C++ template struct has no member named error

The only way I can get this to compile without issuing a warning on no member a when T==B and so on for T==A is to reinterpret_cast within the if statement blocks and access the non-shared members via a pointer. Is there no way to get around this or hint to the compiler otherwise?

This is gcc 4.8.x

enum Type { A, B};                                                                                                                                                                          

template<Type T> struct S { };                                                                                                                                                              
template<> struct S<A> { int x; int a; };                                                                                                                                                   
template<> struct S<B> { int y; int x; int b; };                                                                                                                                            

template<Type T> static void foo(int x)                                                                                                                                                     
{                                                                                                                                                                                           
    // lots of code                                                                                                                                                                         

    S<T> s;                                                                                                                                                                                 
    s.x = someCall();                                                                                                                                                                       

    if (T == A)                                                                                                                                                                             
    {                                                                                                                                                                                       
        s.a = 1;                                                                                                                                                                            
    }                                                                                                                                                                                       
    else if (T == B)                                                                                                                                                                        
    {                                                                                                                                                                                       
        s.y = 2;                                                                                                                                                                            
        s.b = 3;                                                                                                                                                                            
    }                                                                                                                                                                                       

    // a bunch of common code                                                                                                                                                               
}       

Edit: I know about making a dedicated specific functions to handle the specifics but I was hoping to avoid extra boilerplate code.