`declare namespace { using ... }` generates incorrect dts

[JLHwung]

🔎 Search Terms

ambient context, declare namespace, using, await using

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250522#code/CYUwxgNghgTiAEA7KBbEBnADlMCBqUEAlsFAC5ED2i8A3gFDzwCu6RiA5vFAFzeIBPANz0AvkA

💻 Code

declare namespace Validation {
  using a: any;
}

🙁 Actual behavior

In the generated dts, a becomes a const declarator:

declare namespace Validation {
    const a: any;
}

🙂 Expected behavior

The generated dts should be the same as the input

declare namespace Validation {
  using a: any;
}

Additional information about the issue

No response

[jakebailey]

@JLHwung is there a babel issue for this? @Renegade334 seemed to think there was, but I couldn't find it. It would be good for everyone to align on this. (typescript-eslint/typescript-eslint#11244 too)

[JLHwung]

@jakebailey Thanks for the heads-up. There is no such issue yet because currently Babel aligns with tsc: that Babel disallows declare using foo but allows using foo in the namespace. This issue is intended to be TS-only because the generated dts does not look correct. If tsc decides to disallow using foo in ambient context and therefore this issue can be closed because of invalid input, then Babel will follow for sure.