Issue with js.Promise.then() type inference

[jcward]

There is a lot of type magic (Either's, parameters) in js/Promise.hx to mirror the behavior of JavaScript's Promise. Unfortunately, somewhere in all this magic, we lose type inference to the argument of the then callback.

Promise.then() type inference not working: https://try.haxe.org/#aa552

NotAPromise.then() type inference IS working: https://try.haxe.org/#b052a

My brain cannot wrap through all the type magic to know whether this is expected or not, or which exact feature breaks the inference, but from an end user perspective, I'd like Promise.then() to work as well as NotAPromise.then()

[nadako]

This is a more general issue with unifying functions against abstracts (which EitherType is):

abstract Callback(Dynamic) from Int->Void {}

class Main {
	static function main() {
		var f:Int->Void = function(t) $type(t); // Int
		var f2:Callback = function(t) $type(t); // Unknown<0>
	}
}

Probably comes from the fact that with abstracts there can be multiple from/@:from, so we can't simple use one of them as the "expected" when typing the function, so it's typed on its own first and then an abstract cast is chosen.

For the js.Promise case there might be another way to deal with it using @:overload instead of EitherType, I'll look into it.

[jcward]

Thanks for looking into this!

(I decided to separate the 'empty promise' request into its own issue, #6793)

[jdonaldson]

Here's some typescript folks struggling with the same issue:
microsoft/TypeScript#8516
microsoft/TypeScript#3356
microsoft/TypeScript#4260
DefinitelyTyped/DefinitelyTyped#6091