Self Referential Limitation or Bug

SelfReferential.swift

final class Foo {
    //    static let _sharedInstance = self.init() // Why not work?
    static let _sharedInstance = Foo()
    static func sharedInstance() -> Self {
        return _sharedInstance // Why 'Foo' is not convertible to 'Self'? It is `static` so `Self` can only be `Foo`. In fact since the class is final there won't be any subclasses at all.
        //        return self.init() // Why this works?
    }
    
    required init() {
        
    }
}