As we write more Ecmaspeak specs (Streams, 402, Loader), we find the way that ES does branding a bit onerous and opaque. That is, it picks a privileged internal slot and says "if you have this internal slot, you can pass go."

I propose the following instead:

• In a class definition, when defining the internal slots for a class (e.g. "Properties of Map instances" in ES), we can say a magic sentence: "[[MapData]] is Map's branding internal slot."
• We provide two abstract operations, IsBranded(o, Class) and ThrowOnBrandMismatch(o, Class) that do what they say. Example usage:
  • replace steps 2 and 3 of Map.prototype.size with "Perform ? ThrowOnBrandMismatch(M, Map)".
  • replace step 8 of %MapIteratorPrototype%.next with "Assert: IsBranded(m, Map)."

Potential variant: use a small badge in the heading of the method/abstract op, like streams does for throw/nothrow, like brandcheck(Map).

Does this sound interesting? If so I could work on a PR, because streams is getting really sick of defining IsStreamWhatever(x) abstract ops and repeating the boilerplate everywhere. Having ES lead the way would be great.