Narrowing of mapped types fails

[erik-kallen]

Bug Report

Sometimes narrowing of discriminated unions fails with mapped types. See the attached playground

🔎 Search Terms

narrowing mapped types

🕗 Version & Regression Information

Happens in latest. don't know about other versions.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAglC8UDeUCGAuKByVWDcUARpgHYCuAtoRAE4EAepl1NUAvlAD7JqZaH4oAY0wBnYDQCWJAOYMmVWgTYAoFaEhQAcgHsAGgB4AKgD4EUAPIVJwYwBps9LCbxqA9G6gB1HTQDWomiBEPSQQsAQACYqAGZkJOGSOiRQMQCMABRgmDAAlMgqUFCiAO42QgAWUFkAdKj5SIVFwqii0DhY6E3NwsmiOgA2EDUDOjK1hLmuPUWENBCoftPNQq3tAl0zvST9QyNjtUJT3c1zC0tNqqoqHlAAEq1QGtC0NL7BAG4QKcAVOmQyKq-WjQSSBEg6KDzVrJVK+KA2WLxRKwmIAJiymF0hhgJgaTVK5SqtXqBR6qza2FwmxmQj6g2Go3GYBqk2WPTOi3ZLUp-E6JyKdJ2DP2zJqR25s2hFyKVxUQA

💻 Code

type A = { a: 'a'; b: number; x: number } | { a: 'b'; c: string; x: number; }

type NoX<T> = Omit<T, 'x'>;

// Works as expected
function f1(p: A) {
  switch (p.a) {
    case 'a':
      console.log(p.b);
      break;
    case 'b':
      console.log(p.c);
      break;
  }
}

// Has type errors even though there is no reason for it
function f2(p: NoX<A>) {
  switch (p.a) {
    case 'a':
      console.log(p.b);
      break;
    case 'b':
      console.log(p.c);
      break;
  }
}

🙁 Actual behavior

TypeScript does not realize that the member p.b exists if p.a === 'a' in the function f2

🙂 Expected behavior

The function f2 should typechek without errors, since it does not in any way touch or care about the removed member x.

[whzx5byb]

p is narrowed only when it's type is a discriminated union. A is, but Omit<A, 'x'> is not.

To produce your expected behavior you need a "distributive" omit type to produce a discriminated union result:

type DistributiveOmit<T, K extends string | number | symbol> = T extends unknown ? Omit<T, K> : never;
type NoX<T> = DistributiveOmit<T, 'x'>;

[erik-kallen]

Then perhaps this should be considered a feature request to make Omit<A, 'x'> stay a discriminated union if A is one.

The suggested hack is way out of reach for the regular devleoper.

[jcalz]

Duplicate of #31501

[typescript-bot]

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