this is fun:

const f: Foo = {}
f.x = 'hello';
(() => { console.log(f.x.trim()) })(); // works
const g = () => { console.log(f.x.trim()) }; // breaks

I know that TS special-cases IIFEs so that probably isn't surprising, but I only know that from hints and rumors around here. Probably in some github issue, but still. Your example sruprises me too, I've never noticed that.

yeah in general CFA doesn't cross function boundaries,

bro you always be saying what I'm saying.