Status: refused by design
Object?.is is not FunctionalScript. Object?.is is valid JavaScript, and fjs compile refuses it by design: the demo below shows what it prints.
Object is a namespace, not a value: the language reads Object.is and Object.entries as the static functions they name. A guard asks whether a value is null or undefined, and a namespace is never either, so Object?.is and Object?.["is"] are refused at the word. Guarding the function is a call of a value, and stays: Object.is?.(a, b) (static functions).
The row acc.guarded-namespace of refused constructs registers the refusal, and built-in static functions gives its reason.
export default Object?.is(1, 1);
The demo on this page compiles sample.mjs, the module above, with this build's fjs compile, and shows the line it prints and the token it refuses. Below that it shows what JavaScript answers for the same module: true.
Read the static function without the guard, as alternative.f.js does:
export default Object.is(1, 1);
It answers true.
Today fjs compile prints reserved word at the refused token. The demo proposes what it should print instead: the reason at the same token, what JavaScript does there, and the allowed spelling, the line alternative.f.js writes, as help. The page compiles the alternative too, so the suggestion is one the compiler accepts.