For example in a function like this:

function someFunc(err: Error | undefined, data: string | undefined) {
  if (err) throw err;
  data; // Intellisense should show `string`, not `string | undefined`
}

I tried this:

function someFunc<
  Err extends Error | undefined,
  Data extends Err extends Error ? undefined : string
>(err: Err, data: Data) {
  if (err) throw err;
  data.length; // 'data' is possibly 'undefined'
}

It didn’t work.

So the question is if it’s even possible to do something like this, or should I just add another if check for data and move on?