Using Omit weakens type checking

[Bastian]

Bug Report

When using the Omit type, the type-checking does not catch some cases that it previously did.

🔎 Search Terms

Omit inaccurate

🕗 Version & Regression Information

Test in both 3.3.3 and 4.5.0-beta.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface A {
  type: "a";
  payload: string;
  something: string;
}

interface B {
  type: "b";
  payload: boolean;
  something: string;
}

type AB = A | B;

type ABwithoutSomething = Omit<AB, "something">;

const a1: ABwithoutSomething = {
  type: "a",
  payload: true, // This should not work, but it does (unexpected)
};

const a2: AB = {
  type: "a",
  payload: true, // Without the Omit, the compiler complains here (expected)
  something: "",
};

🙁 Actual behavior

When using the Omit type to exclude a single field, the type checking is weaker.

🙂 Expected behavior

When using Omit, the only difference should be that the field is excluded without any other side-effects or weaker typechecks.

[nmain]

Looks like you want Omit to distribute over the union:

https://www.typescriptlang.org/play?ts=4.5.0-beta#code/JYOwLgpgTgZghgYwgAgILIN4ChnLATwAcIAuZAIjnIG4dlC58AbAezgBMyBnMKUAc1q4uLALYQwACwHdeA2gF8sWUJFiIUAIUx0CxMuQBGNOg2ZtOyQyxZMIcEEOQjxUmc7khBWJVj0pUbQBeNGQAH2RNWj8iFAARYB4+QwBXMGAANwgAeVFgMAAeABUAGmQAaWQIAA9IEHYuDz4vcOQQFNFDaFaufE7bAD5kEKKq2oh6xod8ZAB+ZFz84rLyobIQCCyoaP80TQB3fMkWNIBlMQlpFpCEpOBU9KzFwsCy8hdLgXIB6IQWEB4yDgAEYyIFDlITmBzq4rvxhjpcP4DFQSqZGKwOGReCkIGUAPT45BFaSNLjHFJMdhtFhgZD7FhQADWZQeyHyyHYLAgjQAFCkNtViAhIOwAJQ+IRYP4AulwABMYOCiLwsRR5DRuDMmMsOLxyEJyAA6kcoXhJChnmUpCg-qJCMA7FBkHbCEw4KBGhaoCheTVhaKJcILm4vAYNZKgA

See https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types

[MartinJohns]

You expect the omit type to operate distributive, but it doesn't. Look up "distributive omit". You should look at the definition of omit to understand how it works and why it behaves the way it does.

[Bastian]

I see. Thank you very much!