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type Freezable = {
unfreezable: never;
type Frozen<T extends Freezable> = T & {
frozen: true;
function assertFreezable(thing: unknown): asserts thing is Freezable {
if (!thing || (thing as Freezable).unfreezable) {
throw new Error(`Can't freeze that! ❄️`);
function freeze<T extends Freezable>(toFreeze: T): Frozen<T> {
(toFreeze as Frozen<T>).frozen = true;
return toFreeze as Frozen<T>;
const thing = {};
const frozenThing = freeze(thing); //Argument of type '{}' is not assignable to parameter of type 'Freezable'.
//Property 'unfreezable' is missing in type '{}' but required in type 'Freezable'.ts(2345)
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