Re: Factoring SFINAE tests

Below is an attempt to factorize the "SFINAE" idiom. I have tried it on
three compilers and they all complain. As far as I can tell, it's valid
code. The questions are, are the compilers right to reject the code or
is the code right and why ? and has anyone successfully mastered the act
of factorizing SFINAE?
----------------------------------------------------------------------
template <template <typename> class w_Test, typename w_Type>
class sfinae_test
{
     typedef char (&false_value)[1];
     typedef char (&true_value)[2];
     struct base {};
     struct derived : base {};
     static derived & make_derived();
     static w_Type& make_test();
     template <bool i>
     struct H
     {
     };
     template <typename Z>
     static false_value func( Z &, base & );
     template <typename Z>
     static true_value func(Z&, derived&, typename w_Test<Z>::type* =0 );
public:
     static const bool value =
         sizeof(func(make_test(), make_derived())) == sizeof(true_value);
};
// some tests
// is_defined_test will fail if T is not defined
template <typename T> class is_defined_test
{
     typedef char (&type)[ sizeof( T ) ];
};
// has_member_orange will fail if T had no orange
template <typename T> class has_member_orange
{
     static T& make_T();
     public:
     typedef char (&type)[ sizeof( make_T().orange ) ];
};