Protocol using 2.swift

protocol ProtocolB1 {
    init()

    func methodB1()
}

extension ProtocolB1 where Self: ClassB1 {
    func methodB1() {
        print("ProtocolB1 where Self: ClassB1", #function, #line, Self.self)
    }
}

extension ProtocolB1 {
    func methodB1() {
        print("ProtocolB1", #function, #line, Self.self)
    }
}

protocol ProtocolB2: ProtocolB1 {
    associatedtype T

    func methodB2() -> T
}

extension ProtocolB2 where Self == ClassB1 {
    func methodB2() -> Self {
        print("ProtocolB", #function, #line, Self.self)
        return Self()
    }
}

class ClassB1: ProtocolB2 {
    required init() {}
}

class ClassB2: ProtocolB2 {
    required init() {}

    func methodB2() -> ClassB2 {
        print(#function, #line, Self.self)
        return Self()
    }
}

class ClassB3: ProtocolB2 {
    required init() {}
    
    func methodB1() {
        print(#function, #line, Self.self)
    }

    func methodB2() -> ClassB3 {
        print(#function, #line, Self.self)
        return Self()
    }
}


var itemsB1: [ProtocolB1] = [ClassB1(), ClassB2(), ClassB3()]

for item in itemsB1 { item.methodB1() }

print("------------------")

var itemsB2: some Collection = [ClassB1(), ClassB2(), ClassB3()]

for item in itemsB2 {
    if let item = item as? ProtocolB1 {
        item.methodB1()
    }
    if let item = item as? ClassB1 {
        item.methodB2()
    }
    if let item = item as? ClassB2 {
        item.methodB2()
    }
    if let item = item as? ClassB3 {
        item.methodB2()
    }
}

// ProtocolB1 where Self: ClassB1 methodB1() 11 ClassB1
// ProtocolB1 methodB1() 17 ClassB2
// methodB1() 51 ClassB3
// ------------------
// ProtocolB1 where Self: ClassB1 methodB1() 11 ClassB1
// ProtocolB methodB2() 29 ClassB1
// ProtocolB1 methodB1() 17 ClassB2
// methodB2() 42 ClassB2
// methodB1() 51 ClassB3
// methodB2() 55 ClassB3

Protocol using.swift

protocol ProtocolB1 {
    init()

    func methodB1()
}

extension ProtocolB1 where Self: ClassB1 {
    func methodB1() {
        print("ProtocolB1 where Self: ClassB1", #function, Self.self)
    }
}

extension ProtocolB1 {
    func methodB1() {
        print("ProtocolB1", #function, Self.self)
    }
}

protocol ProtocolB2: ProtocolB1 {
    associatedtype T

    func methodB2() -> T
}

extension ProtocolB2 where Self == ClassB1 {
    func methodB2() -> Self {
        print("ProtocolB", #function, Self.self)
        return Self()
    }
}

class ClassB1: ProtocolB2 {
    required init() {}
}

class ClassB2: ProtocolB2 {
    required init() {}

    func methodB2() -> ClassB2 {
        print(#function, Self.self)
        return Self()
    }
}

var itemsB1: [ProtocolB1] = [ClassB1(), ClassB2()]

for item in itemsB1 { item.methodB1() }

print("------------------")

var itemsB2: some Collection = [ClassB1(), ClassB2()]

for item in itemsB2 {
    if let item = item as? ProtocolB1 {
        item.methodB1()
    }
    if let item = item as? ClassB1 {
        item.methodB2()
    }
    if let item = item as? ClassB2 {
        item.methodB2()
    }
}

// ProtocolB1 where Self: ClassB1 methodB1() ClassB1
// ProtocolB1 methodB1() ClassB2
// ------------------
// ProtocolB1 where Self: ClassB1 methodB1() ClassB1
// ProtocolB methodB2() ClassB1
// ProtocolB1 methodB1() ClassB2
// methodB2() ClassB2