Strange Asymmetry: Bug or Feature?

I just found an expression, call it “β”, and a type T, such that:

let x = β as T

is legal, but

let x: T = β

is a error. The asymmetry seems pretty unusual to me—can you think of any such instances?—and I was wondering if it was intentional.

What are `T` and β?
typealias T = AnyObject

β := U.self where U is any type.



import Foundation

let x = NSString()
let y = x as String
let z: String = x // error: 'NSString' is not implicitly convertible to 'String'; did you mean to use 'as' to explicitly convert?

and also the other way around

import Foundation

let x = String()
let y = x as NSString
let z: NSString = x // Cannot convert value of type 'String' to specified type 'NSString'