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.

Inizia con il TypeScript Playground collegato e riproduci il comportamento relativo a buildSchema, FormSchema e InferSharedStoreData. Traccia il motivo per cui il tipo del parametro di dataWithArg diventa InferSharedStoreData; il lavoro è completato quando params viene inferito come lo StoreData derivato dallo store dello schema, mentre le altre proprietà continuano a essere verificate dal sistema dei tipi.