Using Interface class in Qt Object causes linker errors LNK2001 - c++

Question

I'm having strange linkage problems with the following very simple application, which has a class inheriting both QObject and an interface class.
#include <QApplication>
#include <memory>
#include <iostream>
#include <qobject>
class IFoo {
public:
virtual ~IFoo() {}
virtual void foo()=0;
};
class Foo: public QObject, public IFoo
{
Q_OBJECT
public:
explicit Foo(QObject *parent=0): QObject(parent) { std::cout << "foo ctor" << std::endl; }
void foo() override { std::cout << "foo::foo" << std::endl; }
};
std::unique_ptr<IFoo> createFoo() { return std::unique_ptr<IFoo>(new Foo()); }
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
auto foo = createFoo();
return a.exec();
}
This causes the following errors:
LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl Foo::metaObject(void)const " (?metaObject#Foo##UEBAPEBUQMetaObject##XZ)
LNK2001: unresolved external symbol "public: virtual void * __cdecl Foo::qt_metacast(char const *)" (?qt_metacast#Foo##UEAAPEAXPEBD#Z)
LNK2001: unresolved external symbol "public: virtual int __cdecl Foo::qt_metacall(enum QMetaObject::Call,int,void * *)" qt_metacall#Foo##UEAAHW4Call#QMetaObject##HPEAPEAX#Z)
LNK1120: 3 unresolved externals
I have obviously tried run qmake, clean and rebuild without effect. If I make the class Foo a plain C++ class by removing all Qt references, it works correctly.
What could be wrong?