See https://github.com/heycam/webidl/issues/778.
Created attachment 402069 [details] WIP
Created attachment 402071 [details] WIP
Created attachment 402073 [details] Patch
Created attachment 402076 [details] Patch
Created attachment 402102 [details] Patch
Committed r263160: <https://trac.webkit.org/changeset/263160> All reviewed patches have been landed. Closing bug and clearing flags on attachment 402102 [details].
<rdar://problem/64452321>
Very nice :)