Omit is breaking `in` gaurd

[stavalfi]

TypeScript Version: 3.5.2

Search Terms: Omit, guard

Code

type t1 = { a: boolean } & ({ x: number } | { y: string } | {})
const z1: t1 = Date.now() === 123453 ? { a: true, x: 1 } : { a: true }
const z2: Omit<t1, 'a'> = z1
console.log('x' in z2 && z2.x)

Expected behavior:
no errors
Actual behavior:

Error:(235, 29) TS2339: Property 'x' does not exist on type 'never'.

Playground Link:
Link to Playground

---

If I specify the actual type instead of using Omit, everything is working with no errors:

type t1 = { a: boolean } & ({ x: number } | { y: string } | {})
const z1: t1 = Date.now() === 123453 ? { a: true, x: 1 } : { a: true }
const z2: { x: number } | { y: string } | {} = z1
console.log('x' in z2 && z2.x)

[jack-williams]

For this to work you need to implement your mapped types in a distributive way. See #28339 for extensive discussion.

TLDR:

// Omit copied from typescript lib.es5.d.ts file
type UnionKeys<T> = T extends any ? keyof T : never
type DistributivePick<T, K extends UnionKeys<T>> = T extends any ? Pick<T, Extract<keyof T, K>> : never;
type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
type DistributiveOmit<T, K extends UnionKeys<T>> = T extends any ? Omit<T, Extract<keyof T, K>> : never;

type t1 = { a: boolean } & ({ x: number } | { y: string } | {})
const z1: t1 = Date.now() === 123453 ? { a: true, x: 1 } : { a: true }
const z2: DistributiveOmit<t1, 'a'> = z1
console.log('x' in z2 && z2.x)

[stavalfi]

Okay. You gave me a lot to learn before I can respond.

If this is working as Intended, close this issue.