Nested Omit type with same constraints incompatible.

[Chr15t0pher]

Bug Report

🔎 Search Terms

nested Omit, util type

🕗 Version & Regression Information

• This is the behavior in every version i tried on the typescriptlang playground.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Exotic {
  id: string
}

type WithoutExotic<P extends Record<string, unknown> = {}> = Omit<P, 'id'>

type WithExotic<P extends Record<string, unknown> = {}> = WithoutExotic<P> & Exotic

function fn<P extends Record<string, unknown>>(args: WithExotic<P>): WithoutExotic<P>{
  const { id, ...rest} = args
  return rest    // errors, not assignable
}

🙁 Actual behavior

Error occurs at line 12. First line in error message says Type 'Omit<WithExotic<P>, "id">' is not assignable to type 'WithoutExotic<P>'., however, second line shows Type 'Exclude<keyof P, "id">' is not assignable to type 'Exclude<Exclude<keyof P, "id">, "id">'., the assignment direction described is reverse.

Type 'Omit<WithExotic<P>, "id">' is not assignable to type 'WithoutExotic<P>'.
  Type 'Exclude<keyof P, "id">' is not assignable to type 'Exclude<Exclude<keyof P, "id">, "id">'.
    Type 'keyof P' is not assignable to type 'Exclude<Exclude<keyof P, "id">, "id">'.
      Type 'string | number | symbol' is not assignable to type 'Exclude<Exclude<keyof P, "id">, "id">'.
        Type 'string' is not assignable to type 'Exclude<Exclude<keyof P, "id">, "id">'.
          Type 'keyof P' is not assignable to type 'Exclude<keyof P, "id">'.

🙂 Expected behavior

• There should be no error. Omit<WithExotic<P>, "id"> is assignable to type WithoutExotic<P>. Because Omit<Omit<P, K>, K> is equivalent to Omit<P, K> when P and K stay the same.
• Better error messages. The assignment direction of first two error messages is reverse, it should not happened in typescript which allow variance.

[jcalz]

This appears to be a duplicate of issue #37768, which is categorized as a bug. However, I believe it may be more of a design limitation. A similar issue is discussed in #43846, classified as a design limitation, and in #28884, classified as a feature request. So I'm uncertain as to the fate of this issue.

The root problem in all of these cases is that TypeScript struggles to infer much about conditional types that depend on generic type parameters, even in situations where it seems obvious to human developers. Unlike humans, who can reason abstractly and understand the meanings of words like "Exclude" and "Omit," the compiler must rely on a heuristic algorithm that is designed to meet performance constraints.

[typescript-bot]

This issue has been marked as 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.