Not GP, but one the one hand semgrep has a real honest to goodness parser at its core; on the other hand I'd expect C++ to have sufficiently complicated semanti

Not GP, but one the one hand semgrep has a real honest to goodness parser at its core; on the other hand I'd expect C++ to have sufficiently complicated semantics that it needs some understanding of C++ specific mechanics to be useful. Furthermore, you'd need preprocessor and template expansion magic to really get to the bottom of it. Effectively this is the same problem e.g. javacpp has.