Generic arguments gets casted as "unknown" when used as function argument

Typescript version : 5.0.3 (But found on all versions I tested)

Search terms : generic type as function argument unknown

⏯ Playground Link

Playground link with relevant code

💻 Code

Here are the two main generics involved in my case :

type FormSchema<
  StoreKey extends string,
  StoreData extends Record<string, unknown>
> = {
  store: { key: StoreKey; value: () => unknown }[];
  fields: Array<{
    key: string;
    data: StoreData;
    dataWithoutArg: () => StoreData;
    dataWithArg: (params: StoreData) => StoreData;
  }>;
};

type InferSharedStoreData<Store extends { key: string; value: () => unknown }[]> = {
  [K in Store[number] as K["key"]]: Awaited<ReturnType<K["value"]>>;
}

And here's the actual function that casts these :

function buildSchema<
  TSchema extends FormSchema<StoreKey, StoreData>,
  StoreKey extends string,
  StoreData extends Record<string, unknown> = InferSharedStoreData<
    TSchema["store"]
  >
>(schema: TSchema) {
  return schema as TSchema
}

🙁 Actual behavior

The "params" argument of the "dataWithArg" sub-property of "fields" is being typed as InferSharedStoreData, which means the "store" property type of the schema passed as argument of buildSchema is unavailable in the context of the function argument.

the StoreData type is being properly inferred for every other property it's assigned to, except on the case where it is used as a function argument.

🙂 Expected behavior

The function argument "params" should be inferred as StoreData, which itself is InferSharedStoreData<schema['store']>. A complete example is set-up on the playground link above.

リンクされている TypeScript Playground から始め、buildSchema、FormSchema、InferSharedStoreData 周辺の挙動を再現します。dataWithArg の params 型が InferSharedStoreData になる理由を追跡します。params がスキーマの store から導出された StoreData として推論され、他のプロパティは引き続き型チェックされれば完了です。