ExactType

exact.ts

type Args = {
  id?: string | undefined | null;
  firstName?: string | undefined | null;
  parentId?: string | undefined | null;
  different?: string | undefined | null;
};

type Opts = {
  id?: string | undefined | null;
  firstName?: string | undefined | null;
  parent?: string | undefined | null;
};

const a: Args = null!;
// I want parentId and different to be type failures
const o: Opts = a;

function doOpts(o: Opts): void {}

function onlyOpts<T extends Opts>(o: Exact<Opts, T>): void {}
function onlyOpts2<O>(o: Exact<Opts, O>): void {
  doOpts(o);
}

onlyOpts(a);
onlyOpts2(a);
onlyOpts({ firstName: "as" });
onlyOpts({ parent: "as" });

type Impossible<K extends keyof any> = {
  [P in K]: never;
};

// T is the exact type, U is the user's passed type,
type Exact<T, U> = T & U & Impossible<Exclude<keyof U, keyof T>>;