assign value to reference type results in execution of copy constructor

I have this code:

#include <iostream>
#include <string>
#include <unordered_map>

class Y {
public:
    Y()=default;
    Y(const Y&)=delete;
};

class Z{
public:
    Z()=default;
    Z(const Z&)=delete;
    Y y_;
};

class A {
public:
    A()=default;
    A(const A&)=delete;
    std::unordered_map<int, Z> map;
};

class B : public A{
public:
    B()=default;
};

A a;
A * b = new B;

void GetB(B & b) {
    b = *(reinterpret_cast<B *>(&b)); //<== this line causing the error
}

int main()
{
    B b;
    GetB(b);
}

When compiling I get the following error:

error: use of deleted function 'constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const int; _T2 = Z]'
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }

If I change the marked line like so:

void GetB(B * b) {
    b = (reinterpret_cast<B *>(&b));
}

I don't get that error, and the code compile fine. It seems like this error is caused by the fact that I've deleted the copy constructor of class B. But I don't understand why the marked line in the code executes B's copy constructor in the first place?

b is reference, and I expect it to be assigned by reference, and not by value. Does it have something to do with using unordered_map and std::pair? This isn't happening when using vector instead of unordered_map