duplicates:

• SR-4206 Override checking does not properly enforce requirements

Issue Description:

In Swift 5, when we override a method that takes in a generic constraint that adds on another generic constraint, when we call the methods from subclass objects with superclass type. An example snippet is provided below to show this behaviour.

I was able to reproduce this with the latest Swift 5 beta from Xcode 10.2 beta 4. This behaviour does not happen in Swift 4.

class A {}
class SubA<T>: A {}

class B {
  func test<E: A>(_: E) {}
}
class SubB<T>: B {
  override func test<E>(_: E) where E: SubA<T> {
    print(T.self)
  }
}

let subA = SubA<Int>(), subB = SubB<Int>()
let a: A = subA, b: B = subB

subB.test(subA) // works, prints Int
b.test(subA) // works, prints Int
b.test(a) // fails, prints Swift._SwiftObject (instead of Int)