Understanding implicitly defined copy and move constructor c++11 [duplicate]

This question already has an answer here:

• Conditions for automatic generation of default/copy/move ctor and copy/move assignment operator? 3 answers

Consider following code:

class Shape
{
public:
    virtual void introduceMe() = 0;
};

class Triangle : public Shape
{
public:
    void introduceMe() override { std::cout << " I am a triangle \n"; }
};

Moreover consider my following smart pointer:

template<typename T>
class MyUnique_ptr
{
    T* ptr;

public:

    explicit MyUnique_ptr(T* p = nullptr)
    {
        ptr = p;
    }

    ~MyUnique_ptr()
    {
        delete ptr;
    }
}

Furthermore, consider this supplementary piece of code:

MyUnique_ptr<Shape> make_triangle()
{
    MyUnique_ptr<Shape> x(new Triangle);
    return x;  //a- getting an error: implicit defined copy constructor is a deleted function. (But works if I implement a copy constructor)
}

struct Color {
    float g;
};

Color test() {
    Color c;
    return c; //b- why this work whereas a) does not ?
}

And finally, here is how my main look like:

int main(int argc, char** argv)
{
   MyUnique_ptr<Shape> a(new Triangle);
   MyUnique_ptr<Shape> b(make_triangle());  // c- same error here. But works if I implement a move constructor and no copy constructor, why?  
   MyUnique_ptr<Shape> c(a); // d- same error here. But works if copy constructor is explicitly defined.
}

To resume: 1- in a) c) and d) I'm getting errors, saying the implicitly defined copy constructor is a deleted function. Why it is deleted? If I implement a copy constructor then everything works fine. Why the compiler is forcing me to provide a copy constructor?

2- If I implement a move constructor (and no copy constructor) then lines a) and c) will work. Why the compiler is forcing me defined move semantics (constructor)? Are there some cases in c++ (like my example above maybe) where move constructor or move assignment MUST explicitly be defined? Are they actually implicitly defined by the constructor? Thanks