Steps to reproduce:

1. Create a new Qt Widgets Application
2. Open mainwindow.cpp
3. The screen contains error and warning annotations about incomplete type 'Ui::MainWindow'
4. Build the project
5. Important: change the file mainwindow.cpp (e.g. by adding a space)
6. The error and warning messages disappear

The procedure repeats if you e.g. add a Label to the user interface and want to access it in source code. After compiling, the error disappears and code completion works.

Note that this works flawlessly when Clang Code Model is disabled.