ObservableArray is in DistinguishableType production but is not in distinguishable table

[saschanaz]

• https://heycam.github.io/webidl/#prod-DistinguishableType

  DistinguishableType ::
      PrimitiveType Null
      StringType Null
      identifier Null
      sequence < TypeWithExtendedAttributes > Null
      object Null
      symbol Null
      BufferRelatedType Null
      FrozenArray < TypeWithExtendedAttributes > Null
      ObservableArray < TypeWithExtendedAttributes > Null
      RecordType Null

• https://heycam.github.io/webidl/#dfn-distinguishable

If it's intentionally not distinguishable, I think it shouldn't be in DistinguishableType.

[EdgarChen]

And per https://webidl.spec.whatwg.org/#idl-observable-array

Observable array types must only be used as the type of regular attributes defined on an interface

so it is disallowed to be an inner type of sequence, record, FrozenArray, and Promise.

If we could make ObservableArray not a DistinguishableType, but maybe a separate type just for regular attributes, we could handle the restriction in grammar level, which could make the parser implementation a bit simpler.

[annevk]

cc @domenic

[domenic]

I suspect people more grammar-comfortable, like @saschanaz or @EdgarChen, are better positioned to make this change than me. Would either of you be up for it? Happy to help review...

[saschanaz]

Maybe include it in SingleType instead? That will disallow it being in unions and I think it's expected.