How to list Swift types that conform to protocol using reflection?

How does one list types that conform to a protocol for purely Swift based types (i.e. no @objc annotation involved)? I was hoping that a reflection API that allows for this would be provided in the Swift standard library.

Just to be doubly sure, I am aware of the more specialised case of Objective-C or @objc annotated Swift classes conforming to a protocol solvable with Objective-C runtime APIs: How to list all classes conforming to protocol in Swift? – what I am after is the same for an arbitrary Swift type that may be a struct, enum or a class.

Here is my feeble failed attempt at using the Mirror API for the purpose:

protocol Derpable {
    func derp();
}

extension Derpable {
    func derp() {
        print("Herp derp.")
    }
}

enum E: Derpable { }

class C: Derpable { }

struct S: Derpable { }

print(Mirror(reflecting: Derpable.self).children.count) // prints "0\n"