CastCrash.swift

protocol P1 {
    associatedtype X
}
protocol P2 {}

struct Box<T1, T2: P1> {
    struct X {}
}

extension Box.X: P2 where T2.X == String {}

extension String: P1 {
    typealias X = String
}
_ = Box<Int, String>.X.self is P2.Type