I'm not sure about the never[] overload... why is this necessary?

That is because of the most generic typing overload (function all <T, E> (tasks: Array<Task<T, E>>): Task<T[], E>) that was allowing empty arrays but couldn't infer any resolution nor rejection types for them:

I think that is better typed with the never[] case.

I think another option is to default the generics to never in the most general case. It would be:

function all <T = never, E = never> (tasks: Array<Task<T, E>>): Task<T[], E>

I don't have a preference for any of those options.

Well, once we upgrade to 3.x we could use the unknown type. Of those two options I would prefer the second one, but even better is not type it.

When you do

let a1 = [1, 2, 3]
a1.push(4);

you don't specify the type because typescript can infer it. But if you initialize with an empty array you normally specify the type beforehand.

let a2: number[] = [];
a2.push(1)

Having an array of never is not useful. If you don't want to get the {} type infered and prefer never or number you can always specify it

Task.all<never>([])

In conclusion, I would remove the never part.

Ok, I removed that! I think the PR is ready to be merged!