Why can't I have overloading and generics?

[Caligo]

    struct B { }
    struct C { }
    struct D { }

    struct A {

      ref A foo(B item) {
        /* do something special. */
        return this;
      }

      ref A foo(T)(T item) if(is(T == C) || is(T == D)) {
        /* nothing special, do the same for C and D. */
        return this;
      }
    }

Is this unreasonable?  iirc, C++ supports this, but not D.  What's the
reason? Bug?

What's a good solution to this?

1. a generic `foo()` that uses `static if`s?

2. overload `foo()`, even if it means having function bodies that are
exactly same (code duplication).?

3. mixin templates?  I don't know about this because TDPL says it's
experimental, and I've tried and I get weird errors.