std::swap noexcept workaround is inaccurate

[martinmoene]

See expected-dark issue 14.

[martinmoene]

This now is solved by using std(17)::is_nothrow_swappable<> as written in p0323r7.

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A solution to this problem involving ADL can be seen in Vicente J. Botet Escriba's expected.

See also this SO question.

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namespace adl {

using std::swap;

template <class T, class U>
void swap_impl( T& x, U& y ) noexcept( noexcept( swap(x, y) ) )
{
    swap(x, y);
}
}

// expected<T,E>
// expected<void,E>
// member operator=()
// member emplace()
// member swap()

template <class T, class E>
void expected<T,E>::swap( expected& rhs ) noexcept
(
    is_nothrow_move_constructible<T>::value
    && is_nothrow_move_constructible<E>::value 
    && noexcept( adl::swap_impl( declval<T&>(), declval<T&>() ) )
    && noexcept( adl::swap_impl( declval<E&>(), declval<E&>() ) )
)
{
    ...
}