UNPACK should allow recursion that obviously terminates

I want to do this:

{-# LANGUAGE TypeFamilies MagicHash KindSignatures #-}
data Peano = Zero | Succ Peano
data family BigWord (n :: Peano)
data instance BigWord Zero = MachineWord# Word#
data instance BigWord (Succ n) = BigWord {-# UNPACK #-}(BigWord n) {-# UNPACK #-}(BigWord n)

However, the compiler tells me that it is ignoring the UNPACKs, and the only reason I can see is that they are apparently recursive. This exact type of recursion is handled just fine in instance declarations because the recursive instance has fewer type constructors than the head; can't UNPACK do the same thing?

Trac metadata

Trac field	Value
Version	8.0.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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