Omit utility type throws away details of union types

[ducminh-phan]

Bug Report

🔎 Search Terms

omit union broaden types

🕗 Version & Regression Information

• This is the behaviour in every version I tried, and I reviewed the FAQ for entries about union types.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Schedule =
  | {
      scheduled: true;
      scheduleTime: number;
    }
  | {
      scheduled: false;
      scheduleTime: null;
    };

type Item = {
  id: number;
  name: string;
} & Schedule;

// id is auto-generated
// 
// From inspection, CreateItemData is equivalent to
// type CreateItemData = {
//   name: string;
//   scheduled: boolean;
//   scheduleTime: number | null;
// }
type CreateItemData = Omit<Item, "id">;

const data: CreateItemData = {
  name: "item name",
  scheduled: true,
  scheduleTime: null,
};


// @ts-expect-error: This works correctly
const scheduleData: Schedule = {
  scheduled: true,
  scheduleTime: null,
};

🙁 Actual behavior

The line const data: CreateItemData = ... typechecks, as CreateItemData throws away some important data about Schedule type.

🙂 Expected behavior

The line const data: CreateItemData = ... should not typecheck, CreateItemData should be equal to

type CreateItemData =
  | {
      name: string;
      scheduled: true;
      scheduleTime: number;
    }
  | {
      name: string;
      scheduled: false;
      scheduleTime: null;
    };

[fatcerberus]

Omit does not distribute over unions. See #49659 (comment)

[ducminh-phan]

Thanks for pointing me to #49659. @RyanCavanaugh Should we bring DistributedOmit into TS's core utility types?

[MartinJohns]

Should we bring DistributedOmit into TS's core utility types?

See #39522 (comment):

We've opted to not include utility type aliases in the lib unless they're required for declaration emit purposes.

Also #49656 (comment):

One of the reasons we don't like adding type aliases to the built-in lib -- we can't "upgrade" them!