> * sum types (would be really nice)

That seems really difficult to retrofit, and to not really fit the language either.

I've been thinking something like sealed interfaces would fit better: Go already has fallible downcasts (and RTTI which takes the role of discriminant), and type switches can play the role of your `case` statement:

    func do[T any](i option[T]) {
        switch v := i.(type) {
        case T:
            fmt.Printf("Got a %v", v)
        case nothing: // or `nil` could be a (pseudo)-type
            fmt.Printf("was empty!")
        // no default necessary because the check is complete
        }
    }

The only pieces missing are a way to "seal in" a set of types, and add completeness check to type switches for those.