Ohh that is awesome , thank you so much ! I can confirm that using the BuildConfiguration . bAllowLTCG flag my issue is solved for Shipping builds . Now that leads me to some more questions , if you don ' t mind Why does this flag only works in Shipping builds , but not Development builds ? Why would I not want to enable this flag for such Development builds ? Is there a way to force this flag also for Development builds ? Moreover , when I compile a Debug build of my . lib and use that , then I get the following error when compiling the UE 4 projet with it , even when using the Development build configurationerror LNK 2038 mismatch detected for ' RuntimeLibrary ' value ' MDdDynamicDebug ' doesn ' t match value ' MDDynamicRelease ' in CatAlphaDemo . h . objAny ideas how to solve that problem too ? In any case thanks again for your help !