Currently the pragma only supports single-constructor types. So in the following example it will simply be ignored:

data A = 
  A1 Char |
  A2 {-# UNPACK #-} !B

data B = 
  B1 Int |
  B2 Bool

However it seems to be easily solvable by changing the type A to something like the following during unpacking:

data A =
  A1 Char |
  A2_1 Int |    -- from B1
  A2_2 Bool     -- from B2

Am I missing something? Why is it not implemented?