C ++：隐式成员函数

考虑下面我编写的代码：

#include <iostream>

using namespace std;

class Sample{
    int a;
    public:
    Sample(){a=0;cout << "Sample Created (Default Constructor)" << endl;}
    Sample(int n):a(n){cout << "Sample Created" << endl;}
    ~Sample(){ cout << "Sample destroyed" << endl;}
    Sample(Sample& s){a=s.getA(); cout << "Sample Copy Constructor called" << endl;}
    Sample& operator= (Sample& s){this->a=s.getA(); cout << "Assignment Operator Called" << endl;return (*this);}
    void setA(int n){ a=n;}
    int getA(){return a;}
    };

class Test{
    Sample k;
    public:
    Test(){ cout << "Test Created(Default Constructor)" << endl;}
    Test(Sample& S):k(S){ cout << "Test Created" << endl;}
    ~Test(){cout << "Test Destroyed" << endl;}
    Test& operator= (Test& test){ k = test.getK(); cout << "Test Assignement Operator called" << endl; return (*this); } // Here 1
    Test(Test& test){k=test.getK();cout << "Test Copy Constructor called" << endl;}
    Sample getK(){return k;} // Here 2
    void setK(Sample& s){k=s;}
    };

int main()
{
    Sample a1(5);
    //Sample a2,a4;
    //a2=a1=a4;
    //Sample a3(a2);
    Test b1(a1);
    Test b2=b1;
    //b2=b1;

    return 0;
    }

编译时出现以下错误：

$ g++ -Wall Interview.cpp -o Interview
Interview.cpp: In member function `Test& Test::operator=(Test&)':
Interview.cpp:23: error: no match for 'operator=' in '((Test*)this)->Test::k = Test::getK()()'
Interview.cpp:12: note: candidates are: Sample& Sample::operator=(Sample&)
Interview.cpp: In copy constructor `Test::Test(Test&)':
Interview.cpp:24: error: no match for 'operator=' in '((Test*)this)->Test::k = Test::getK()()'
Interview.cpp:12: note: candidates are: Sample& Sample::operator=(Sample&)

当我对here 2as进行更改时，Sample& getK(){return k;}它可以完美编译。

有人可以解释为什么吗？

此外，在here 1如果该函数被定义为Test& operator= (const Test& test){ k = test.getK(); cout << "Test Assignement Operator called" << endl; return (*this); }

我遇到错误-

$ g++ -Wall Interview.cpp -o Interview
Interview.cpp: In member function `Test& Test::operator=(const Test&)':
Interview.cpp:23: error: passing `const Test' as `this' argument of `Sample& Test::getK()' discards qualifiers

为什么这样？