[cfe-dev] swap, noexcept and non-movable types

[Howard Hinnant]

Consider this C++0x code:

template <class T>
T&&
declval() noexcept;

template <class T>
struct some_trait
{
    static const bool value = false;
};

template <class T>
void swap(T& x, T& y) noexcept(some_trait<T>::value)
{
    T tmp(static_cast<T&&>(x));
    x = static_cast<T&&>(y);
    y = static_cast<T&&>(tmp);
}

template <class T, unsigned N>
struct array
{
    T data[N];

    void swap(array& a) noexcept(noexcept(swap(declval<T&>(), declval<T&>())));
};

struct DefaultOnly
{
    DefaultOnly() = default;
    DefaultOnly(const DefaultOnly&) = delete;
    DefaultOnly& operator=(const DefaultOnly&) = delete;
    ~DefaultOnly() = default;
};

int main()
{
    array<DefaultOnly, 1> a, b;
}

We currently do not compile this code.  The errors are:

test.cpp:14:7: error: call to deleted constructor of 'DefaultOnly'
    T tmp(static_cast<T&&>(x));
      ^   ~~~~~~~~~~~~~~~~~~~
test.cpp:24:43: note: in instantiation of function template specialization 'swap<DefaultOnly>' requested here
    void swap(array& a) noexcept(noexcept(swap(declval<T&>(), declval<T&>())));
                                          ^
test.cpp:30:5: note: function has been explicitly marked deleted here
    DefaultOnly(const DefaultOnly&) = delete;
    ^
test.cpp:15:7: error: overload resolution selected deleted operator '='
    x = static_cast<T&&>(y);
    ~ ^ ~~~~~~~~~~~~~~~~~~~
test.cpp:31:18: note: candidate function has been explicitly deleted
    DefaultOnly& operator=(const DefaultOnly&) = delete;
                 ^
2 errors generated.

I recently learned that a recent gcc 4.7 snapshot /does/ compile this code.

So I'm asking the language lawyers here:  should we compile this code or not?

The underlying question is whether or not swap<DefaultOnly>() should be instantiated.  It is not called except as an unevaluated operand.

Thanks,
Howard