This source break is severable and if it's not worth it, we can drop it from the proposal.
However, as @xwu alluded to, this is not as drastic in practice as it seems. As proposed:
let x = Int("7") // Optional(7)
let y = Float("2") // Optional(2.0)
let z = Int(ascii: "z", radix: 36) // Optional(35)