Stop using user-assignable type generics in the types of functions that don't do anything with them

[Avaq]

This is an alternative to #230 made possible by the introduction of the unknown type.

See also #334 (comment).
See also #370 (comment).

[Avaq]

TypeScript 3.4 introduces higher order type inference from generic functions. This might make the need for using anything but an undetermined type variable/generic unnecessary. I should do some experiments with this.

[Avaq]

^{In response to #370 (comment)}

I tried a simple thing myself, and it seems never and unknown have the same issues, but in opposite orders.

With never/undefined

Here the problem is that we're not allowed to assign anything to never (the same goes for undefined). So when we're trying to assign Future<string, never> to Future<never, number>, it breaks on the rejection branch.

With unknown

Here the problem is the other way around: we're not allowed to assign unknown to anything. So when we're trying to assign Future<string, unknown> to Future<unknown, number>, it breaks on the resolution branch.

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In conclusion, it seems that neither never nor unknown possess the desired behaviour. I will continue exploring.

[Avaq]

It seems that never has the behaviour we want, but only when chain uses a union to define the type of the left branch. The downside is that this will allow users to mix their types through usage of chain.

[codingedgar]

Yes i agree that neither of the types work right now.