Yes, I know the compiler will generate this error message.  What I try
to ask is how can I get around with this if I want different f() being
called for different type of parameter (e.g. overloading)  .. by
putting a where constraint.

For example, in a non-generic world, this is valid:

static class foo
{
   public static void f(double dummy)  { ....}
   public static void f(Array dummy) { ....}
}

Can I do sth similar in the generic world?  I guess one way is just to
create one f() and do dispatch inside the code by checking
tyeof(dummy), but it will be very messy / inconvenient.  So I am
trying to seek an alternatively approach.