type SceneSessionData = {
    current?: string
    expires?: number
    state?: {
        lastMessageId?: number
    }
}

type MyContext = Context & {
    session: {
        __scenes?: SceneSessionData
    }
    scene: Scenes.SceneContextScene<MyContext, SceneSessionData>
}

    get state() {
        return (this.session.state ??= {})
    }

    set state(value) {
        this.session.state = { ...value }
    }

export default class SceneContextScene<C extends SessionContext<SceneSession<D>>, D extends SceneSessionData = SceneSessionData> {
	// ...
    get session(): D;
    get state(): object;
    set state(value: object);
    // ...
}

export default class SceneContextScene<C extends SessionContext<SceneSession<D>>, D extends SceneSessionData = SceneSessionData> {
    // ...
    get session(): D;
    get state(): D["state"];
    set state(value: D["state"]);
    // ...
}