> a static_cast that bypasses private inheritance (because the alternative would be to fall back to a reinterpret_cast, which is wrong if the static_cast needs to apply an offset to the pointer)

That may well have been it, then. I would think that if it could have been expressed naturally, it would have been.

(I don't think I ever used private inheritance in my own C++ code. I'm not a huge fan of inheritance at all, so.)