OneOf.swift

// we generate the boundary with `A | B` or directly OneOf<A, B>
enum OneOf<...T> {

    ...case $#(T)

    // Bikeshedding variadic enum casses:
    // we might need an index to set the value?
    init(index: Int, value: T) {

        self = .$index(value)
    }
}

/// Usage:
/// A | B | C == OneOf<A, B, C>
func |<T, U>(_: T.Type, _: U.Type) -> OneOf<T, U>.Type {

    // I also use the proposal to remove `.self` magic here
    return OneOf<T, U>
}

// Here is how to merge OneOf into a single dimension
func |<...T, U>(_: OneOf<...T>.Type, _: U.Type) -> OneOf<...T, U>.Type {

    // Copy and merge types into the new `OneOf` type
    return OneOf<...T, U>
}

func |<T, ...U>(_: T.Type, _: OneOf<...U>.Type) -> OneOf<T, ...U>.Type {

    // Copy and merge types into the new `OneOf` type
    return OneOf<T, ...U>
}

func |<...T, ...U>(_: OneOf<...T>.Type, _: OneOf<...U>.Type) -> OneOf<...T, ...U>.Type {

    // Copy and merge types into the new `OneOf` type
    return OneOf<...T, ...U>
}