(food: Grass) { print("moo") }
}

struct Goat: Animal {
    func feed(food: Grass) { print("bah") }
}

let grassEaters = [AnyAnimal(Cow()), AnyAnimal(Goat())]
for animal in grassEaters {
    animal.feed(Grass())
}

Now i wanted to give a default implementation in the protocol Animal like below

extension Animal {

    func feed(food: Food) -> Void {
        print("unknown")
    }
}

And when i removed the function from the struct Cow i get the error message that Cow doesn't conform to Protocol Animal.

Does that mean you cannot have Type Erasures and Default Implementation together?. Is there any way where i can do TypeErasure and also keep a Default Implementation?

The problem is unrelated to the type erasure and you'll get the same error message if you remove the struct AnyAnimal<Food> definition.

If you remove the feed() method from struct Cow then the compiler cannot infer the associated type Food. So either you use a concrete type in the default implementation:

extension Animal {
    func feed(food: Grass) -> Void {
        print("unknown")
    }
}

struct Cow: Animal {
}

or you define a type alias Food for each type using the default implementation:

extension Animal {
    func feed(food: Food) -> Void {
        print("unknown")
    }
}

struct Cow: Animal {
    typealias Food = Grass
}

It is also possible to define a default type for Food in the protocol:

protocol Animal {
    associatedtype Food = Grass
    func feed(food: Food) -> Void
}


extension Animal {
    func feed(food: Food) -> Void {
        print("unknown")
    }
}

struct Cow: Animal {
}