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

[Sean Hunt]

On 05/31/11 11:26, Sean Hunt wrote:
> On 05/31/11 10:53, Howard Hinnant wrote:
>> 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
>
> We definitely shouldn't be instantiating swap here. Looks like a bug.
> I'll poke at it a bit.
>
> Sean

Fixed in r132350.

Sean