'Sendable' class 'ABC' cannot inherit from another class other than 'NSObject'

I got this error in a package which compiled perfectly up to recently.

There is a public inner class in it that inherit form an open master class in an other package. Sure, if I cut the inheritance the problem is also cut off.

Someone would have and idea on it?
Thanks in advance

Does anybody have an idea about this issue. I have found a bug report (#2146) that reported a similar problem.