Errors

Optional (?T) and failable (T!) types are working declared types. null coerces to ?T, a plain T coerces to ?T, and the postfix ? operator propagates an optional operand inside a function that itself returns an optional.

fn halve(value: ?i32): ?i32 {
    let inner: i32 = value?;
    inner / 2
}

The recovery vocabulary around those types is not implemented. fail, with, catch, must, and throw remain specification-only. Keep examples to optionals and explicit branching until error values land.

as is a real cast that lowers to an LLVM conversion, covering numeric pairs and raw opaque <-> *T; other conversions are rejected and there is no narrowing overflow check. is supports only is null, which requires an optional operand; any other operand reports a dedicated diagnostic. Flow-sensitive narrowing after an is null check does not exist yet, so E3005 is reported where non-null proof is required.

The planned rules are in the Error Handling reference; the current boundary is in Implementation Status.