The override is invalid - an overridden method is not allowed to have generic requirements that are not imposed by the base method. Previously, we used to crash however this is now resolved in Swift 5.2. Your example diagnoses as expected and does not crash anymore:

error: overridden declaration is here
    class func work<Input>(input: Input) {
               ^
error: overridden method 'work' has generic signature <Input where Input : TitleProvider> which is incompatible with base method's generic signature <Input>; expected generic signature to be <Input>

Comment by Alexander Orlov (JIRA)

First, yes, not compiling that code would be way better than crashing in runtime. But I used more specific constraints than base method's. Moreover, if you remove `doWork()` method from the code and will use `work` directly — it will work even now, as expected and with expected result.

Maybe you could consider to allow us to use more specific constrains in overridden methods, because we have that ability with generic classes already 🙂. Thus, now I will need to create separate generic class which actually allows me to do what I wanted and works perfectly.

P.S. Sorry for duplicating, hadn't found any others by the moment I created my own.

The problem is that you're violating Liskov:

protocol Proto {
  var value: Int { get }
}

class Base {
  func foo<T>(arg: T) {}
}


class Derived: Base {
  override func foo<T: Proto>(arg: T) {
    print(arg.value)
  }
}

class Test {}

let base: Base = Derived()
base.foo(arg: Test()) // Uh oh

Comment by Alexander Orlov (JIRA)

I understand that. I am worrying about generic class inheritance and their constraints. I mean, what I should expect about them in future versions?

@swift-ci create