Make GHC generics capable of handling unboxed types

Currently, GHC generics will not accept any unboxed constructor arguments:

$ ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
λ> :set -XMagicHash -XDeriveGeneric
λ> :m + GHC.Generics GHC.Exts
λ> data IntHash = IntHash Int# deriving Generic

<interactive>:4:38:
    Can't make a derived instance of ‘Generic IntHash’:
      IntHash must not have unlifted or polymorphic arguments
    In the data declaration for ‘IntHash’

This makes GHC generics strictly less powerful than deriving clauses in some scenarios, as typeclasses like Eq, Ord, and Show are capable of special-casing certain unboxed types:

λ> :set -ddump-deriv
λ> data IntHash = IntHash Int# deriving Show

==================== Derived instances ====================
Derived instances:
  instance GHC.Show.Show Ghci1.IntHash where
    GHC.Show.showsPrec a_a1OJ (Ghci1.IntHash b1_a1OK)
      = GHC.Show.showParen
          ((a_a1OJ GHC.Classes.>= 11))
          ((GHC.Base..)
             (GHC.Show.showString "IntHash ")
             (GHC.Show.showsPrec 11 (GHC.Types.I# b1_a1OK)))
    GHC.Show.showList = GHC.Show.showList__ (GHC.Show.showsPrec 0)

As discussed on the GHC devs mailing list previously, Andres suggested adding new generic representation types to mark where Int#, Char#, etc. are used (suggesting the name UInt to represent Int#).

Trac metadata

Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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CC	dreixel, kosmikus
Operating system
Architecture