Require a default value for optional dictionary member of another dictionary?

[saschanaz]

What is the issue with the Web IDL Standard?

A version of #602 for dictionary members. Consider:

dictionary Foo {
  Bar bar;
};

dictionary Bar {};

Given we treat an empty dictionary as nothing-is-passed in arguments, one can expect the same for dictionary; not passing bar would be same as passing {}. But it's not always the case - e.g. in DisplayMediaStreamOptions.video not passing a value may also mean true, or in GPUBindGroupLayoutEntry not passing {} means disabling the feature.

Might want to do the same work as #620 so that such dictionary would set an explicit non-{} default value, and otherwise explicitly get {}.

Context: gpuweb/gpuweb#5249

[kainino0x]

There are probably details in WebIDL that I don't understand, but this seems consistent with #602 already. My understanding - please check if this is correct - is:

• In a dictionary:
  • dictionary Foo { Bar bar = {}; }; means that Foo {} == { bar: undefined } == { bar: {} }
  • dictionary Foo { Bar bar; }; means that Foo {} == { bar: undefined }, distinct from { bar: {} }
• In a function:
  • foo(optional Bar bar = {}); means that foo() == foo(undefined) == foo({})
  • foo(optional Bar bar); is not valid syntax

I'm also not entirely sure how this connects with needing to allow Foo { bar: false } or { bar: true } which we were discussing for GPUBindGroupLayoutEntry.