Interesting query. At first I wondered how this situation ever arises,
then I remembered my VFR flight into Troutdale (TTD) which is in a
class D airspace that partially underlies and actually seems to invade
the overlying Class C of Portland (PDX). I was on flight following
from the southwest and couldn't find the TTD airport, so PDX approach
was kindly giving me help... "it's about 2 miles at 12 o'clock."
(Don't laugh; they're not always easy to find.) When he told me to
contact TTD tower, I got a rather exasperated reply on my callin that I
should have called them earlier. I meekly replied that I was with PDX
approach until then, and he said "Oh well, that was ok."
So, I guess I technically busted the Class D by not contacting them
while I was still talking to approach (within Class C) but was already
within 5 miles of the Class D airport. Frankly, since I was on radar
with approach control I felt it was safer for everyone to stay with
them until told to contact the local tower at TTD.