relates to:

• SR-6279 Failing cast from Any to Optional

Issue Description:

Description

Consider a concrete type T extending a protocol P (class T : protocol P)
Cast an instance of T? (optional) to Any, and then downcast it :

• it seems valid to downcast to T (i.e : it works)

• on the contrary, downcasting to P yields a runtime exception :

  Could not cast value of type 'Swift.Optional<T>' to 'P'

Is there any reason why downcasting to a class and downcasting to a protocol implemented by the same class should yield different results ?
I think one should either get an exception in both cases, or no exception at all.

Rmq : this issue is not purely theoretical ; I bumped into it when sending optional values to a Swift pod making heavy use of generics (and heavy use of downcasts...)

Test case

Paste this in the main.swift of an XCode 7.3 OSX Command Line Tool project :

protocol FooProtocol {
    func doSomething()
}

class Foo: FooProtocol {
    func doSomething() {
        print("hello")
    }
}

let optionalFoo : Foo? = Foo()
let anyValue: Any = optionalFoo
    
let foo1 = anyValue as! Foo
foo1.doSomething()                  // prints 'hello', ok

let foo2 = anyValue as! FooProtocol // Could not cast value of type 'Swift.Optional<Foo>' to 'FooProtocol'
foo2.doSomething()