Obscure "type cannot be used to index type" error when a mapped type is nested into another mapped type

[shau-kote]

Bug Report

🔎 Search Terms

• mapped type
• nested mapped type

🕗 Version & Regression Information

• It reproduces in TS of versions from 3.8.3 to 5.0.4 (there are other compiler errors in earlier versions).
• It reproduces in Nightly (v5.2.0-dev.20230516) as well.
• I failed to find out a description of similar cases in the FAQ.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Values<T extends object> = T[keyof T]

const ENTITY_CONFIG = {
  'entity1': {
    'step1':  {
      body: { aaa: 1 }
    },
    'step2':  {
      body: { bbb: 1 }
    },
  },
  'entity2': {
    'step1':  {
      body: { ccc: 1 }
    },
    'step2':  {
      body: { ddd: 1 }
    },
  }
} as const

type EntityConfig = typeof ENTITY_CONFIG

type EntityType = Values<{
  [T in keyof EntityConfig]: {
    entityName: T,

    step: Values<{
      [S in keyof EntityConfig[T]]: {
        stepName: S
        body: EntityConfig[T][S]['body'] // <-- TS2536 error
      }
    }>
  }
}>

🙁 Actual behavior

I see an error around the EntityConfig[T][S]['body'] type expression with the following error message: Type '"body"' cannot be used to index type '{ readonly entity1: { readonly step1: { readonly body: { readonly aaa: 1; }; }; readonly step2: { readonly body: { readonly bbb: 1; }; }; }; readonly entity2: { readonly step1: { readonly body: { readonly ccc: 1; }; }; readonly step2: { ...; }; }; }[T][S]'..

Firstly, I don't expect any errors in my type definition. Although it's possible of course that my mental model is wrong, or I don't understand some compiler limitations.

Secondly, the error message is quite obscure to me, it doesn't help to understand what I do wrong and how to fix it.

🙂 Expected behavior

I expect that the EntityType will be successfully constructed and will be something like this:

type EntityType = {
  entityName: 'entity1'
  step: {
    stepName: 'step1'
    body: { aaa: 1 }
  } | {
    stepName: 'step2'
    body: { bbb: 1 }
  }
} | {
  entityName: 'entity2'
  step: {
    stepName: 'step1'
    body: { ccc: 1 }
  } | {
    stepName: 'step2'
    body: { ddd: 1 }
  }
}

In fact, it seems to me based on my "tests" in the playground above that the EntityType type construction works well, despite the error in the type definition.

[shau-kote]

Also, I found out that it's possible to kinda suppress the error by using the EntityConfig[T][S] extends infer U ? U extends { body: infer B } ? B : never : never expression for specifying the type of the body property, but it seems to me as a weird and should-be-needless hack.

[shau-kote]

Lastly, I tried to explicitly declare that all steps of all entities should have a body declaration — so it's safe to index any step definition by the 'body type.
But still no luck.

Playground Link

type Values<T extends object> = T[keyof T]

type Config = {
  readonly [entityName: string]: {
    readonly [stepName: string]: {
      readonly body: object
    }
  }
}

const ENTITY_CONFIG = {
  'entity1': {
    'step1':  {
      body: { aaa: 1 }
    },
    'step2':  {
      body: { bbb: 1 }
    },
  },
  'entity2': {
    'step1':  {
      body: { ccc: 1 }
    },
    'step2':  {
      body: { ddd: 1 }
    },
  }
} satisfies Config

type EntityConfig = typeof ENTITY_CONFIG

type EntityType = Values<{
  [T in keyof EntityConfig]: {
    entityName: T,

    step: Values<{
      [S in keyof EntityConfig[T]]: {
        stepName: S
        body: EntityConfig[T][S]['body']
      }
    }>
  }
}>

[Andarist]

It's likely a duplicate of #21760

[shau-kote]

Thank you for the quick answer, Mateusz Burzyński (@Andarist)!

It really looks like a related problem. It's very sad that there's no visible success in finding a solution in this area.

Could you please also let me know if you're aware of any other (maybe better) workarounds except the double extends infer I described above?

[Andarist]

You could try using something like this if you think that it makes your life easier (TS playground):

type Tail<T extends any[]> = T extends [any, ...infer R] ? R : never;
type GetProp<T, K> = K extends keyof T ? T[K] : never;
type GetPath<T, P extends any[]> = P extends [any]
  ? GetProp<T, P[0]>
  : GetPath<GetProp<T, P[0]>, Tail<P>>;

type EntityType = Values<{
  [T in keyof EntityConfig]: {
    entityName: T;

    step: Values<{
      [S in keyof EntityConfig[T]]: {
        stepName: S;
        body: GetPath<EntityConfig, [T, S, "body"]>;
      };
    }>;
  };
}>;

[shau-kote]

Thank you for the advice, Mateusz Burzyński (@Andarist)! I'll think about it — it looks great from the usage side, but the mutually-recursive implementation... oh, gods. 🥲

[shau-kote]

I'm not sure whether it will be proper to close this issue now, but feel free to do it if it is.

[Andarist]

I'm not sure whether it will be proper to close this issue now, but feel free to do it if it is.

I don't have such privileges - I think that the issue can be closed though (since it's a duplicate). Might be worth posting your case in that main thread though, this way it's likely that it will be added as a test case when that issue gets fixed.