I am reading the source code of std::pair implemented by GCC. And I got confused by the redundant typedef as below. I know I must miss something, but just a few functions use first_type and second_type, like the move assignment operator. Why doesn’t it just use _T1 and _T2? Thank you.

template<typename _T1, typename _T2>
struct pair
: public __pair_base<_T1, _T2>
{
    typedef _T1 first_type;    ///< The type of the `first` member
    typedef _T2 second_type;   ///< The type of the `second` member

    _T1 first;                 ///< The first member
    _T2 second;                ///< The second member

    /// Move assignment operator
    constexpr pair&
    operator=(pair&& __p)
    noexcept(_S_nothrow_assignable<_T1, _T2>())
    requires (_S_assignable<_T1, _T2>())
    {
        first = std::forward<first_type>(__p.first);
        second = std::forward<second_type>(__p.second);
        return *this;
    }
};