Can you use RTTI to determine a union member?

[Janice Caron]

Suppose I have

union U
{
    A a;
    B b;
    C c;
}

and a function f(U u)

is there any way to determine at runtime, which union member has been
assigned? I guess I want to do something like

void f(U u)
{
    if (is(u.a.typeof == A)) /* stuff */
    else if (is(u.a.typeof == B)) /* stuff */
    else if (is(u.a.typeof == C)) /* stuff */
}

except obviously that won't work because typeof is a compile time
thing, not a run time thing, and so it will always return A.

I don't suppose there's a runtime solution? If not, I guess I'll just
do what I've been doing all along (that is, store the choice in an
enum outside the union).