rvalue assign from boost::optional<const T&> doesn't work

[russell.mcclellan@…]

consider the following code:

#include "boost/optional.hpp"
#include <iostream>

struct a 
{
    int p;
};

boost::optional<const a&> bar() {
    static a b;
    b.p = 3;
    return b;
}

int main(int, char**)
{
    boost::optional<a> baz;
    baz = bar(); // uh-oh!
    std::cout<<baz->p<<std::endl;
    return 0;
}

My expectation is that the line marked "uh-oh" should compile correctly and invoke a's copy constructor to fill in baz's a. What actually happens is that I get a compile error saying that I can't static cast from const a& to a&& inside the move assignment operator for optional.