Type-level laws: "function arguments should never intersect"

never-intersect.ts

export type If<T, U, True, False> = [T] extends [U] ? True : False;
export type IfDef<T, Yep, Nope> = If<T, never, Nope, Yep>;
export type Intersect<A extends {}, B extends {}> =
  Pick<A, Exclude<keyof A, Exclude<keyof A, keyof B>>> extends { [x: string]: never } ?
  never :
  Pick<A, Exclude<keyof A, Exclude<keyof A, keyof B>>>;

const f = <
  A extends {},
  B extends {},
  // Typelevel law: A and B should not intersect
  NeverIntersect = IfDef<Intersect<A, B>, never, {}>
>(a: A & NeverIntersect, b: B & NeverIntersect): A & B & NeverIntersect => ({ ...a, ...b });

interface IA {
  foo: string;
}

interface IB {
  bar: number;
}

const a1: IA = { foo: 'foo' };
const b1: IB = { bar: 42 };

const c1 = f<IA, IB>(a1, b1); // c1 :: IA & IB
const c2 = f<IA, IA>(a1, a1); // Argument of type 'IA' is not assignable to parameter of type 'never'