Dependent name and scope

Does anyone know what is going on with the compiler when certain dependent functions are in scope after vs before use? I'm using a close copy of Stroustrup's example in C++ 4th Ed Page 747. In the DEP_NAME example g and Q can be declared after the template function f and are in scope, however in the non DEP_NAME example the opposite is true.

Appreciate your guidance!

#define DEP_NAME
#ifdef DEP_NAME
template<typename T> T f(T a)
{
    return g(a); // OK: a is a dependent name and therefore so is g
}
// can be declared after f
class Q { };
Q g(Q e)
{
    return e;
}
#else
// must be declared before f
class Q { };
Q g(Q e)
{
    return e;
}
template<typename T> T f(T a)
{
    return g(Q{});
}
#endif

int main(int argc, char *argv[])
{
    Q z = f(Q{});
    return 0;
}