export interface IFruits {
    tropical: {
        papaya: number;
        pineapple: number;
        guava: number;
    };

    stone: {
        peach: number;
        nectarine: number;
    };

    berry: {
        strawberry: number;
        blueberry: number;
    };
}

/**
 * Aggregating all the second level keys (the fruits) into a single union type.
 */
export type FruitKey = {
    [category in keyof IFruits]: keyof IFruits[category];
}[keyof IFruits];

/**
 * Helper to pick only fruit categories (first level) that are required for the requested fruit keys.
 */
type RequiredFruitCategories<K extends FruitKey> = {
    [category in keyof IFruits]: Extract<K, keyof IFruits[category]> extends never ? never : category;
}[keyof IFruits];


/**
 * Takes a nested slice of the IFruits interface depending on the fruit keys (second level) that
 * we ask for -- essentially a nested `Pick`.  So `FruitsSubset<"papaya" | "peach">` ends up being:
 * {
 *      tropical: {
 *          papaya: number;
 *      };
 *      stone: {
 *          peach: number;
 *      };
 * }
 */
type FruitsSubset<K extends FruitKey> = {
    [category in RequiredFruitCategories<K>]: Pick<
        IFruits[category],
        Extract<K, keyof IFruits[category]>
    >;
};

/**
 * Just wrapping the above in an interface for other interfaces (i.e. React props) to extend.
 */
interface FruitsSubsetOpts<K extends FruitKey> {
    fruits: FruitsSubset<K>;
}

type MyFavoriteFruitsSubset = FruitsSubset<"papaya" | "peach">;
// Inferring the key from above -- this ends up being `"papaya" | "peach"` on TS 4.1.5 but `never` on 4.2.3
type MyFavoriteFruitsSubsetKey = MyFavoriteFruitsSubset extends FruitsSubset<infer K> ? K : never;

type MyFavoriteFruitsSubsetOpts = FruitsSubsetOpts<"papaya" | "peach">;
// Inferring the key from above -- this ends up being `"papaya" | "peach"` on both TS 4.1.5 and 4.2.3
// Pretty confusing because FruitsSubsetOpts is just a wrapper around FruitsSubset!
type MyFavoriteFruitsSubsetOptsKey = MyFavoriteFruitsSubsetOpts extends FruitsSubsetOpts<infer K> ? K : never;