Why is clang not optimizing this with NRVO?

I'm trying to reason why a reasonably good C++ 11 compiler (clang) is not optimizing this code, and wondering if anybody here has opinions.

#include <iostream>
#define SLOW

struct A {
  A() {}
  ~A() { std::cout << "A d'tor\n"; }
  A(const A&) { std::cout << "A copy\n"; }
  A(A&&) { std::cout << "A move\n"; }
  A &operator =(A) { std::cout << "A copy assignment\n"; return *this; }
};

struct B {
  // Using move on a sink. 
  // Nice talk at Going Native 2013 by Sean Parent.
  B(A foo) : a_(std::move(foo)) {}  
  A a_;
};

A MakeA() {
  return A();
}

B MakeB() {  
 // The key bits are in here
#ifdef SLOW
  A a(MakeA());
  return B(a);
#else
  return B(MakeA());
#endif
}

int main() {
  std::cout << "Hello World!\n";
  B obj = MakeB();
  std::cout << &obj << "\n";
  return 0;
}

If I run this with #define SLOW commented out and optimized with -s I get

Hello World!
A move
A d'tor
0x7fff5fbff9f0
A d'tor

which is expected.

If I run this with #define SLOW enabled and optimized with -s I get:

Hello World!
A copy
A move
A d'tor
A d'tor
0x7fff5fbff9e8
A d'tor

Which obviously isn't as nice. So the question is:

Why am I not seeing a NRVO optimization applied in the "SLOW" case? I know that the compiler is not required to apply NRVO, but this would seem to be such a common simple case.

In general I try to encourage code of the "SLOW" style because I find it much easier to debug.