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

从链接的 TypeScript Playground 复现开始，检查类型检查器如何处理映射类型的 as 子句和条件约束。跟踪为什么 AcceptNonNullable<T[K]> 上的约束未被识别，然后确认示例不再报告错误 2344，同时保留筛选行为。