Why is my defaulted move constructor not noexcept?

I believe the problem is that you defaulted move constructor of D is private. Try to make it public.

In fact it has nothing to do with noexcept; static_assert would fail also with std::is_move_constructible because the move constructor is private. So just declare it as public.

class D {
public:
    D(D&&) = default;
};

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