Indentation is a good point, you need a proper parse for that. But for that you don't need the ability to consult symbol tables, or resolve nasty C++ template metaprogramming stuff. A context-free grammar suffices, and C++ has that. C++ also has some very complicated (undecidable) rules for what programs are well-formed and should be accepted by the compiler, but the editor's indentation tool doesn't need to worry about those.