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

Avi Kivity avi@scylladb.com
Sat Apr 22 19:24:00 GMT 2017


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?



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