A result type with a literal discriminant, and every branch typed without a single assertion.
type Result =
| { ok: true; data: Order }
| { ok: false; error: string; retryable: boolean }
if (result.ok) {
result.data // Order
result.error // error: does not exist
} else {
result.retryable // boolean
}The discriminant has to be a literal type on every member, and a member missing it silently disables narrowing for the whole union — which produces errors that read as if the union itself is wrong. Using a boolean as the discriminant works and reads worse than a string once there are three or more cases.