Cannot use keyof on one type to index fields in another type with the same keys, but different types for the values

[dbinnersley]

Bug Report

🔎 Search Terms

keyof types

🕗 Version & Regression Information

4.2.3 when running on my own machine
4.5.2 when running on ts playground

⏯ Playground Link

Playground link with relevant code

💻 Code

export type KeysOfType<T, U, B = false> = {
  [P in keyof T]: B extends true
    ? T[P] extends U
      ? U extends T[P]
        ? P
        : never
      : never
    : T[P] extends U
    ? P
    : never;
}[keyof T];

export type BigIntKeys<T> = KeysOfType<T, bigint, true>

export type CompatType<T> = Omit<T, BigIntKeys<T>> &
  {
    [Property in BigIntKeys<T>]: string;
  };

export function compatModel<T>(model: T): CompatType<T> {
  const compat: Partial<CompatType<T>> = {};
  for (const k of Object.keys(model) as Array<keyof T>) {
    const v = model[k];
    compat[k] = typeof v === "bigint" ? v.toString() : v;
  }
  return compat as CompatType<T>;
};

🙁 Actual behavior

The final compat[k] shows a type error indicating Type 'keyof T' cannot be used to index type 'Partial<CompatType<T>>

🙂 Expected behavior

The keys from T should be allowed to index CompatType<T> since the keys contained in both of the types are identical. The only difference is the values which have changed from bigints to strings.

[jcalz]

Duplicate of #28884.

[RyanCavanaugh]

Nice find @jcalz