Overrides whose parameters widen and whose results convert into the
inherited ones keep the inherited slot, which converts around the
override's own body; Error values take code writes, `in` tests and
Object.assign; and a few gaps behind generic wrappers close:
- Method overrides: each inherited argument converts into the
override's parameter (equal, wider, or a checked extraction from an
`any` slot) and the override's result converts into the inherited one
(narrower, a convertible union, a widening Promise payload, anything
convertible over an `any` base). The slot %C.m keeps the inherited
ABI and forwards to %C.m$own; TypeScript callers typed with the
subclass call $own directly when nothing overrides below. Async
overrides of sync and async bases, and JavaScript overrides of
builtin or TypeScript bases (zod's ZodError.toString) are included;
a toString slot converts its result with ToString.
- `err.code = value` on Error values in TypeScript: strings set the
runtime code slot (and the error's checked-dynamic node, if any); other
values clear it and keep the dynamic own property.
- `'key' in err` with any receiver: declared members answer true;
otherwise a class test over the declaring subclasses, a runtime lookup
of the error's own properties, or an Object.prototype member.
- `Object.assign(err, ...sources)` onto an Error target copies onto the
error, re-reads name, message, stack, cause and code into the typed
slots, and returns the error.
- `Error & { ... }` intersections whose extra members have checkable
types are Error values; those members read and write the error's own
properties, each read checked at its declared type.
- Catch bindings typed `any` read as checked-dynamic values (member
reads and writes, assertions); a multi-statement type guard with an
`unknown` parameter takes the caught value as an ordinary argument;
Error members of a binding narrowed to an Error-extending interface
read through the same value.
- Lambda parameters typed `any` by their default are checked-dynamic
slots; forwarded-only rest tuples with optional positions forward
their slots; `value as never` from an untyped value into a typed
parameter takes the checked extraction to the parameter's type.
- Lexical `this` stops at generic instance frames, so an instance no
longer sees the receiver of the caller that triggered it. The fence
for `this` captured through a plain function now points at the
`this` keyword.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
