I have the following

export type WithRequired<T, K extends keyof T> = T & { [P in K]-?: T[P] };

type A = {
  f1?: string;
};

function test(a: A): WithRequired<A, 'f1'> {
  if (a.f1) {
    return a;
  } else {
    throw Error();
  }
}

however, this gives an error

Type 'A' is not assignable to type 'WithRequired<A, "f1">'.
  Type 'A' is not assignable to type '{ f1: string; }'.
    Types of property 'f1' are incompatible.
      Type 'string | undefined' is not assignable to type 'string'.
        Type 'undefined' is not assignable to type 'string'.

I guess I would have expected this to work, since in the context of this branch, the compiler knows that a.f1 is not undefined….and in some contexts, typescript will do this correctly.

I guess I have two questions

1. why doesn’t this work? (to understand the compiler better)
2. is there a way to make something like this work without a cast?