sigill.swift

import Foundation 

protocol P {
  typealias PT: NSObject = Self
  static func vendSpecific() -> PT?
}

extension P where Self: NSObject, PT == Self {
  static func vendSpecific() -> PT? {
    return nil
  }
}

class A: NSObject, P {
}

class B: A {
  typealias PT = B
}

func compilerSIGILL() {
  B.vendSpecific()
}

Without the typealias in Class B, you can't call B.vendSpecific() b/c B doesn't conform to the P extension's constraints.

With the typealias, the compiler SIGILLs because SIL verification failed.