[exactOptionalPropertyTypes] Unexpected 'undefined' arises in error message

[lukehoban]

🔎 Search Terms

"not assignable", "never", "undefined", "exactOptionalPropertyTypes"

🕗 Version & Regression Information

4.3.5
'undefined' error instead of 'never' ; the value undefined is considered valid
4.4.4, 5.0.4
Expected behavior
5.1.6, 5.7.2
'undefined' error instead of 'never' ; the value undefined triggers the 'never' message
Nightly
not available when testing

⏯ Playground Link

https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true&ts=5.7.2#code/TwDgpgBAKlC8UCwAoKUDeUCGAuKAdgK4C2ARhAE4A0UZA-PgRAG5VQC+UAPqultkaFWVWmSakKlTgG5UqAMYB7AgGdgUYPjiIMfHPgAM1feKiHUHGUA

💻 Code

type T = 
  { a: number, b?: never } |
  { a?: never, b: number };

const t: T = { //  Types of property 'b' are incompatible. Type 'number' is not assignable to type 'undefined'
  a: 0,
  b: 0
};

🙁 Actual behavior

Type 'number' is not assignable to type 'undefined' is not relevant, since undefined cannot be the type of the t[b].

🙂 Expected behavior

The error should be about type 'number' not being assignable to type 'never'. It is correctly displayed if the value undefined is used (after the regression at least).

Additional information about the issue

No response