[exactOptionalPropertyTypes] Unexpected 'undefined' arises in error message

[ghostinpeace]

🔎 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/C4TwDgpgBAKlC8UCwAoKUDeUCGAuKAdgK4C2ARhAE4A0UZA-PgRAG5VQC+UAPqultkaFWVWmSakKlTgG5UqAMYB7AgGdgUYPjiIMfHPgAM1feKiHUHGUA

💻 Code

--exactOptionalPropertyTypes

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).

[jcalz]

You might want to explicitly mention that this is a problem because you're using --exactOptionalPropertyTypes; otherwise undefined would be allowed and the error message wouldn't be misleading.