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

[Howard Hinnant]

On May 31, 2011, at 4:00 PM, Sean Hunt wrote:

> 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.

Thanks!

Howard