Adding type for conversion by boost::lexical_cast (segfault)

I want to convert my type back and forth between string representation and actual data using boost::lexical_cast. This is what I'm trying to do:

#include <iostream>
#include <boost/lexical_cast.hpp>
#include <string>

struct Foo
{
    Foo() = default;

    Foo(const std::string& str)
        : str(str)
    {
    }

    friend std::istream& operator>>(std::istream& is, Foo& foo);
    friend std::ostream& operator<<(std::ostream& os, const Foo& foo);

    private:
        std::string str;
};

// For conversion to Foo.
std::istream& operator>>(std::istream& is, Foo& foo)
{
    return is >> foo.str;
}

// For conversion from Foo.
std::ostream& operator<<(std::ostream& os, const Foo& foo)
{
    return os << foo.str;
}

int main()
{
    boost::lexical_cast<Foo>("ok");
    boost::lexical_cast<std::string>(Foo("ok"));
}

But this results in a segfault. Why?

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[Documentation](https://www.boost.org/doc/libs/1_73_0/doc/html/boost_lexical_cast.html)