Issue Description:

My idea is in a base class's static method to create an instance of a subtype and then pass it as a parameter of the created type to a provided closure:

class Foo {

{{ required init() {}}}

{{ class func createSubtype(initialize: (Self) -> Void) -> Self {}}

{{ let new = Self()}}

{{ initialize(new)}}

{{ return new}}

{{}}}

class Bar: Foo {

{{ var v: Int?}}

}

let bar = Bar.createSubtype() { new in

{{ new.v = 1}}

{{}}}

Doing that the Swift compiler yields an error Value of type 'Self' has no member 'v'. But 'print(type(of: o))' in the closure gives 'Bar'. According to [Expanding Swift Self to class members and value types|https://github.com/apple/swift-evolution/blob/master/proposals/0068-universal-self.md] implemented in Swift 5.1 using 'Self' should be appropriate here.