A class constructor's `return expr` was evaluated and dropped, so
`new C()` answered the constructed instance even where Node answers the
returned object (`if (C.instance) return C.instance;` built a second
instance).
A return that yields an instance of the class itself stores it in a
per-class result slot, read and cleared by one helper at every
construction site (direct new, class values, the checked dynamic
construction dispatchers, record-to-class coercion). A return of any
other object refuses with SC1090 (compile time in TypeScript, a
deferred fence in JavaScript); a JavaScript value typed only at run time
is tested when it returns. super() into a class that can return an
existing instance refuses, since Node would rebind the subclass's this.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
