IceDane

  declare const brand: unique symbol;
  type NonEmptyString = string & { readonly [brand]: 'NonEmptyString' };

  // the ONLY non-cast way to produce one
  export function nonEmptyString(s: string): NonEmptyString | undefined {
    return s.length > 0 ? (s as NonEmptyString) : undefined;
  }

  export type { NonEmptyString };

  const m = { name: "weavejester", active: true };

  const mStar = { ...m, number: 3 };
  //    ^? const mStar: { number: number, name: string, active: boolean }

  const x = mStar.number;
  //    ^? const x: number