class IName{
public:
virtual QString GetName() const = 0;
}

class ITaskName{
public:
virtual QString GetName() const = 0;
}

class CName: virtual public IName{
public:
virtual QString GetName() const { return m_name; }
}

class CTask: virtual public CName, virtual public ITaskName
{
public:
using CName::GetName;
}

class CTask
...
virtual const QString GetName() const { return CName::GetName(); }
...

/usr/bin/g++ --version
g++ (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4

/usr/bin/g++ -g -O0 -Wall -Wextra -m64 -pipe -fexceptions -fvisibility=default -fPIC -DQT_DEPRECATED_WARNINGS -DQT_CORE_LIB -I/home/abbapoh/Qt/5.9/gcc_64/include -I/home/abbapoh/Qt/5.9/gcc_64/include/QtCore -I/home/abbapoh/Qt/5.9/gcc_64/mkspecs/linux-g++ -I/home/abbapoh/programming/build-cpptest-Desktop_Qt_5_9_0_GCC_64bit-Debug/qtc_Desktop__baa544a4-debug/cpptest.qtc-Desktop--baa544a4.063e55c3/qt.headers -std=c++0x -o /home/abbapoh/programming/build-cpptest-Desktop_Qt_5_9_0_GCC_64bit-Debug/qtc_Desktop__baa544a4-debug/cpptest.qtc-Desktop--baa544a4.063e55c3/.obj/3a52ce780950d4d9/main.cpp.o -c /home/abbapoh/programming/cpptest/main.cpp
/home/abbapoh/programming/cpptest/main.cpp: In function ‘void g()’:
/home/abbapoh/programming/cpptest/main.cpp:43:8: warning: unused variable ‘dr’ [-Wunused-variable]
     D& dr = dynamic_cast<D&>(*bp); // fails
        ^
/home/abbapoh/programming/cpptest/main.cpp: In function ‘int main(int, char**)’:
/home/abbapoh/programming/cpptest/main.cpp:75:11: error: cannot declare variable ‘t’ to be of abstract type ‘CTask’
     CTask t;
           ^
/home/abbapoh/programming/cpptest/main.cpp:65:7: note:   because the following virtual functions are pure within ‘CTask’:
 class CTask: virtual public CName, virtual public ITaskName
       ^
/home/abbapoh/programming/cpptest/main.cpp:57:17: note:    virtual QString ITaskName::GetName() const
 virtual QString GetName() const = 0;
                 ^

Однако компилятор упорно сообщает, что не может создать абстракный класс CTask.