Template dependent false

I have a template struct which can't be used directly, only specializations are allowed. And I want to use static_assert to show meaningful error message. I can't just type static_assert(false, "error"); since false isn't template dependent and compiler may show error message even if the template is never used.

My solution:

template<class>
struct AlwaysFalse : std::false_type{};

#define DEPENDENT_FALSE(arg) AlwaysFalse<decltype(sizeof(arg))>::value

template<class T>
struct Foo{
    static_assert(DEPENDENT_FALSE(T), "You must use specialization!");
};

template<int i>
struct Bar{
    static_assert(DEPENDENT_FALSE(i), "You must use specialization!");
};

But I'm not sure about realization DEPENDENT_FALSE. Because MSVC doesn't treat sizeof(arg) as template dependent expression(unlike GCC), but decltype(sizeof(arg)) is fine.

Can somebody explain this behavior in terms of standard? Is it portable?