In assignment Type Union is resolved as Intersection

[askirmas]

TypeScript Version: Nightly

Search Terms:
Type Union, Type check during assignment
Expected behavior:
Some error i.e.

19:7 - error TS2339: Property 'c' does not exist on type 'tAB'.

19 c: false,

Actual behavior:
exits with status 0

Related Issues:

Code

type tAB = {
  a: string
  b: number
}
type tAC = {
  a: string
  c: boolean
}

type tUnion = tAB | tAC

//@ts-ignore Type '{}' is not assignable to type 'tUnion'
const empty: tUnion = {}

//No ts-error but should be
const union: tUnion = {
  a: '',
  b: 0,
  c: false,
}
//@ts-ignore  Property 'b' does not exist on type 'tAC'
union.b

Output

"use strict";
//@ts-ignore Type '{}' is not assignable to type 'tUnion'
const empty = {};
//No ts-error but should be
const union = {
    a: '',
    b: 0,
    c: false,
};
//@ts-ignore  Property 'b' does not exist on type 'tAC'
union.b;

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[RyanCavanaugh]

Unions are not mutually exclusive. You have a valid tAB and a valid tAC; no type violation has occurred.

[askirmas]

Updated. Had a typo in example

---

I see that they are exclusive - working like operator XOR or JsonSchema's oneOf

Playground Link

type tA = {
  a: string
}
type tB = {
  b: number
}

type tUnion = tA | tB

const union: tUnion = {
  a: '',
  b: 0,
}

if ('b' in union)
  // type hint as const union: tB
  if ('a' in union)
    //TSError: Property 'a' does not exist on type 'never'.ts(2339)
    union.a

[askirmas]

https://en.wikipedia.org/wiki/Union_type also thinks that better to have XOR rather than mathematical union.
Therefore OR can be done by tA | tB | tA & tB

[askirmas]

@RyanCavanaugh so it is not working as intended

[typescript-bot]

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

[askirmas]

How to reopen it?

[RyanCavanaugh]

@askirmas This is the intended behavior; truly it is.

[askirmas]

@rictic RyanCavanaugh So please explain why here type is resolved to never
#38159 (comment)

[rictic]

I don't think you meant to at me, but consider: https://www.typescriptlang.org/play/#code/C4TwDgpgBMCCUF4oG8CwAoKUCGAuKAzsAE4CWAdgOYYC+GokMAQoihlgEb7kCuAthwjFaGeuGjAAquVIB7cqzhQAPsxVQAFEoBkzAJSj0AY3lEoPGfPxTLCpGkw58AcmcAadlC5QADB-R06BikAGaazhzOUBTmtgaOAPQJMOJQABYUwDgEUCbkZhZy5NZMnqHh2FExhfLxWFhJACoAygCixMSyxPgACp2QxKBQzpVQACayEDnkslkQAB6kZvIpjM7kEABuQs4AdMAEGgBMAMwnAJx19TXku9hAA

[askirmas]

Oh, miss in mention, sorry.
I know that OR pattern - just a bit up #38159 (comment). My question in XOR that doesn't work in assignment statement, means object example can be constructed duck: Bird|Fish = {fly, swim}. Check Expected behaviour in issue body