override specifier as template parameter - is it valid?

I have the following piece of code that does not compile under Visual C++ 2015, but does under GCC 4.8.4. I'm wondering which is right? The code in question is here below:

template <class T> class ATemplate;
template <class R, class A1>
    struct ATemplate<R(A1)>{ };

int main()
{
    ATemplate<void(int)> x;
//    ATemplate<void(int)override> y; //---Does not compile!!!
    return 0;
}

Is it wrong to use override as specifier here below (or const). Similar code exists in the GMock library, where macro expansion is used to generate the template parameter (including override), as well as the actual function signature.

Kind regards,

Werner