{-# UNPACK #-} support for size-indexed types

I don't know too much of the theory, but it seems to me that {-# UNPACK #-} should be able to work for something like this:

data Tup (l::[*]) where
    Nil  :: HList '[]
    Cons :: e -> {-# UNPACK #-} !(HList l) -> HList (e ': l)

This is essentially just HList from the HList package but strict in the second Cons argument. What I would want is for Tup '[a,b,c] to be represented in just the same way as (a,b,c). When possible, I'd want the getters/setters/lenses to be optimized to index into the unpacked structure rather than walking along it.

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Trac field	Value
Version	7.11
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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