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).

Open the linked TypeScript Playground and reproduce the error with genericFunction, BaseSpec, and the differing args objects. Investigate why keyof Spec is widened to string in this generic inference case, then verify that the example type-checks and that the inferred result is "someKey".