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Handle conversion failures in 'upon settling'. #791

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Handle conversion failures in 'upon settling'. #791

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11 changes: 9 additions & 2 deletions index.bs
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Expand Up @@ -8236,8 +8236,15 @@ IDL [=promise type=] values are represented by ECMAScript [=PromiseCapability=]
steps:

1. Let |onFulfilledSteps| be the following steps given argument |V|:
1. Let |value| be the result of [=converted to an IDL value|converting=] |V| to an IDL
value of type |T|.
1. Try running the following step:
1. Let |value| be the result of [=converted to an IDL value|converting=] |V| to an IDL
value of type |T|.

And then, if <a lt="an exception was thrown">an exception |exception| was thrown</a>:

1. If there are steps that are required to be run if the promise was rejected, let
|result| be the result of performing those steps, given |exception|.
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|result| is unused. Should it be returned, instead of throwing exception?

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Hm, good question. This matters if:

  1. we want to do a type conversion
  2. the returned promise is used (former "transforming")
  3. we want to do something else than passing the rejection along

Are you aware of an existing case to inform the decision? From #782 (comment), waitUntil/complete fail (1), respondWith/updateWith fail (2), WebAssembly fails (3).

Writing this, I'm realizing that we never actually pass the rejection along, the way WebAssembly needs. Should we pass undefined instead of onRejectedSteps if there's no steps?

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Are you aware of an existing case to inform the decision?

I'm not aware of any. However I generally think aligning with promise.then() will be more intuitive for spec authors, if we have that degree of freedom. So I'd prefer that if possible.

Writing this, I'm realizing that we never actually pass the rejection along, the way WebAssembly needs. Should we pass undefined instead of onRejectedSteps if there's no steps?

Definitely.

1. [=ECMAScript/throw=] |exception|.
1. If there are no steps that are required to be run if the promise was fulfilled, then
return <emu-val>undefined</emu-val>.
1. Let |result| be the result of performing any steps that were required to be run if
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