问是什么阻止编译器优化重新排序强异常安全的代码？

EN

#include <algorithm> // std::swap

template< typename T1, typename T2 >
class Cargill_Widget
{
public:
    Cargill_Widget& operator=( Cargill_Widget const& r_other )
    {
        using std::swap;

        T1 temp_t1( r_other.m_t1 ); // may throw
        T2 temp_t2( r_other.m_t2 ); // may throw
        /* The strong exception-safety line */
        swap( m_t1, temp_t1 ); // no throw
        swap( m_t2, temp_t2 ); // no throw

        return *this;
    }

private:
    T1 m_t1;
    T2 m_t2;
};