TypeScript Fails to Infer Specific Generic Types as Expected

[unclesmiley]

I'm currently facing an issue with TypeScript where it seems to be unable to correctly infer types in a specific scenario involving generics, enums, and inheritance.

Here is a simplified version of my code:

enum Status {
  UNDEFINED = 0,
  SUCCESS = 1,
  FAIL = 2,
}

class Result<StatusT extends Status = Status, Data = unknown, Code = string | number> {
  constructor(
    public status?: StatusT,
    public data?: Data,
    public code?: Code,
  ) {}
}

abstract class Task {
  async done(...args: any[]) {
    while (true) {
      const result = await this.execute(...args);
      if (result.status === Status.SUCCESS) {
        return result;
      }
    }
  }

  abstract execute(...args: any[]): Promise<{
    [K in Status]: Result<K>;
  }[Status]>
}

class TestTask extends Task {
  // (method) TestTask.execute(): Promise<Result<Status.SUCCESS, {}, number> | Result<Status.FAIL, string, string | number>>
  async execute () {
    if (Math.random() > 0.5) {
      return new Result(Status.SUCCESS, {}, 0);
    } else {
      return new Result(Status.FAIL, "error");
    }
  }
}

const test = new TestTask()

// got: `Promise<Result<Status.SUCCESS, unknown, string | number>>`
// but i want: `Promise<Result<Status.SUCCESS, {}, number>>`
test.done();

TypeScript only infers the return type of done() as Promise<Result<Status.SUCCESS, unknown, string | number>>, while I expect it to infer Promise<Result<Status.SUCCESS, {}, number>>.

Why does TypeScript fail to infer the specific generic types as I expect in the TestTask class? Is this a limitation of TypeScript's type inference system, or am I missing something in how I should annotate or structure my types or code?

[jcalz]

This isn't an appropriate place to ask this question, and you haven't properly filled out the issue template. Please close this to spare the TS team from having to do so. Please stick to Stack Overflow or some other more appropriate place if you're asking questions.

[RyanCavanaugh]

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