cast :: UnsafeMutablePointer<T> -> UnsafeMutablePointer<U> in Swift?

repl.swift

 13> let y : UnsafeMutablePointer<UInt8> = unsafeDowncast(malloc(50))
repl.swift:13:39: error: generic parameter 'T' could not be inferred
let y : UnsafeMutablePointer<UInt8> = unsafeDowncast(malloc(50))
                                      ^
Swift.unsafeDowncast:12:13: note: in call to function 'unsafeDowncast'
public func unsafeDowncast<T : AnyObject>(x: AnyObject) -> T
            ^

 13> let y : UnsafeMutablePointer<UInt8> = unsafeDowncast<UnsafeMutablePointer<UInt8>>(malloc(50))
repl.swift:13:39: error: cannot explicitly specialize a generic function
let y : UnsafeMutablePointer<UInt8> = unsafeDowncast<UnsafeMutablePointer<UInt8>>(malloc(50))
                                      ^
repl.swift:13:53: note: while parsing this '<' as a type parameter bracket
let y : UnsafeMutablePointer<UInt8> = unsafeDowncast<UnsafeMutablePointer<UInt8>>(malloc(50))


 16> let y : UnsafeMutablePointer<UInt8> = malloc(50) as UnsafeMutablePointer<UInt8>
repl.swift:16:39: error: cannot convert value of type 'UnsafeMutablePointer<Void>' (aka 'UnsafeMutablePointer<()>') to type 'UnsafeMutablePointer<UInt8>' in coercion
let y : UnsafeMutablePointer<UInt8> = malloc(50) as UnsafeMutablePointer<UInt8>
                                      ^~~~~~~~~~


 20> let y : UnsafeMutablePointer<UInt8> = malloc(50) as! UnsafeMutablePointer<UInt8>
y: UnsafeMutablePointer<UInt8> = <extracting data from value failed>

Execution interrupted. Enter code to recover and continue.
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