This is only 20% relevant, but I've been messing with the Vim extension YouCompleteMe, expecting very intelligent completion based on the syntax of the language.

However, at least for C++ code, it seems like just a dumb symbol-lookup, suggesting only other symbols in the file, regardless of whether they syntactically make sense or not.

I must be doing something wrong but I don't know what. It seems to do a better job with Python, where it knows about keywords and built in functions.

(fyi, compiled with Clang Complete enabled).