No error on self-referencing type parameter of non-generic mapped type

🔎 Search Terms

mapped type circular self reference type parameter

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20250801#code/C4TwDgpgBAKhDOwoF4oGsIgPYDMoG8AoKKAbQAUoBLAO3U1ynIF0AuKAVxrRqwHcahAL4BuQoQAmEAMYAbAIYAnaNKw1EUYAmDs4iMYQgAPMFkVIcXacCpqoAcwjA9wABQBKAsSjLgHRXRa+sKEQA

💻 Code

type Test = keyof {
  [P in keyof P]: unknown
};

declare const test: Test;

export function getTest() {
  return test;
}

🙁 Actual behavior

[P in keyof P] doesn't error anyhow but it's clearly circular. Furthermore, the generated .d.ts here is:

export declare function getTest(): keyof P;

P leaked here and this is invalid

🙂 Expected behavior

I'd expect this to produce a circularity error and at the very least the generated declaration file should not contain a leaked type variable

Additional information about the issue

cc @jfet97 who originally mentioned part of this to me in a DM 😉

Start with the linked TypeScript Playground repro and verify both the missing circularity diagnostic and the leaked P in the generated declaration. Trace how the compiler handles [P in keyof P] and declaration output for this case. Done means the circular reference is reported and the emitted declaration contains no leaked type variable.