This is the mail archive of the
gcc-help@gcc.gnu.org
mailing list for the GCC project.
Re: std::experimental::optional::swap() vs std::swap()
On 22 April 2017 at 21:34, Jonathan Wakely <jwakely.gcc@gmail.com> 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;