This is an old, old issue, and it's not due to migration but due to inheritance. `<T>keys(o: T) => keyof T` is actually quite incorrect, because you can't guar

This is an old, old issue, and it's not due to migration but due to inheritance. `<T>keys(o: T) => keyof T` is actually quite incorrect, because you can't guarentee that keys() doesn't return a value not in `keyof T`, a key could be from a subclass of T. I think TypeScript is doing the right thing here in not claiming something that's very easily violated.