🤔 I don't understand what's being asked for here. Also, the playground link and code snippet aren't the same code. Could you please edit the post so they're the same, and provide an explanation for why the private notReadonly?: number; shouldn't report any errors?

notReadonlyFoo is clearly reassigned in the testMethodFoo() method, but eslint instructs to mark it as readonly. It is possible to reach the code with the assignment after type guard. If you use classic type guard like normalTypeGuard(value) there is no error, but it is present if you use a hack to typeguard a class member (like here)

Ah, got it - thanks. That makes more sense. This does seem like a bug.

I found the original code snippets a little confusing so I made a smaller reproduction case: https://typescript-eslint.io/play/#ts=5.1.6&fileType=.tsx&code=MYGwhgzhAEAqCmEAu0DeAoa0AOBXARiAJbDQC2YAnvvAPwBc0AdrmTQE4Dc6mO7RANzBJ4zAPZIASvDAATMUxCUAcmPYUQDZqw7de2fkJHipM%2BYsqwAFkQhaWbeF14B6F9ACy8R%2B2gByJglpOQUla1s-aFtmeAEnaHYZKCIAcyZ4WU5yMHYAayiUSGgAA0SQi2KAOmgeLDxCEmgRZFhKbHgAcVwc2QgACgBKNF4sIgAzaD6kGwhK4Ct4YFzVdTAQQaGMLG2mmcrA03KlFY1oAF5oAAZuHYBfWu3xyenbOYWl1vaunvD%2Bgc2RtsXrMDsFzGEZucrjdtvcsPd9IZhKJ5otlmoNINhjtEkhcOwmLtXhRqKIAIRnC64JiyeBjIjpTK8BF1JHGVEfNqdbrsWS-QaMYFRGBCgBkaGypMYDg40Fu2O2uPxhOBlRJNGgFKpNLpDIyMPutyAA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Y6RAM0Wls4CGAEwD2TeIXRRe0EdEjgwAXxBKgA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylAGYBDVCXTgwAXxCygA&tokens=false

class Test {
  public maybe?: number;

  private notReadonlyNormal?: number;

  private notReadonlyThis?: number;
  // Member 'notReadonlyThis' is never reassigned; mark it as `readonly`. 

  public testTypeGuards() {
    if (this.checkNormal()) {
      this.notReadonlyNormal = 0;
    }

    if (this.checkTypeGuardThis()) {
      this.notReadonlyThis = 0;
    }
  }

  private checkNormal() {
    return this.maybe !== undefined;
  }

  private checkTypeGuardThis(): this is this & { maybe: number } {
    return this.maybe !== undefined;
  }
}