Wow, thanks for the detailed response! I wasn’t expecting a cmake bug…
The 'single evaluation of $<TARGET_PROPERTY:combined,COMPILE_DEFINITIONS>’ is very awkward. I think in cmake 3.15+ the FILTER generator expression would work. However, I can’t think of a way of achieving anything similiar in 3.12, as the list-processing generator expressions are more limited in that version. I’m going to sleep on it, but in the meantime, if anyone has any idea how this might be achieved in 3.12, I’d be incredibly grateful!