This feels like it has the potential to become a pretty subtle sharp edge for macro implementors. The difficulty we're having in naming this parameter is indicative of what a narrow concept we're trying to describe, and I'm wondering if we might be better served by revisiting the discussion of whether extensions that appear in the same source file as the original type declaration ought to be fully privileged and behave exactly like being part of the original type.
In past discussions I've been against such a change, but if we're getting to the point of abstracting over these restrictions in the macro layer, I might be convinced that it's time to relax them.