Acceptor / User Pattern for TypeScript

main.ts

interface Acceptor<T extends string> {
	accept(x: UserOf<T>): any;
	allow<T1 extends string>(x: T1): Acceptor<T|T1>;
}

interface UserOf<T extends string> {
	_brand: T;
}

function use<T extends string>(name: T): UserOf<T> { return null!; }
function combine<T1 extends string, T2 extends string>(x: UserOf<T1>, y: UserOf<T2>): UserOf<T1|T2> { return null!; }

const a = use("a");
const b = use("b");
const ab = combine(a, b);

let f: Acceptor<never> = null!;
f.allow("a").allow("b").accept(ab);
f.allow("a").accept(ab);
f.allow("b").accept(ab);