C++中关于返回局部变量的引用的问题

Question

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

#ifndef MYTIME0_H_
#define MYTIME0_H_

class Time
{
private:
    int hours;
    int minutes;
public:
    Time();
    Time(int h, int m = 0);
    void AddMin(int m);
    void AddHr(int h);
    void Reset(int h = 0, int m = 0);
     Time &Sum(const Time & t) const;//！！
    void Show() const;
};
#endif

//头文件实现函数
// mytime0.cpp  -- implementing Time methods
#include <iostream>
#include "mytime0.h"

Time::Time()
{
    hours = minutes = 0;
}

Time::Time(int h, int m )
{
    hours = h;
    minutes = m;
}

void Time::AddMin(int m)
{
    minutes += m;
    hours += minutes / 60;
    minutes %= 60;
}

void Time::AddHr(int h)
{
    hours += h;
}

void Time::Reset(int h, int m)
{
    hours = h;
    minutes = m;
}

Time &Time::Sum(const Time & t) const    //！！
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}

void Time::Show() const
{
    std::cout << hours << " hours, " << minutes << " minutes";
}


// usetime0.cpp -- using the first draft of the Time class
// compile usetime0.cpp and mytime0.cpp together
#include <iostream>
#include "mytime0.h"

int main()
{
    using std::cout;
    using std::endl;
    Time planning;
    Time coding(2, 40);
    Time fixing(5, 55);
    Time total;

    cout << "planning time = ";
    planning.Show();
    cout << endl;
 
    cout << "coding time = ";
    coding.Show();
    cout << endl;
    
    cout << "fixing time = ";
    fixing.Show();
    cout << endl;

    total = coding.Sum(fixing);
    cout << "coding.Sum(fixing) = ";
      total.Show();
    cout << endl;
    // std::cin.get();
    return 0;
}

运行结果及报错内容

类方法Sum（），在里面创建了一个sum局部变量，但返回了它的引用，按理说应该是错误的。
但是却能跑通，结果也正确。这是为什么？

我的解答思路和尝试过的方法

我想要达到的结果

Answer 1

那只是侥幸能跑，并不代表每次都能正常跑，各个编译器上都能跑