Swift: Extension on [?] to produce [?] possible?

Question

In Swift, I have a custom struct with this basic premise:

A wrapper struct that can contain any type that conforms to BinaryInteger suc

Answer 1

Martin R's answer is a good solution. An alternative that doesn't require an extra marker protocol is this: write an unconstrained extension on Collection, and in that extension, define a generic function that's constrained to where Element == SomeType?:

extension Collection {
    func values() -> [T?] where Element == SomeType? {
        return map( { $0?.value })
    }
}

This works:

let arr: [SomeType?] = [SomeType(value: 123), SomeType(value: 456)]
arr.values() // [Optional(123), Optional(456)]

You'll notice that I used a func instead of a computed property. I couldn't get the generic syntax right. Isn't this supposed to work?

extension Collection {
    // error: consecutive declarations on a line must be separated by ';'
    var values: [T?] where Element == SomeType? {
        return self.map( { $0?.value })
    }
}