CMAKE_CXX_STANDARD vs target_compile_features?
Solution 1:
The newer alternative is definitely
target_compile_features(Foo PUBLIC cxx_std_20)
And with this you can and should remove the old set(CMAKE_CXX_STANDARD*).
However the new version has an issue if you also want to disable compiler extensions with set(CMAKE_CXX_EXTENSIONS OFF). Its not possible with the new syntax as far as I know and combining both of them does not work either.
And the second part of your question: Yes, cmake will recognize the dependencies and put the compiler in c++-20 mode whenever necessary.