The following leads to an error with this.#map:

type MapType<K, V> = K extends object ? (Map<K, V> | WeakMap<K,V>) : Map<K,V>

export function getOrSet<K, V> (map: MapType<K, V>, key: K, create: (k: K) => V): V {
    return create(key)
}

export class Select<T extends object>  {
  #map = new WeakMap<T, any>()
    observable (item: T) { return getOrSet(this.#map, item, () => 'any') }
}

But this is incorrect because T extends object and the error goes away when we change it to WeakMap<object, any>.

Seems like a TS bug or I’m missing something.

Is there any fix/workaround here?

Reproduction on TS Playground