Thank you for your time edford, unfortunately I already saw this generic answer and it didn't help me.The project was in fact well configured, I solved the problem inside the code.The class of my application framework which extended the MySuperClass object from (S) didn't have the suffixe "S".I do not know why it was coded like that since it is not my code, neither why it worked before XCode 11.4, since it did not seem logic from the beginning, but whatever, I just needed to change the code from this :extension MySuperClass {}to this :extension S.MySuperClass {}"Fun" fact, in the extension of MySuperClass from (O), I did not have "O.MySuperClass" either, but adding it did not solve the problem.

Same here, I have to use Xcode 12.0.1 but I have these incompatibilities. Having two classes with the same names from two independent librairies should definitely not be an issue, a fix would be appreciated.