Re: std::experimental::optional::swap() vs std::swap()

[Jonathan Wakely <jwakely.gcc@xxxxxxxxx>]

On 22 April 2017 at 21:34, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
> On 22 April 2017 at 20:24, Avi Kivity wrote:
>> libstdc++'s optional::swap reads as follows:
>>
>>
>>       void
>>       swap(optional& __other)
>>       noexcept(is_nothrow_move_constructible<_Tp>()
>>                && noexcept(swap(declval<_Tp&>(), declval<_Tp&>())))
>>       {
>>
>>
>> When compiling with clang, it complains that swap() (called from the
>> noexcept operator) accepts only one argument.  Isn't it correct? shouldn't
>> single-argument member swap() hide the two-argument non-member swap?
>>
>>
>> A few lines below, libstdc++ continues:
>>
>>
>>         using std::swap;
>>
>>         if (this->_M_is_engaged() && __other._M_is_engaged())
>>           swap(this->_M_get(), __other._M_get());
>>         else if (this->_M_is_engaged())
>>           {
>>             __other._M_construct(std::move(this->_M_get()));
>>             this->_M_destruct();
>>           }
>>         else if (__other._M_is_engaged())
>>           {
>>             this->_M_construct(std::move(__other._M_get()));
>>             __other._M_destruct();
>>           }
>>       }
>>
>> So it's explicitly bringing std::swap into scope here, but it's too late for
>> the expression in the noexcept operator.
>>
>>
>> Is this a bug in libstdc++ (and in gcc for not detecting it), or in clang?
>
> Yes, it's a libstdc++ bug, similar to
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63860
>
> It's a G++ bug for not detecting it (we do lookup in exception
> specifications incorrectly).
>
> We have __is_nothrow_swappable now, so should be using that there,
> could you report it to bugzilla please?

The fix is:

--- a/libstdc++-v3/include/experimental/optional
+++ b/libstdc++-v3/include/experimental/optional
@@ -690,7 +690,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void
       swap(optional& __other)
       noexcept(is_nothrow_move_constructible<_Tp>()
-               && noexcept(swap(declval<_Tp&>(), declval<_Tp&>())))
+               && __is_nothrow_swappable<_Tp>::value)
       {
         using std::swap;