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).

打开链接的 TypeScript Playground，使用 genericFunction、BaseSpec 和不同的 args 对象重现该错误。调查为什么在这种泛型推断情况下 keyof Spec 会被扩大为 string，然后验证示例能够通过类型检查，并且推断出的结果是 "someKey"。