construct boost::variant from parameters pack issue

Question

I tried to write function that allows me to construct a boost::variant<> from a parameters pack. In other words, I'm attempting to construct one of the boost::variant<>::types type, whose constructor's signature matches the pack, and then convert to boost::variant<>. I naively wrote the following:

#!/usr/bin/env sh -vex
WARN="-W -Wall -Wextra"
INCLUDE="-isystem /c/libs/boost-trunk"
OPT="-Ofast"
g++ -x c++ - -std=gnu++1y $INCLUDE $WARN $OPT -o a <<__EOF && ./a && echo -e "\e[1;32msucceeded\e[0m" || echo -e "\e[1;31mfailed\e[0m"
#include <boost/assert.hpp>
#include <boost/variant.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/find_if.hpp>
#include <boost/mpl/deref.hpp>
#include <boost/type_traits.hpp>

#include <type_traits>

#include <cstdlib>

struct A
{
    A() = default;
};

static_assert( std::is_constructible< A >::value,           " A()"        );
static_assert(!std::is_constructible< A, int >::value,      "!A(int)"     );
static_assert(!std::is_constructible< A, int, int >::value, "!A(int, int)");

struct B
{
    B(int) {}
};

static_assert(!std::is_constructible< B >::value,           "!B()"        );
static_assert( std::is_constructible< B, int >::value,      " B(int)"     );
static_assert(!std::is_constructible< B, int, int >::value, "!B(int, int)");

struct C
{
    C(int, int) {}
};

static_assert(!std::is_constructible< C >::value,           "!C()"        );
static_assert(!std::is_constructible< C, int >::value,      "!C(int)"     );
static_assert( std::is_constructible< C, int, int >::value, " C(int, int)");

using V = boost::variant< A, B, C >;

template< typename V,
          typename ...O >
inline constexpr
V construct(O &&... _o)
{
    using types = typename V::types;
    using predicate = std::is_constructible< boost::mpl::_1, O... >; // I think, that the problem is exactly here.
    using iter = typename boost::mpl::find_if< types, predicate >::type;
    using deduced_type = typename boost::mpl::deref< iter >::type;
#if 1
    return deduced_type(std::forward< O >(_o)...);
#else
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wmissing-field-initializers"
    return deduced_type{std::forward< O >(_o)...}; // second part of the question
#pragma GCC diagnostic pop
#endif

}

int main()
{
    V a = construct< V >();
    BOOST_ASSERT(a.type() == typeid(A));
    V b = construct< V >(0);
    BOOST_ASSERT(b.type() == typeid(B));
    V c = construct< V >(0, 0);
    BOOST_ASSERT(c.type() == typeid(C));
    return EXIT_SUCCESS;
}
__EOF

It produces a lovely bundle of error messages, the basic meaning of which, as I understand, is that the std::is_constructible is not compatible with the boost::mpl:: library (or more exactly is not compatible with placeholders from boost::mpl::). In turn, the boost:: library itself does not contain the (supposedly compatible) boost::is_constructible type trait.

Another related problem is to deal with classes like struct A {}; struct B {int _1;}; struct C {int _1; int _2;}; and to construct them via curly braces, not parenthesis. The main aspect of the last-mentioned problem is absence of type trait, which deduces whether or no some type can be constructed from some pack of arguments. The solution does not works with boost::mpl::.