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Regarding **Update 2**. It has been brought to my attention that if you explicitly-default the move constructor, it will be defined as deleted by the condition

> for the move constructor, a non-static data member or direct or virtual base class with a type that does not have a move constructor and is not trivially copyable.

In the following, the move constructor will be defined as **deleted**, because `CopyOnly` is not trivially copyable. 

    struct Question {
        std::vector<int> data_;
        CopyOnly         copyOnly_;

        Question(Question&&) = default;
    };