Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=5.9.2&fileType=.tsx&code=JYOwLgpgTgZghgYwgAgJIFEpQPZQDwAqAInGHAHzIDeAsAFDLLQ5QBcyYUArhANz2MAJqTjtiI-nQC%2B9egmwgAzmGQIAFhAQBrVIswtkAXmSESZADTkAFHBABPMGtABzdlxBaQ2AO4gAlOy2Dk4gzsjAimj6uKYilIaUtAzIUBBgXFAgyFYCHHYADhDYMMhBji5GhsYA5NgARgBWmmDVyABkbbkAhF1lIWEdudXMuK2gpfbloe2dyWAFRSV9LgB0I1CVNXXY2AA2ELatg8nLoWtYuLl%2BklKScgrKTAAenIhg0VBmcEYm4hbW6wA-G4PF5fAFkH9vgAfZAgLi7XZGRK5VLpTKqDTaXQfWJkAEXKB%2BZCApiElbCMjIdjwxE3XhAA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUFtQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0ipWsRFCAtonyJoqDJCXQO0SODABfELqA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

interface IError<TData> {
  error: true;
  data: TData;
}

const checkIsError = <TData>(anything: unknown): anything is IError<TData> => {
  return (
    typeof anything === 'object' &&
    !!anything &&
    'error' in anything &&
    typeof anything.error === 'boolean' &&
    anything.error
  );
};

const extractErrorData = <TData>(error?: unknown): TData | null => {
  return checkIsError<TData>(error) ? error.data : null;
};

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
    "@typescript-eslint/<rule-name>": ["error", ...<options>],
  },
};

tsconfig

{
  "compilerOptions": {
    // ...
  }
}

Expected Result

I didn't expect the error on TData, because it is passed to the type guard and is needed to provide the return type of the extractErrorData function.

Actual Result

I receive an error on the generic TData of extractErrorData saying Type parameter TData is used only once in the function signature.

Additional Info

Hi,
I read the docs and I tried to search for a similar case, but didn't find any answers. My apologies if this has been covered or falls into a described case and I simply failed to understand it.

Now onto the problem:
I understand the suggestion to replace all usages of the type parameter with its constraint (unknown, in this case), but that would mean I can't type the data field of the object. I understand there's no runtime guarantee of the structure of the data field (as in: I'm not performing any checks on the structure of the data field). But from a type descriptions perspective, shouldn't it work, just like type guards do?

Then, why does the following work?

const extractErrorData = <TData,>(error?: unknown): IError<TData> | null => {
  return checkIsError<TData>(error) ? error : null;
};

Here, the returned type is IError<TData> instead of TData. The same checks are applied to TData, but there's no alert from the rule.
(Here's a playground link)

Should the rule stop alerting on my original case? Or should the rule also alert for this second case? Or am I seeing this wrong?

Any help is greatly appreciated, thank you!