SomeBC.swift

protocol A {}

protocol B {
	typealias AType
	func setAn(a: AType)
}

struct SomeB: B {
	typealias AType = A
	func setAn(a: AType)
	{
	}
}

struct SomeC {
	func doWith(a: A){
		let b = SomeB()
		b.setAn(a) // no error, here
	}
}