keyof FilteringMappedType can't compute its constraint

[Andarist]

🔎 Search Terms

keyof index constraint filtering mapped

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-dev.20231027#code/C4TwDgpgBAaghgGwK4QM4B4AqA+KBeKTAbQGsIQB7AM0IF0BuAKEdEigAUAnCgNwEsAJhAEBBAMbAKnfFADejKFFScxALiXBOfAHYBzJooQVdfNVCTaS2igHdtTAL5NGO4BE5U4Y6AGVgcNwBZLwALHQgAYQptKj5dLHFJTlQoCAAPN20BFK5eQWFEqVx5RR0eCjJ1EsUlFXVMQuSiACJlMWaGBSgnRgdmITEEOE5oKgsJPmilCGAkMASJKRT0zOyoACUIMSkBdFRNHV0AGnNLaztsbAAKAH0qrq8k1HuaqCIAaSgdKDJKGgbFslaPVGqgPp1FD0HABKF5QMQjAIQYJiMLaCDqK5dRTbGJxdR+JEotGRaKxeLYmrwZBodDVV6KD5fbQ-cjUQigqBwFKfABkGi0emBckpDNqZneBjFNSMJjMAKe4KlYp6KuwlOwRy60PwuHKgkcTCAA

💻 Code

type Values<T> = T[keyof T];

type ProvidedActor = {
  src: string;
  logic: unknown;
};

interface StateMachineConfig<TActors extends ProvidedActor> {
  invoke: {
    src: TActors["src"];
  };
}

declare function setup<TActors extends Record<string, unknown>>(_: {
  actors: {
    [K in keyof TActors]: TActors[K];
  };
}): {
  createMachine: (
    config: StateMachineConfig<
      Values<{
        [K in keyof TActors as K & string]: {
          src: K;
          logic: TActors[K];
        };
      }>
    >,
  ) => void;
};

🙁 Actual behavior

Type 'Values<{ [K in keyof TActors as K & string]: { src: K; logic: TActors[K]; }; }>' does not satisfy the constraint 'ProvidedActor'.
  Types of property 'src' are incompatible.
    Type 'keyof { [K in keyof TActors as K & string]: { src: K; logic: TActors[K]; }; }' is not assignable to type 'string'.
      Type 'string | number | symbol' is not assignable to type 'string'.
        Type 'number' is not assignable to type 'string'.(2344)

🙂 Expected behavior

It should typecheck

Additional information about the issue

Almost the same thing typechecks OK if we refactor this to [K in keyof TActors & string].

[StreetStrider]

I think I've found this as well (2025, [email protected])

To narrow down:
https://www.typescriptlang.org/play/?#code/FAFwngDgpgBASjAvPKBjA9gJwCYB4DOImAlgHYDmANDAIalgB8oksAKgIxIwDWUY6AM3jBm0GAHEoIANJ8YuWWBhQAHiCils+GIRIUGXRaLYAmLpJl9cvfkLhNjMVgGYuAbxgBtGIphkefILwMAC6AFwSUorWgXYGAL4i4GIWcGhY2PJpqMpqGlooGDgERGRUtPQMBsge3r7+NkHZoREW0YoJjqwALOZS2Rm49sBAA

type R = Record<string, any>
type T1 = keyof R

type GetKey <Key extends string> = Key
type T2 = GetKey<keyof R>

type T3 = { [ Key in keyof R ]: GetKey<keyof R> }

type GetRecord <Rec extends Record<string, any>> = { [ Key in keyof Rec ]: GetKey<Key> }
type T4 = GetRecord<R>

where T1, T2 and T3 compute, while GetRecord not. T4 also compute, but it is based on GetRecord, so error would be present.

Type 'Key' does not satisfy the constraint 'string'.
  Type 'keyof Rec' is not assignable to type 'string'.
    Type 'string | number | symbol' is not assignable to type 'string'.
      Type 'number' is not assignable to type 'string'.

[Andarist]

Your case is working as expected. The constraint describes minimal requirement for the type - this is a valid type argument for your GetRecord: Record<PropertyKey, any>. So, as you can see, internally GetRecord can't assume that Key will be assignable to string

[StreetStrider]

It seemes you are right. I've double-checked and Record creates somewhat open type, so
type T5 = Record<number, any> extends Record<string, any> ? true : false is actually true. Object can be indexed by both.