type Exact<A, B extends A> = A extends B
  ? B
  : B extends Record<any, any>
    ? { [K in keyof B]: K extends keyof A ? Exact<A[K], B[K]> : never }
    : B extends Array<any>
      ? { [K in keyof B]: K extends keyof A ? Exact<A[K], B[K]> : never }
      : never;

function invariantize<A, R>(
  fun: (a: A) => R
): <B extends A>(a: Exact<A, B>) => R {
  return fun as any;
}