Fix predicate's defaulted copy constructor for C++03 while also fixing #276.

[Romain-Geissler-1A]

Hi,

This fixes the issue I introduced in #276 (sorry for the breakage).

This time I tested not directly with boost, but using compiler explorer. This is the code I tested:

#include "boost/config.hpp"    

struct predicate {
    BOOST_DEFAULTED_FUNCTION(predicate(const predicate& rhs), : m_ti(rhs.m_ti) {})
    BOOST_DELETED_FUNCTION(predicate & operator=(const predicate & rhs))
public:
    const void * const m_ti;
    /*
    bool operator()(helper_value_type const &rhs) const {
        return m_ti == rhs.first;
    }
    */
    predicate(const void * ti) :
        m_ti(ti)
    {}  
};

void f(const predicate& a)
{
    predicate b(a);
    (void)b;
}

I tried with flags -Wall -Wextra -Werror -std=gnu++03 and -Wall -Wextra -Werror -std=gnu++23, and with both gcc and clang x86_64 (trunk version in both cases). There is no compilation error in all these 4 different variants.

Compiler explorer link for clang/C++03: https://godbolt.org/z/PhPGsMdoM

Cheers, Romain

[Romain-Geissler-1A]

Note: given that you use Boost filesystem in your tests (but not in the core library), and given that Boost filesystem 1.84 will drop C++03 support, you may hit some issues testing C++03 in the future.

Same for Boost variant (which is used only in variant.hpp), and potentially other libraries not fully necessary for the core of boost serialization.

[robertramey]

merged and tested