windows – Qt create and use self-made static library - c++

Question

There are a few questions which seem to be similar, but nothing really helps me out. I want to create a static library inside a project and use it in the same project, but linking error occurs.
A good example, which meets my conditions very well is attached to the Qt Ticket QTBUG-45706 https://bugreports.qt.io/browse/QTBUG-45706. In a simple explanation, we have an app which should use some self-made libraries. Just modifiy a few things to see my problem.
app -> main.cpp
#include <QCoreApplication>
#include <lib.h>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Lib l1;
return a.exec();
}
lib.pro
CONFIG += staticlib
If you now compile the project, you will see the following error
main.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl Lib2::Lib2(void)" (__imp_??0Lib2##QEAA#XZ) referenced in function main
Use Qt Creator 4.0.3 based on Qt 5.6.1, qmake with mscv2013
What is needed to bring this to work?
CLARIFY:
The project structure is as follow:
subdirs_test.pro (subdir project)
\- app (app project, includes lib and lib2)
\-- app.pro
\-- main.cpp
\- lib (static library)
\-- lib.pro
\-- lib.h
\-- lib_global.h
\-- lib.cpp
\- lib2 (static library)
\-- lib2.pro
\-- lib2.h
\-- lib2_global.h
\-- lib2.cpp
The 'app' project should use the classes from lib and lib2, which are static libraries.
As suggested, use the "Add Library..." doesn't change a thing. In my case, this code will be generated.
win32:CONFIG(release, debug|release): LIBS += -L$$OUT_PWD/../lib/release/ -llib
else:win32:CONFIG(debug, debug|release): LIBS += -L$$OUT_PWD/../lib/debug/ -llib
else:unix: LIBS += -L$$OUT_PWD/../lib/ -llib
INCLUDEPATH += $$PWD/../lib
DEPENDPATH += $$PWD/../lib
win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$OUT_PWD/../lib/release/liblib.a
else:win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$OUT_PWD/../lib/debug/liblib.a
else:win32:!win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$OUT_PWD/../lib/release/lib.lib
else:win32:!win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$OUT_PWD/../lib/debug/lib.lib
else:unix: PRE_TARGETDEPS += $$OUT_PWD/../lib/liblib.a

Answer 1

You can use QtCreators Subdirs project. Here's a detailed step by step instructions how to achieve that with QtCreator.
Pick Subdirs Project from the New Project wizard menu.
Add Subrojects by clicking on created Subdirs project with right
mouse button and selecting New Subproject....
By following wizards you should have a GUI or console Subproject and
a library Subproject. Then click on subproject where you want to link
your library subproject with right mouse button and select Add
Library....
Select Internal library in the dialog and you will be prompted to
choose library you want to add.
Make sure your library subproject is included before gui/console
subproject as subdir project will fail to build.
TEMPLATE = subdirs
SUBDIRS += \
LibProject \
CoreProject

Answer 2

Can you try doing next steps:
Right button on project
Add library
Choose type (external or other)
Set flag on static, like this picture