[exactOptionalPropertyTypes] Unexpected 'undefined' arises in error message

🔎 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).

提供された TypeScript Playground リンクの診断を、exactOptionalPropertyTypes 設定と報告にある union type を使って再現します。記載されたコンパイラーバージョン間で診断を比較します。完了の条件は、number の代入が undefined ではなく never との非互換性を報告し、undefined のケースでは期待される動作が維持されることです。