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Here is an example:

data family Complex a
data instance Complex Double = CD {-# UNPACK #-} !Double
                                  {-# UNPACK #-} !Double

data T = T {-# UNPACK #-} !(Complex Double)

We would like T to just store two Double# but that doesn't happen. Contrast this with

data Complex_Double = CD {-# UNPACK #-} !Double
                         {-# UNPACK #-} !Double

data T = T {-# UNPACK #-} !Complex_Double

Trac metadata

Trac field	Value
Version	6.13
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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