keyof on generics arbitrarily returns `string` rather than the inferred values depending on the generic

Bug Report

🔎 Search Terms

keyof generic string

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about it

⏯ Playground Link

Playground link with relevant code

💻 Code

export type Item = {
  args?: Record<string, string>;
};

export type BaseSpec = Record<string, {
  items: Item[];
}>;

export function genericFunction<Spec extends BaseSpec>(spec: Spec): keyof Spec {
  return {} as any;
}

const AAAAA: `someKey` = genericFunction({
  someKey: {
    items: [{
      args: {},
    }, {
      args: {
        name: `foo`,
      },
    }],
  },
});

🙁 Actual behavior

Type 'string' is not assignable to type '"someKey"'.

🙂 Expected behavior

No errors

💻 Additional Information

Weirdly, it seems to work if all args objects that are specified have exactly the same content. Removing an arg object seems to work, and it works as well if we add name: "foo" into the first arg object (or remove it from the second one).

リンク先の TypeScript Playground を開き、genericFunction、BaseSpec、および異なる args オブジェクトを使ってエラーを再現してください。このジェネリック推論のケースで keyof Spec が string に拡大される理由を調査し、その後、例が型チェックを通過し、推論された結果が "someKey" になることを確認してください。