macOS playground playing with opaque types & protocols.

OpaqueTypesPlayground.swift

// paste into new macOS playground
// Xcode 11.3 on 10.15.2

import Cocoa

protocol InfoTypeProtocol {}

protocol Base {
	associatedtype InfoType: InfoTypeProtocol
	var info: InfoType { get set }
}

struct A<T: InfoTypeProtocol>: Base {
	typealias InfoType = T
	
	var info: InfoType
	var diff: String
}
struct B<T: InfoTypeProtocol>: Base {
	typealias InfoType = T
	var info: InfoType
	var other: Int
}

extension String: InfoTypeProtocol {}
extension Int: InfoTypeProtocol {}

func test<T:Base>(_ val: T) {
	switch val.info {
	case is String:
		print("string")
	case is Int:
		print("int")
	default:
		print("unknown")
	}
}

let a = A<String>(info: "hello", diff: "a-string")
let ai = A<Int>(info: 42, diff: "42 info")


test(a)     // "string"
test(ai)    // "int"

// this work also:

// function returns the protocol defined associated type field using opaque types.
// enough type information for switch statement below
// (without the `some` keyword this doesn't work)

func test2<T: Base>(_ val: T) -> some InfoTypeProtocol {
	return val.info
}

let z = test2(ai)

switch z {
case is String:
	print("string!", z)
case is Int:
	print("Int!", z)
default:
	print("unknown")
}

// prints: "Int! 42"