Just a question on your discussion. Why not to let mpl::if choose an appropriate overload, like:

//... somewher in c

private:
inline void functionality_impl()const/or non const
{
      //make impl here
}

inline void f()
{
     f<typename mpl::if_c<SYNCH, ScopedCriticalSection, void*>::type>();
}

template<class T>
void f();

void f<ScopedCriticalSection>()
{
    scs(cs);
    functionality_impl();
}

void f<void*>()
{
   functionlity_impl();
}

Greetings,
Ovanes