qmake doesn't invoke uic - c++

Question

I'm aware that there is a couple of questions asked online on similar issues but unfortunately none of them helped me solving this relatively simple-looking one, as the title suggests.
I set up a minimal Qt project in which the problem occurs:
├── main.cpp
├── mainwindow.ui
└── qt.pro
main.cpp:
#include <QtWidgets/QtWidgets>
#include "ui_mainwindow.h"
int main () {
return 0;
}
qt.pro:
TEMPLATE = app
TARGET = qt-qmake-uic-problem
INCLUDEPATH += .
# Input
FORMS += mainwindow.ui
SOURCES += main.cpp
The file mainwindow.ui is the default MainWindow Form provided by Qt Creator.
Now if I run
qmake qt.pro
then a Makefile is created causing this error when make gets executed:
[developer#kdb qt-qmake-uic-problem]$ make
g++ -c -pipe -O2 -march=i686 -mtune=generic -O2 -pipe -fstack-protector-strong -Wall -W -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/qt -isystem /usr/include/qt/QtGui -isystem /usr/include/qt/QtCore -I. -I/usr/lib/qt/mkspecs/linux-g++ -o main.o main.cpp
main.cpp:2:28: fatal error: ui_mainwindow.ui: No such file or directory
#include "ui_mainwindow.h"
^
compilation terminated.
make: *** [Makefile:298: main.o] Error 1
According to my best knowledge ui_mainwindow.h must be generated either by qmake or by the Makefile - not by myself invoking uic manually. I must note that something similar happens with moc as well and doing my research I haven't found an explanation neither in the documentation nor in similar issues, although it would be strange if this installation of qmake were broken - I'm running Arch Linux and got Qt SDK from the default repository.
Thank you very much for your help in advice!

Answer 1

Assuming you are using QT-5:
If you want to use UI elements linke a QMainWindow or other QT widgets, you need to enable widgets generally in your project configuration:
QT += widgets
This then activates the processing of the FORMS content like mainwindow.ui in your example, and causes that your code gets linked with the correct libraries.
See also here: http://doc.qt.io/qt-5/qtwidgets-index.html