Somewhat surprising that TypeScript doesn't flag this as an error. (And yes, `strictFunctionChecks` is enabled.)

I wonder why?

Playground link: typescriptlang.org/play?noImplici…

I'm guessing it's because the type of `this` can only be observed from inside the function. So it's not unsafe to allow the use of an arrow function here, and it would overly restrictive to disallow it.