Fix ECMAScript to IDL Promise type conversion

[ExE-Boss]

Most notably, this fixes the bug where rejected promises were being converted into resolved promises and that Promise.resolve(x) returns x when x is a Promise object from the same realm as the Promise.resolve function, whereas WebIDL requires the conversion to always return a new Promise object.

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Note that there still is a small timing difference for non‑void promises due to the fact that the IDL conversion and promise reaction are being performed in separate then callbacks, whereas WebIDL runs them in the same then callback. (See the “to react to a Promise<T>” algorithm)

Fixing this minor timing discrepancy would require the type conversion to be performed manually in implementation classes or overriding the then method of the converted Promise instance.

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P.S.: If you still want then to be called with two arguments, then the correct callback is:

reason => {
	throw reason;
}

Which is the default behaviour when null or undefined is passed.

[domenic]

Can you remove the T conversion entirely? As I pointed out, it does not exist in the Web IDL spec.

[ExE-Boss]

It does exist in the “to react to a Promise<T>” algorithm.

[domenic]

Right. When the spec reacts to it, it can do the conversion. We shouldn't do it automatically, especially since nobody reacts to return types.

[domenic]

I'm sorry, perhaps I wasn't clear enough. The type conversion logic should not live in webidl2js at all.

[domenic]

Thank you! Confirmed this fixes whatwg/streams in combination with #194.

When we add promise-accepting APIs to jsdom we can consider adding utils.transformPromise or something which does the appropriate type conversions.