Conditional type flow types missing in the template in mapped types with as clause

🔎 Search Terms

as clause renaming filtering mapped type conditional type flow

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250222#code/C4TwDgpgBAggxnCZgDkD2A7FBXANrgQwCNcIAeAFSggA9gIMATAZygG8BfAPigF4oKAbgBQw0JAERmwSj35thUKAG0A0lACWGKAGsIINADMBUAqwpqAutToMW7DlAD8UdQC4oGCADcIAJ0sPeERkdCw8QhJyC1VLLhEOESA

💻 Code

type AcceptNonNullable<T extends {}> = T;

type Test<T> = {
  [K in keyof T as T[K] extends {} ? K : never]: AcceptNonNullable<T[K]>;
};

🙁 Actual behavior

AcceptNonNullable<T[K]> errors with:

Type 'T[K]' does not satisfy the constraint '{}'.
  Type 'T[keyof T]' is not assignable to type '{}'.
    Type 'T[string] | T[number] | T[symbol]' is not assignable to type '{}'.
      Type 'T[string]' is not assignable to type '{}'.(2344)

🙂 Expected behavior

It doesn't have to error. It's known that T[K] has to extend {} at this position

Additional information about the issue

No response

リンクされた TypeScript Playground の再現例から始め、型チェッカーが mapped type の as 句と条件付き制約をどのように処理するかを調べてください。AcceptNonNullable<T[K]> に対する制約が認識されない理由を追跡し、その後、フィルタリング動作を維持したまま、例でエラー 2344 が報告されなくなったことを確認してください。