Why does this work if I test for the class, and not for the protocol?

ProtocolPlayground.swift

//: Playground - noun: a place where people can play

import Foundation

protocol MyProtocol {
    var value: Int! { get set }
}

class MyClass : MyProtocol {
    var value : Int!
}

var foo : Any?

if let conformingProtocol = foo as? MyProtocol {
//  Playground execution failed: error: MyPlayground.playground:17:30: error: cannot assign to property: 'conformingProtocol' is a 'let' constant
    conformingProtocol.value = 5
//  ~~~~~~~~~~~~~~~~~~       ^
}

if let conformingClass = foo as? MyClass {
    conformingClass.value = 42
}

Solution is simple: The protocol could be implemented with a struct, and a struct property will not be mutable.

Therefore.

protocol MyProtocol : class