No error on self-referencing type parameter of non-generic mapped type · Microsoft/TypeScript#62176

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BugDomain: Mapped TypesHelp Wanted

Description

mapped type circular self reference type parameter

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

declare const test: Test;

export function getTest() {
  return test;
}

[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

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

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

Tech stack: typescript
Domain: developer experience
Issue type: bug
Difficulty: 4
Estimated time: 3-5 days
Activity status: fresh
Clarity: clear
Prerequisites: TypeScript type system basicsMapped typesType parameter scoping
Newbie friendliness: 20
Research direction: Investigate the TypeScript compiler's handling of mapped types and type parameter resolution. The issue is about a missing circularity error for `[P in keyof P]` in a non generic mapped type. Look into the type checker's logic for detecting self referencing type variables. The playground link shows the code. The fix should ensure that such constructs produce a circularity error and prevent the type variable from leaking into declaration files.