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#include <iostream>
 
#include <boost/optional.hpp>
 
struct A
{
 
};
 
int main()
{
    boost::optional <A> opt;
    opt = {}; // nope!
    if (opt)
        std::cout << "is initialized?\n";
    opt = A{};
    if (opt)
        std::cout << "is initialized now!\n";
    return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:13:12: error: ambiguous overload for ‘operator=’ (operand types are ‘boost::optional<A>’ and ‘<brace-enclosed initializer list>’)
     opt = {}; // nope!
            ^
In file included from /usr/include/boost/optional.hpp:15:0,
                 from prog.cpp:3:
/usr/include/boost/optional/optional.hpp:776:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(const boost::optional<T>&) [with T = A]
     optional& operator= ( optional const& rhs )
               ^~~~~~~~
/usr/include/boost/optional/optional.hpp:784:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(boost::optional<T>&&) [with T = A]
     optional& operator= ( optional && rhs )
               ^~~~~~~~
/usr/include/boost/optional/optional.hpp:794:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(boost::optional<T>::argument_type) [with T = A; boost::optional<T>::argument_type = const A&]
     optional& operator= ( argument_type val )
               ^~~~~~~~
/usr/include/boost/optional/optional.hpp:802:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(boost::optional<T>::rval_reference_type) [with T = A; boost::optional<T>::rval_reference_type = A&&]
     optional& operator= ( rval_reference_type val )
               ^~~~~~~~

Standard output is empty