型制約をつけると具体型が必要になる.swift

TypeConforming.swift

protocol Creature { }
protocol Animal: Creature { }
struct Dog: Animal { }
struct Cat: Animal { }


// OK
class Hoge<A> { }
let hoge = Hoge<Animal>() // OK

// NG
// error: using 'Animal' as a concrete type conforming to protocol 'Creature' is not supported
class Fuga<A: Creature> { }
let fuga = Fuga<Animal>() // NG

// OK
class Piyo<A> { }
extension Piyo where A: Creature { }
let piyo = Piyo<Animal>() // OK