This is issue was raised while I was trying to do some experimental implementation of the following WebIDL: dictionary ImageBitmapBufferSourceInit { required BufferSource buffer; required long offset; required long length; required ImageBitmapFormat format; required ImageFormatPixelLayout layout; }; typedef (HTMLImageElement or HTMLVideoElement or HTMLCanvasElement or Blob or ImageData or CanvasRenderingContext2D or ImageBitmap or ImageBitmapBufferSourceInit) ImageBitmapSource; Quote from bz's comment at bug1141979 comment57: "...if you have an owning union that has a dictionary member and that dictionary has members that need to be traversed/unlinked, the codegen currently doesn't know how to output the right traverse/unlink bits."