Strange generic? autoclosure?

autclosure.swift

struct Err: Error {}
open class ClosureContainer<Closure, Value> {
    var arg: Closure?
}

func unwrap<T>(_ expression: @autoclosure () throws -> T?) throws -> T {
    guard let value = try expression() else { throw Err() }
    return value
}

let handler = ClosureContainer<(String) -> Void, Void>()

handler.arg = { v in print("CALLBACK : \(v)") }

// NOT WORKING! // crash
 let callback = try unwrap(handler.arg)

// WORKING // no crash
// let callback = handler.arg!

print(callback("123"))