[swift-users] Unexpected Optional unwrapping in generic fn

[Michael Gardner via swift-users]

I'm having trouble understanding why the following code is trying to unwrap an 
Optional.

////////

class Proxy<T> {
        var value:T
        init(_ value:T) {
                self.value = value
        }
}

func setter<T>(p:Proxy<T>) -> Any? -> () {
        return {p.value = $0 as! T}
}

var i = Proxy<Int?>(0), s = setter(i)

s(2)
i.value // => 2
s(nil) // => fatal error: unexpectedly found nil while unwrapping an Optional 
value

////////

My understanding is that since T is `Int?` here, the setter should be doing 
(nil as! Int?), which doesn't cause an error. What am I missing?
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