RTCSessionDescriptionInit vs "local" RTCLocalSessionDescriptionInit about webrtc-pc

[jan-ivar]

RTCSessionDescriptionInit differs from RTCLocalSessionDescriptionInit by its type member being required:

dictionary RTCSessionDescriptionInit {
  required RTCSdpType type;
  DOMString sdp = "";
};

vs.

dictionary RTCLocalSessionDescriptionInit {
  RTCSdpType type;
  DOMString sdp = "";
};

This is what allows description to be optional in sLD but not in sRD:

  Promise<undefined> setLocalDescription(optional RTCLocalSessionDescriptionInit description = {});
  Promise<undefined> setRemoteDescription(RTCSessionDescriptionInit description);

...which is observable:

pc.setLocalDescription(); // fine
pc.setLocalDescription({type: "rollback"}); // fine
pc.setRemoteDescription({type: "rollback"}); // also fine
pc.setRemoteDescription(); // TypeError

RTCPeerConnection.prototype.setLocalDescription.length // 0
RTCPeerConnection.prototype.setRemoteDescription.length // 1

So this is by design.

Problem: Dart Language web bindings which are generated from IDL is having an dart-lang/web#178 where RTCLocalSessionDescriptionInit and RTCSessionDescriptionInit are unique, and neither class implements the other. This seems inconsistent with MDN and actual usage.

WebIDL dictionaries are not classes or interfaces: "an operation that accepts a dictionary as an argument will perform a one-time conversion from the given JavaScript value into the dictionary, based on the current properties of the JavaScript object"

IOW member-compatible inputs are valid, making RTCSessionDescriptionInit valid input to a method expecting RTCLocalSessionDescriptionInit.

If the Dart Language web bindings cannot express this then that seems like a limitation of those bindings.