Type Inference Fails with Generic Types

🔎 Search Terms

error recursive generic types

🕗 Version & Regression Information

• This changed between versions v5.7.0-beta and v4.1.5

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.3#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-62flSiQXQJXQNqUAAO40NEdCCHAPB0J4dgUSquqYoacrdZQvU6iq-U9SwOqUIZxlzGZmBxYa4aJbQwApZBUDEbMZg8KJ4nQFwsl5B1nHmJh2EJDg1klOQBFhV4PDdEdnWqmNI2CgNQ26lNcgmeSukXUFpALXYwPHe9fXHCMUxaEwABSAQ1NtwBiVBwClfZj5FPZjnMFArnwu5nlBKEPC+RwB4iGA77uiF-waO5Zj4nUPgKjwNO0Y4M7ZPongiGzt0DVFMUABIQHAdAsJ9TPoCzEuUILrPYHAjVA8DFFFCLYtmJq2sU1TOBK9EDNSyzpAlEdTBFCIZv6TCrrAAyMAgAACq1wDmdgZguzwbt8wC21i0k0Sfu5pTc3YXs+zdAJFPLUAa3QWva7HiuNWbIMUfHvsaFjTmPeRUB2w7zuu+76ui3QZS9ioinnSnhsc1krih-FIPHZnUcaEs2pLHnT3UbTUAMRlkVENFrNxb3qsF26RcR+Zz5mD21dCPIddmCHV0t63wP+BAUk8JPu-7+bzBk7bbo4J77TFR3DduGH0-2z8jtz5fUC63kmAu6jZj3qGrI23REeJ02A8JZ0giAUo+digAAZgqFDPkA-MXQTygMsnYIoTtbJQC+ppMks1QElDInmR0qYXQXw9lAJ2YZ2BAA

💻 Code

type BaseItem<T = any> = { id: T }

interface Snapshot<T extends BaseItem<I> = BaseItem, I = any> {
  id: string,
  time: number,
  collectionName: string,
  items: T[],
}

const selector: FlatQuery<Snapshot<BaseItem>> = { collectionName: 'asdf' }
// ^^
// No error ✅

function test<ItemType extends BaseItem<IdType>, IdType = any>() {
  type ItemKeys = DotNotationKeys<Snapshot<ItemType, IdType>>
  // ^^
  // "id" | "time" | "collectionName" | "items" | `items.${string}`
  
  type CollectionNameType = Flatten<Get<Snapshot<ItemType, IdType>, 'collectionName'>>
  // ^^
  // "string"

  const genericSelector: FlatQuery<Snapshot<ItemType, IdType>> = { collectionName: 'asdf' }
  // ^^
  // Error ❌: Type '{ collectionName: string; }' is not assignable to type 'FlatQuery<Snapshot<ItemType, IdType>>'.
}

// Utility type to check if a type is an array or object.
type IsObject<T> = T extends object ? (T extends Array<any> ? false : true) : false

// Recursive type to generate dot-notation keys
type DotNotationKeys<T> = {
  [K in keyof T & (string | number)]:
  T[K] extends Array<infer U>
  // If it's an array, include both the index and the $ wildcard
    ? `${K}` | `${K}.$` | `${K}.${DotNotationKeys<U>}`
  // If it's an object, recurse into it
    : IsObject<T[K]> extends true
      ? `${K}` | `${K}.${DotNotationKeys<T[K]>}`
      : `${K}` // Base case: Just return the key
}[keyof T & (string | number)]

type Split<S extends string, Delimiter extends string> =
  S extends `${infer Head}${Delimiter}${infer Tail}`
    ? [Head, ...Split<Tail, Delimiter>]
    : [S]

type GetTypeByParts<T, Parts extends readonly string[]> =
  Parts extends [infer Head, ...infer Tail]
    ? Head extends keyof T
      ? GetTypeByParts<T[Head], Extract<Tail, string[]>>
      : Head extends '$'
        ? T extends Array<infer U>
          ? GetTypeByParts<U, Extract<Tail, string[]>>
          : never
        : never
    : T

type Get<T, Path extends string> =
  GetTypeByParts<T, Split<Path, '.'>>

type Flatten<T> = T extends any[] ? T[0] : T

type FlatQuery<T> = {
  [P in DotNotationKeys<T>]?: Flatten<Get<T, P>>
}

🙁 Actual behavior

Typescript isn't able to assign { collectionName: 'asdf' } when using generic types:

function test<ItemType extends BaseItem<IdType>, IdType = any>() {
  const selector: FlatQuery<Snapshot<ItemType, IdType>> = { collectionName: 'asdf' }
  // ^^
  // Type '{ collectionName: string; }' is not assignable to type 'FlatQuery<Snapshot<ItemType, IdType>>'.
}

Without generic types it work seamlessly:

const selector: FlatQuery<Snapshot<BaseItem>> = { collectionName: 'asdf' }

🙂 Expected behavior

{ collectionName: 'asdf' } should be successfully assignable to FlatQuery<Snapshot<ItemType, IdType>> using generic types.

Additional information about the issue

Related issue: https://github.com/maxnowack/signaldb/pull/1030

リンクされている TypeScript Playground から始め、BaseItem、Snapshot、Get、FlatQuery を含む再帰的なジェネリックの例を縮小し、ジェネリックではない代入と比較してください。ジェネリックな { collectionName: 'asdf' } の代入が、報告されている期待される動作を弱めることなく受け入れられれば完了です。