Usage of boost::optional

This question already has an answer here:

• retrieving an object from boost::optional 2 answers

I get a compilation error when trying to use the value return, look in the code below.

The error is: error: conversion from ‘boost::optional’ to non-scalar type ‘A’ requested

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

struct A
{
    int x = 0;
    int y = 0;
};

boost::optional<A> f_not_set()
{
    boost::optional<A> result;

   return result;
}

int main(int argc, char** argv)
{
    auto a = f_not_set();

    if(a)
    {
        A a1 = a;  // <<<--- COMPILE ERROR
    }
}

return 0;

}