"Infinite" loop with ForwardIndexType and `distanceTo`

ForwardIndexType.swift

struct AIndex: ForwardIndexType {
    let index: Int

    func successor() -> AIndex {
        return AIndex(index: self.index + 1)
    }
}

func == (lhs: AIndex, rhs: AIndex) -> Bool {
    return lhs.index == rhs.index
}

// Sit and wait...
AIndex(index: 999).distanceTo(AIndex(index: 1))