What would you do in the case of pointers to functions, virtual functions, functions implemented with inline assembly, and functions for which the source is not available?
Just like we don't know if the derived class overrides a given function, we could do something similar i suppose? Similar could be applied to functions without source, after all we don't know until linking that the provided function has the right signature. I am sorry if these doesn't make sense for a compiler writer, i have no idea about the process :)
