boost::optional - using boost::in_place for the object which constuctor takes other object by the reference

I try to use boost::in_place for the non-movable and non-copyable object which constructor takes other object by the reference:

struct A
{
};

struct B
{
    B(A& a): a_(a){}

    B(B const &) = delete;
    B(B&&) = delete;
    B& operator=(B const &) = delete;
    B& operator=(B&) = delete;

    A& a_;
};

int main()
{
    A a;
    boost::optional<B> op(boost::in_place(a));

    return 0;
}

The code doesn't compile: binding reference of type ‘A&’ to ‘const A’ discards qualifiers

How to fix that ?