Comment by lmitchell

The only thing 'unspecified' guarantees you in this context is 'safe to destroy'. It specifically does not guarantee the safety of any other member functions - only the destructor. So either push_back() or pop_back() could potentially cause UB here (specifically, it's quite possible that the move swaps some internal pointers for nullptr, so you end up dereferencing null here - but morally, it's just never okay to continue using a moved-from object).