Demonstrate covariant return thunk in C++

Disassembly

__ZN13FancyReturner7BaseRefEv: # the regular
00000001000014a0        pushq   %rbp
00000001000014a1        movq    %rsp, %rbp
00000001000014a4        movq    %rdi, 0xfffffffffffffff8(%rbp)
00000001000014a8        movq    0xfffffffffffffff8(%rbp), %rdi
00000001000014ac        addq    $0x8, %rdi
00000001000014b3        movq    %rdi, %rax
00000001000014b6        popq    %rbp
00000001000014b7        ret
00000001000014b8        nopl    (%rax,%rax)

...

__ZTch0_h8_N13FancyReturner7BaseRefEv: # the thunk
0000000100001b70        pushq   %rbp
0000000100001b71        movq    %rsp, %rbp
0000000100001b74        subq    $0x10, %rsp
0000000100001b78        movq    %rdi, 0xfffffffffffffff8(%rbp)
0000000100001b7c        movq    0xfffffffffffffff8(%rbp), %rdi
0000000100001b80        callq   0x100001c68
0000000100001b85        addq    $0x8, %rax
0000000100001b8b        addq    $0x10, %rsp
0000000100001b8f        popq    %rbp
0000000100001b90        ret
0000000100001b91        nopw    %cs:(%rax,%rax)

main.cpp

#include <iostream>

using namespace std;

struct TheBase {
public:
  int a;
};

class FancySubclass : public TheBase {
public:
  virtual int GetA() {
    return a;
  }
};

class BaseReturner {
public:
  virtual TheBase & BaseRef() = 0;
};

class FancyReturner : public BaseReturner {
public:
  virtual FancySubclass & BaseRef() {
    return instance;
  }
  
  FancySubclass instance;
};

int main() {
  FancyReturner fancyRet;
  BaseReturner & baseRet = fancyRet;
  cout << "baseRet is " << (uintptr_t)&baseRet.BaseRef() << endl;
  cout << "fancyRet is " << (uintptr_t)&fancyRet.BaseRef() << endl;
  return 0;
}