MPL占位符在默认模板参数中替换

我遇到一些麻烦，理解MPL占位符。
有人可以向我解释为什么此代码无法编译？

我希望数字0，1＆amp;2要打印，但是当编译器试图确定包装器的默认模板参数的类型时，似乎占位符并不能被实际类型替换。

#include <iostream>
#include <boost/mpl/inherit.hpp>
#include <boost/mpl/inherit_linearly.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/vector.hpp>
template <typename T> struct Traits;
template<> struct Traits<int>  { typedef boost::mpl::int_<0> type; };
template<> struct Traits<char> { typedef boost::mpl::int_<1> type; };
template<> struct Traits<bool> { typedef boost::mpl::int_<2> type; };
template <typename T, typename Type=typename Traits<T>::type > struct Wrapper
{
    Wrapper() { std::cout << "Value: " << Type::value << std::endl; }
    T value;
};
int main()
{
    typedef boost::mpl::inherit_linearly<
                boost::mpl::vector<int, char, bool>,
                boost::mpl::inherit<boost::mpl::_1, Wrapper<boost::mpl::_2> > >::type Object;
    Object obj;
    return 0;
}

这是GCC-4.1.2的错误（我知道...工作中的旧编译器）

# g++4 -I ../boost test.cpp -o test
test.cpp: In function 'int main()':
test.cpp:24: error: invalid use of undefined type 'struct Traits<mpl_::arg<2> >'
test.cpp:7: error: declaration of 'struct Traits<mpl_::arg<2> >'
test.cpp:24: error: template argument 2 is invalid
test.cpp:24: error: template argument 2 is invalid
test.cpp:24: error: template argument 2 is invalid
test.cpp:24: error: expected initializer before 'Object'
test.cpp:26: error: 'Object' was not declared in this scope
test.cpp:26: error: expected `;' before 'obj'

编辑：在下面的Acorbe回答之后，我制作了我的示例程序的一种变体，以说明为什么他的建议解决方案不符合我的需求。这澄清了我的追求。在这种情况下，我希望打印文本type_a，type_b，type_a。G 错误是相同的。

#include <iostream>
#include <boost/mpl/inherit.hpp>
#include <boost/mpl/inherit_linearly.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/vector.hpp>
enum WrapperType { TYPE_A, TYPE_B };
template <typename T> struct Traits;
template<> struct Traits<int>  { typedef boost::mpl::int_<TYPE_A> type; };
template<> struct Traits<char> { typedef boost::mpl::int_<TYPE_B> type; };
template<> struct Traits<bool> { typedef boost::mpl::int_<TYPE_A> type; };
template <typename T, typename Type=typename Traits<T>::type > struct Wrapper;
template <typename T>
struct Wrapper<T, boost::mpl::int_<TYPE_A> >
{
    Wrapper() : value (0) { std::cout << "TYPE_A" << std::endl; }
    T value;
};
template <typename T>
struct Wrapper<T, boost::mpl::int_<TYPE_B> >
{
    Wrapper() { std::cout << "TYPE_B" << std::endl; }
    T value;
};
int main()
{
    typedef boost::mpl::inherit_linearly<
                boost::mpl::vector<int, char, bool>,
                boost::mpl::inherit<boost::mpl::_1, Wrapper<boost::mpl::_2> > >::type Object;
    Object obj;
    return 0;
}

这是GCC-4.5.4

# g++ -I ../boost test.cpp -o test
test.cpp: In functie ‘int main()’:
test.cpp:37:79: fout: invalid use of incomplete type ‘struct Traits<mpl_::arg<2> >’
test.cpp:9:34: fout: declaration of ‘struct Traits<mpl_::arg<2> >’
test.cpp:37:79: fout: template argument 2 is invalid
test.cpp:37:81: fout: template argument 2 is invalid
test.cpp:37:83: fout: template argument 2 is invalid
test.cpp:37:91: fout: expected initializer before ‘Object’
test.cpp:39:9: fout: ‘Object’ was not declared in this scope
test.cpp:39:16: fout: expected ‘;’ before ‘obj’

这是（部分）Clang -3.1的错误：

test.cpp:15:50: error: implicit instantiation of undefined template 'Traits<mpl_::arg<2> >'
    template <typename T, typename Type=typename Traits<T>::type > struct Wrapper;
                                                 ^
test.cpp:37:57: note: in instantiation of default argument for 'Wrapper<mpl_::arg<2> >' required here
                    boost::mpl::inherit<boost::mpl::_1, Wrapper<boost::mpl::_2> > >::type Object;
                                                        ^~~~~~~~~~~~~~~~~~~~~~~
test.cpp:9:34: note: template is declared here
    template <typename T> struct Traits;