Steps to reproduce:
1. Open the attached simple project where a class derives from QObject and has Q_OBJECT macro inside its body.
2. Compile it with GCC (G++) with -Wuseless-cast (already in CMakeLists.txt).

Expected result:
Successful compilation with no warnings or errors.

Actual result:
The compiler displays a warning in the MOC generated function
    Foo::qt_metacast(const char *_clname)
in line
    return static_cast<void*>(const_cast< Foo*>(this));

Full warning:

D:\dev\project\test\useless-cast-test\main.cpp:13: In file included from D:/dev/project/test/useless-cast-test/main.cpp:13:0:
D:\dev\project\test\build-useless-cast-test-Qt_5_9_1_MinGW_x64_GCC_7_2-Debug\useless-cast-test_autogen\include\main.moc:-1: In member function 'virtual void* Foo::qt_metacast(const char*)':
D:\dev\project\test\build-useless-cast-test-Qt_5_9_1_MinGW_x64_GCC_7_2-Debug\useless-cast-test_autogen\include\main.moc:79: warning: useless cast to type 'Foo*' [-Wuseless-cast]
return static_cast<void*>(const_cast< Foo*>(this));
                                                                              ^

The problem is worse when you compile with -Werror to treat every warning as error, because then it's not obvious how to fix this issue.