Warning: defaulted move assignment operator of X will move assign virtual base class Y multiple times

Question

I\'m catching a warning under Clang when testing a library under C++11. I\'ve never come across the warning before and searching is not providing too much in the way of reading

Answer 1

The standard allows implementations to choose a simple but sometimes broken way to handle memberwise assignment in the presence of virtual bases.

http://en.cppreference.com/w/cpp/language/move_assignment:

As with copy assignment, it is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined move assignment operator.

This is particularly nasty for move assignment, since it may mean assigning from an already moved-from member.