Multiple tagged union with undefined can't catch error

[Kwongiseok]

Bug Report

TypeScript can't catch error about Multiple tagged union (more 3 tagged union) with undefined

🔎 Search Terms

Multiple tagged union , tagged union with undefined , undefined union

🕗 Version & Regression Information

TypeScript Version : 4.6

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• This is a bug
• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

type T = 1 | 2 | 3

interface P {
  a: T
}
interface A extends P {
  a: 1
  b: undefined
}
interface B extends P {
  a: 2
  b: number
}
interface C extends P {
  a: 3
}
type Q = A | B | C

const a: Q = { a: 1, b: 1 } // Good :  TypeScript catch error, b must be undefined

const c: Q = { a: 3, b: 1 } // Bad : TypeScript didn't catch any error, TS compiler passed it.

🙁 Actual behavior

variable a is error
variable c is not error

🙂 Expected behavior

I expected variable c is error when a is 3 variable b shouldn't exits

[MartinJohns]

I expected variable c is error when a is 3 variable b shouldn't exits

See: https://github.com/microsoft/TypeScript/wiki/FAQ#what-is-structural-typing

Duplicate of #20863. Generally the excess-property check does not work for union types.

[Kwongiseok]

type T = 1 | 3

interface P {
  a: T
}
interface A extends P {
  a: 1
  b: undefined
}

interface C extends P {
  a: 3
}

type Q = A | C


const a: Q = {a: 3, b: 4} //  It catch error but type inference is { a: C, b: A}
const b: Q = {a: 1 , b: undefined} // It is Okay  { a: A, b: A }  

I think In this case variable a must should pass type check Okay....

why 2 tagged union is okay??

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.