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

[Avi Kivity <avi@xxxxxxxxxxxx>]

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?