c – 在Clang vs. GCC与MSVC中的SFINAE和能见度检查 – 这是正确的吗？

template<class = void> struct is_default_constructible;
template<> struct is_default_constructible<>
{
protected:
    // Put base typedefs here to avoid pollution
    struct twoc { char a,b; };
    template<bool> struct test { typedef char type; };
    template<class T> static T declval();
};
template<> struct is_default_constructible<>::test<true> { typedef twoc type; };
template<class T> struct is_default_constructible : is_default_constructible<>
{
private:
    template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U*);
    template<class U> static char sfinae(...);
public:
    static bool const value = sizeof(sfinae<T>(0)) > 1;
};

当我在GCC中测试它(-std = c 03)时,它返回0,因为构造函数是不可见的：

class Test { test(); };

int main()
{
    return is_default_constructible<Test>::value;
}

当我在Visual C中测试它时(不同的版本都有相同的行为),我回来1.

当我在Clang中测试它(也是-std = c 03)时,我得到：

error: calling a private constructor of class 'Test'
template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U *);
                                                      ^
note: while substituTing explicitly-specified template arguments into function template 'sfinae' 
static bool const value = sizeof(sfinae<T>(0)) > 1;
                                 ^
note: in instantiation of template class 'is_default_constructible<Test>' requested here
return is_default_constructible<Test>::value;
       ^
error: calling a private constructor of class 'Test'
template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U *);
                                                      ^
note: while substituTing deduced template arguments into function template 'sfinae' [with U = Test]
static bool const value = sizeof(sfinae<T>(0)) > 1;
                                 ^
note: in instantiation of template class 'is_default_constructible<Test>' requested here
return is_default_constructible<Test>::value;

哪个编译器是正确的,为什么？