Undefined reference error when linking dynamic and static libraries consequently - c++

Question

I have a QT project with 4 subprojects with the following names: exec (executable), sharedlib (dynamic library), sharedlib2 (other one dynamic library) and staticlib. All this is linked consequently as follows:
exec->sharedlib2->sharedlib->staticlib
In the static library is defined Staticlib class.
Now, when compiling all of this, I'm getting undefined reference error when trying to use that class (in exec project, main.cpp):
/home/user/temp/libproj/libproj/exec/main.cpp:9: error: undefined reference to `Staticlib::Staticlib()'
What am I doing wrong?
staticlib.h:
#ifndef STATICLIB_H
#define STATICLIB_H
#include <QDebug>
class Staticlib
{
public:
Staticlib();
void debugprint();
};
#endif // STATICLIB_H
sharedlib.pro:
QMAKE_LFLAGS += -Wl,--whole-archive,--allow-multiple-definition
LIBS += -L$$OUT_PWD/../staticlib/
LIBS += -lstaticlib
sharedlib2.pro:
LIBS += -L$$OUT_PWD/../sharedlib/
LIBS += -lsharedlib
exec.pro:
INCLUDEPATH += $$PWD/../staticlib
#doesnt work
LIBS += -L$$OUT_PWD/../sharedlib2
LIBS += -lsharedlib2
#works well
#//LIBS += -L$$OUT_PWD/../sharedlib/
#//LIBS += -lsharedlib
exec, main.cpp:
#include <QCoreApplication>
#include "staticlib.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Staticlib statlib;
statlib.debugprint();
return a.exec();
}
staticlib.pro
QT -= gui
TEMPLATE = lib
CONFIG += staticlib
CONFIG += c++17
SOURCES += \
staticlib.cpp
HEADERS += \
staticlib.h
staticlib.cpp
#include "staticlib.h"
#include <QDebug>
Staticlib::Staticlib()
{
}
void Staticlib::debugprint()
{
qDebug() << "Staticlib::debugprint()";
}