Symptom:

1. Run qmake to generate the Makefile
2. Change QMAKE_CXXFLAGS inside a .pri file that is included in the .pro file
3. Run "make"

The Makefile is correctly regenerated and CXXFLAGS in the Makefile reflects the change.

However, the C++ source files were not re-compiled.

Analysis:

The .o files in the generated Makefile do not depend on the Makefile itself. So even though the Makefile is regenerated, this does not trigger a remake of the object files.

Comment:

I hope this is considered a bug. If the answer is that one is supposed to re-run qmake after a change of a .pri file, my question would be: Why is there a rule to regenerate the Makefile in the first place?

Files to reproduce the problem:
$ tree
.
├── hello.pro
├── main.cpp
└── other.pri

// main.cpp
int main(void)

{ return 42; }

#: hello.pro
include(other.pri)
SOURCES += main.cpp

#: other.pri
linux

{ QMAKE_CXXFLAGS_WARN_ON += -Wno-unused-function #: QMAKE_CXXFLAGS_WARN_ON += -W__THIS_DIAGNOSTIC_DOES_NOT_EXIST__ }

How to reproduce:

$ qmake

$ make
g++ -c -pipe -O2 -Wall -W -Wno-unused-function -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -isystem /usr/include/libdrm -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
g++ -Wl,-O1 -o hello main.o -lQt5Gui -lQt5Core /usr/lib/x86_64-linux-gnu/libGL.so -lpthread

$ ... change other.pri to include the -W__THIS_DIAGNOSTIC_DOES_NOT_EXIST ...

$ make
/usr/lib/qt5/bin/qmake -o Makefile hello.pro
make: Nothing to be done for 'first'.

As you see the Makefile was regenerated, but the main.cpp was not recompiled.