How to create generic functions within a non-generic struct

Using Swift
7

Was trying to make a simple example, bad choice. Here is a real world example without any generics - I keep think I should be able to make it some or mostly generic:

typealias XClosure = ((NSArray?) -> Void)
typealias YClosure = ((Any?, Error?) -> Void)

struct Str {
func callAsFunction(val: @escaping XClosure) -> XClosure {
return wrapper(val)
}
func callAsFunction(val: @escaping YClosure) -> YClosure {
return wrapper(val)
}

private func wrapper(_ originalHandler: @escaping XClosure) -> XClosure {
    let newHandler: XClosure = { a in
        DispatchQueue.main.async {
            originalHandler(a)
        }
    }
    return newHandler
}
private func wrapper(_ originalHandler: @escaping YClosure) -> YClosure {
    let newHandler: YClosure = { a, b in
        DispatchQueue.main.async {
            originalHandler(a, b)
        }
    }
    return newHandler
}

}