把pair的源码拷贝出来，起个新名字叫MyPair

template<class _Ty1,class _Ty2>struct MyPair
{	// store a MyPair of valuesusing first_type = _Ty1;using second_type = _Ty2;//
// 本文主要是改造这一行
//MyPair& operator=(const volatile MyPair&) = delete;template<class _Uty1 = _Ty1,class _Uty2 = _Ty2,enable_if_t<conjunction_v<is_default_constructible<_Uty1>,is_default_constructible<_Uty2>,_Is_implicitly_default_constructible<_Uty1>,_Is_implicitly_default_constructible<_Uty2>>, int> = 0>constexpr MyPair()_NOEXCEPT_COND(is_nothrow_default_constructible_v<_Uty1>&& is_nothrow_default_constructible_v<_Uty2>)	// strengthened: first(), second(){}template<class _Uty1 = _Ty1,class _Uty2 = _Ty2,enable_if_t<conjunction_v<is_default_constructible<_Uty1>,is_default_constructible<_Uty2>,negation<conjunction<_Is_implicitly_default_constructible<_Uty1>,_Is_implicitly_default_constructible<_Uty2>>>>, int> = 0>constexpr explicit MyPair()_NOEXCEPT_COND(is_nothrow_default_constructible_v<_Uty1>&& is_nothrow_default_constructible_v<_Uty2>)	// strengthened: first(), second(){}template<class _Uty1 = _Ty1,class _Uty2 = _Ty2,enable_if_t<conjunction_v<is_copy_constructible<_Uty1>,is_copy_constructible<_Uty2>,is_convertible<const _Uty1&, _Uty1>,is_convertible<const _Uty2&, _Uty2>>, int> = 0>constexpr MyPair(const _Ty1& _Val1, const _Ty2& _Val2)_NOEXCEPT_COND(is_nothrow_copy_constructible_v<_Uty1>&& is_nothrow_copy_constructible_v<_Uty2>)	// strengthened: first(_Val1), second(_Val2){}template<class _Uty1 = _Ty1,class _Uty2 = _Ty2,enable_if_t<conjunction_v<is_copy_constructible<_Uty1>,is_copy_constructible<_Uty2>,negation<conjunction<is_convertible<const _Uty1&, _Uty1>,is_convertible<const _Uty2&, _Uty2>>>>, int> = 0>constexpr explicit MyPair(const _Ty1& _Val1, const _Ty2& _Val2)_NOEXCEPT_COND(is_nothrow_copy_constructible_v<_Uty1>&& is_nothrow_copy_constructible_v<_Uty2>)	// strengthened: first(_Val1), second(_Val2){}template<class _Other1,class _Other2,enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>,is_constructible<_Ty2, _Other2>,is_convertible<_Other1, _Ty1>,is_convertible<_Other2, _Ty2>>, int> = 0>constexpr MyPair(_Other1&& _Val1, _Other2&& _Val2)_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>&& is_nothrow_constructible_v<_Ty2, _Other2>)	// strengthened: first(_STD forward<_Other1>(_Val1)),second(_STD forward<_Other2>(_Val2)){}template<class _Other1,class _Other2,enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>,is_constructible<_Ty2, _Other2>,negation<conjunction<is_convertible<_Other1, _Ty1>,is_convertible<_Other2, _Ty2>>>>, int> = 0>constexpr explicit MyPair(_Other1&& _Val1, _Other2&& _Val2)_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>&& is_nothrow_constructible_v<_Ty2, _Other2>)	// strengthened: first(_STD forward<_Other1>(_Val1)),second(_STD forward<_Other2>(_Val2)){}MyPair(const MyPair&) = default;MyPair(MyPair&&) = default;template<class _Other1,class _Other2,enable_if_t<conjunction_v<is_constructible<_Ty1, const _Other1&>,is_constructible<_Ty2, const _Other2&>,is_convertible<const _Other1&, _Ty1>,is_convertible<const _Other2&, _Ty2>>, int> = 0>constexpr MyPair(const MyPair<_Other1, _Other2>& _Right)_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, const _Other1&>&& is_nothrow_constructible_v<_Ty2, const _Other2&>)	// strengthened: first(_Right.first), second(_Right.second){}template<class _Other1,class _Other2,enable_if_t<conjunction_v<is_constructible<_Ty1, const _Other1&>,is_constructible<_Ty2, const _Other2&>,negation<conjunction<is_convertible<const _Other1&, _Ty1>,is_convertible<const _Other2&, _Ty2>>>>, int> = 0>constexpr explicit MyPair(const MyPair<_Other1, _Other2>& _Right)_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, const _Other1&>&& is_nothrow_constructible_v<_Ty2, const _Other2&>)	// strengthened: first(_Right.first), second(_Right.second){}template<class _Other1,class _Other2,enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>,is_constructible<_Ty2, _Other2>,is_convertible<_Other1, _Ty1>,is_convertible<_Other2, _Ty2>>, int> = 0>constexpr MyPair(MyPair<_Other1, _Other2>&& _Right)_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>&& is_nothrow_constructible_v<_Ty2, _Other2>)	// strengthened: first(_STD forward<_Other1>(_Right.first)),second(_STD forward<_Other2>(_Right.second)){}template<class _Other1,class _Other2,enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>,is_constructible<_Ty2, _Other2>,negation<conjunction<is_convertible<_Other1, _Ty1>,is_convertible<_Other2, _Ty2>>>>, int> = 0>constexpr explicit MyPair(MyPair<_Other1, _Other2>&& _Right)_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>&& is_nothrow_constructible_v<_Ty2, _Other2>)	// strengthened: first(_STD forward<_Other1>(_Right.first)),second(_STD forward<_Other2>(_Right.second)){}template<class _Tuple1,class _Tuple2,size_t... _Indexes1,size_t... _Indexes2> inlineMyPair(_Tuple1& _Val1,_Tuple2& _Val2,index_sequence<_Indexes1...>,index_sequence<_Indexes2...>);template<class... _Types1,class... _Types2> inlineMyPair(piecewise_construct_t,tuple<_Types1...> _Val1,tuple<_Types2...> _Val2);template<class _Other1 = _Ty1,class _Other2 = _Ty2,enable_if_t<conjunction_v<is_assignable<_Ty1&, const _Other1&>,is_assignable<_Ty2&, const _Other2&>>, int> = 0>MyPair& operator=(const MyPair<_Other1, _Other2>& _Right)_NOEXCEPT_COND(is_nothrow_assignable_v<_Ty1&, const _Other1&>&& is_nothrow_assignable_v<_Ty2&, const _Other2&>)	// strengthened{first = _Right.first;second = _Right.second;return (*this);}template<class _Other1 = _Ty1,class _Other2 = _Ty2,enable_if_t<conjunction_v<is_assignable<_Ty1&, _Other1>,is_assignable<_Ty2&, _Other2>>, int> = 0>MyPair& operator=(MyPair<_Other1, _Other2>&& _Right)_NOEXCEPT_COND(is_nothrow_assignable_v<_Ty1&, _Other1>&& is_nothrow_assignable_v<_Ty2&, _Other2>)	// strengthened{first = _STD forward<_Other1>(_Right.first);second = _STD forward<_Other2>(_Right.second);return (*this);}void swap(MyPair& _Right)_NOEXCEPT_COND(_Is_nothrow_swappable<_Ty1>::value&& _Is_nothrow_swappable<_Ty2>::value){if (this != _STD addressof(_Right)){	// different, worth swapping_Swap_adl(first, _Right.first);_Swap_adl(second, _Right.second);}}_Ty1 first;		// the first stored value_Ty2 second;	// the second stored value
};

测试代码：
MyPair<const int, int> t0;
MyPair<int, int> t1;
t0 = t1;编译错误：
error C2679: 二进制“=”: 没有找到接受“MyPair<int,int>”类型的右操作数的运算符(或没有可接受的转换)
note: 可能是“MyPair<const int,int> &MyPair<const int,int>::operator =(volatile const MyPair<const int,int> &)”
note: 尝试匹配参数列表“(MyPair<const int,int>, MyPair<int,int>)”时

尝试把delete的MyPair的operator=给个实现呢：
把MyPair& operator=(const volatile MyPair&)=delete;
改为：
MyPair& operator=(const volatile MyPair&)
{return (*this);
}编译错误为：
error C2679: 二进制“=”: 没有找到接受“MyPair<int,int>”类型的右操作数的运算符(或没有可接受的转换)
note: 可能是“MyPair<const int,int> &MyPair<const int,int>::operator =(volatile const MyPair<const int,int> &)”
note: 尝试匹配参数列表“(MyPair<const int,int>, MyPair<int,int>)”时因为左边的MyPair实例化的是<const int,int>，而
MyPair& operator=(const volatile MyPair&)里括号里的MyPair里实例化的是<int,int>，
按照函数的写法，这两者应该一样。但此时不一样，所以编译错误。

下面编译成功。也就是把operator=采用模板函数，括号里的形参进行单独推导把MyPair& operator=(const volatile MyPair&) = delete;
改为：template<typename T, typename U>MyPair& operator=(const volatile MyPair<T, U>& _Right){//first = _Right.first;//second = _Right.second;return (*this);}

template<typename T, typename U>MyPair& operator=(const volatile MyPair<T, U>& _Right){
//强行把this的first的const给去掉，就可以被参数给赋值了using decayType = std::decay_t<_Ty1>;decayType& _first = const_cast<decayType&>(first);_first = _Right.first;second = _Right.second;return (*this);}测试代码：
编译通过，t0的内容就是(3,4)，被成功赋值MyPair<const int, int> t0{1,2};
MyPair<int, int> t1{ 3,4 };
t0 = t1;
return 1;