Generic parameter ’T’ cannot be bound to non-@objc protocol type ‘Protocol’.

ExtendedProtocol

protocol BaseProtocol {}
protocol ExtendedProtocol: BaseProtocol {}

class A<T: BaseProtocol> {
	private let t: T

	init(a: T) { self.t = a }
}

class B<T: ExtendedProtocol> {
	private let a: A<T>

	init(a: A<T>) { self.a = a }
}

class ExtendedClass: ExtendedProtocol {}

// This isn't possible either :(
let a: A<ExtendedProtocol> = A(a: ExtendedClass())
let b: B<ExtendedProtocol> = B(a: a)

Generic Protocol Parameters

protocol Protocol {}

class A<T: Protocol> {
	private let array: [T]

	init(a: [T]) { self.array = a }
}

class Class1: Protocol {}
class Class2: Protocol {}

let array: [Protocol] = [
	Class1(),
	Class2()
]

// Generic parameter ’T’ cannot be bound to non-@objc protocol type ‘Protocol’.
let a: A<Protocol> = A(a: array)

// Unfortunately `T` can only be a class type, not a protocol.
// But `Class1` and `Class2` are only related by a protocol.