About cloning:

Cloned functions still expect their own impl function.  In current winreg, OpenKey and OpenKeyEx literally are the same function by two names; is the best approach for this to define winreg_OpenKeyEx_impl as `return winreg_OpenKey_impl(module, key, sub_key, reserved, access);`?

As stated in #20226, that patch works fine with my conversions.  Once the 20226 patch lands, I'll get a comprehensive patch for this issue posted.  Until then, I've branched the sandbox repo yet again; future_clinic_20172 contains the 20226 patch, along with the necessary updates to these files and further changes using the new features that aren't in trunk yet.