What about conditionally compiled code, based on preprocessor definitions? And what happens when you change one of those definitions in the editor? Visual Studio, for example, will re-scan the source in the background.

And what about tokens created using the token pasting operator ## from include files that are included multiple times (e.g. code generation hackery)? Visual Studio will at least locate the include file which introduced the token, and of course will be able to complete on the token, aware of its type etc.