I extended a bit my the simplest project ever I met the issue that QDateTime is not properly parsed.
First manifestation is "bad" colorizing. Class name has the same color as preprocesor directive. Second manifestation is that when I move mouse cursor over this class name then nothing happen.
Because QDateTime is located in QtCore I also added into Includes/Imports (in project configuration) path to QtCore (for me this is /usr/include/Qt/QtCore where is bunch of headers) and reparsed the project. Not helped.

Below please find the only one function from this project. For your convenience I also attached archive containing this project.

#include <QCoreApplication>
#include <QDateTime>
#include <QString>

int main(int argc, char** argv)
{
	QCoreApplication app(argc, argv);

	QDateTime dt;
	dt.addDays(1);

	QString s;

	return app.exec();
}

Please note that when I write "dt." then I'm getting the list of method from QDateTime class.
Of course method "addDays" is also not parsed and when I move mouse cursor over it then nothing happen.

Reproducible: Always

Steps to Reproduce:
1. Clear ~/.cache/kdevduchain cache
2. Run KDevelop
3. Open attached project
4. Open file main.cpp
5. Move mouse cursor over QDateTime

Actual Results:  
nothing happen

Expected Results:  
should be shown tool tip referring to QDateTime

Kate, KTextEditor, KDevelop, KDevplatform cloned at 04.11.2015

I'm not sure, but maybe the reason is the same like in Bug 354846 - QCollator class name is not colorized in global function