Support syntax for specifying that an optional record<K, V> has a default value of an empty record<K, V>

[weinig]

What problem are you trying to solve?

There is currently syntax to specify a default value empty sequence (= []) and a default value default dictionary (= {}), but no way to specify a default value empty record<K, V>.

This is needed to eliminate the invalid syntax used by WebGPU's GPUDeviceDescriptor, which tries to the default dictionary ( = {}) to default initialize the record<K, V> member requiredLimits:

dictionary GPUDeviceDescriptor
         : GPUObjectDescriptorBase {
    sequence<GPUFeatureName> requiredFeatures = [];
    record<DOMString, (GPUSize64 or undefined)> requiredLimits = {};
    GPUQueueDescriptor defaultQueue = {};
};

(I have also filed gpuweb/gpuweb#5529 on WebGPU to remove the invalid syntax).

What solutions exist today?

Today, one can use prose for this.

How would you solve it?

Make either = [] or = {} valid for record<K, V>.

[annevk]

@saschanaz @noamr thoughts for Gecko and Chromium? I think we should do this because removing the distinction between missing and empty seems worthwhile. One less thing for specification algorithms to normalize.

[noamr]

@domfarolino

[domfarolino]

@caseq is our main bindings lead at this point, what do you think Andrey? This seems like a good idea; it's probably not too difficult.

[saschanaz]

Sounds reasonable to me. Cc @EdgarChen

[caseq]

Sounds reasonable to me, I can prototype it.

[domfarolino]

Chromium is prototyping this in https://crrev.com/c/7539470; assuming that goes well, we should support this imminently.

[weinig]

We added support for = {} for records in WebKit as an extension and have not run into any problems so far.

[domfarolino]

Make either = [] or = {} valid for record<K, V>.

We're interpreting this as we should pick one (mutually exclusive), and not add both. We're adding support for = {} as opposed to = [], since the latter doesn't seem to make as much sense. Hopefully we're interpreting this right.

[weinig]

Yeah. We picked ‘= {}’ as well since it seemed to fit better.