map.ts

interface Mapping<K, V> {
  _k: K;
  _v: V;
}

type P<K, M extends Mapping<K, any>> = M extends Mapping<K, infer V>
  ? V
  : never;

interface XMap<T extends Mapping<any, any> = Mapping<never, undefined>> {
  set<K, V>(key: K, val: V): XMap<T | Mapping<K, V>>;
  get<K>(key: K): P<K, T> | undefined;
}

declare const map: XMap;
const a = map.set("foo", 123);
const b = a.set(456, true);
const num = b.get("foo");
const bool = b.get(456);