Why Swift requires override of designated initializer of generic superclass?

Question

According to Apple's documentation Swift does not necessary require override of initializer. In a following code example Bar inherits initializer of Foo:

class Foo {
  let value: Int
  init(value: Int = 5) {
    self.value = value
  }
}

class Bar: Foo {
}

As soon as we add some generic into Foo such as class Foo<T> { Xcode provides us a error Initializer does not override a designated initializer from its superclass. Is there a documentation or swift evolution discussion that explains why it is happening?

---

Update. It seems that generic is not a major cause for override requirement. Here are an option how to define a class with generic that does not require override of designated initializer:

protocol FooProtocol {
    associatedtype T
}

class Foo<U>: FooProtocol {
    typealias T = U

    let value: Int
    init(value: Int, otherValue: T) {
        self.value = value
        self.otherValue = otherValue
    }
}

class Bar: Foo<Int> {
}

However there is another interesting observation of behavior. Defining initializer like following cause override requirement:

init(value: Int = 5) {
    self.value = value
}

The funny thing thing that adding one more parameter as following into such designated initializer cause this override requirement to disappear:

init(value: Int = 5, otherValue: T) {
    self.value = value
}

Update 2. I can not find a logical explanation to this behavior, at this point I reported it as Compiler bug — https://bugs.swift.org/browse/SR-1375

Answer 1

I actually filled a bug report for inheriting from generic class:

It was back in November last year and didn't get an answer yet, so ¯_(ツ)_/¯