Enable PolyKinds in GHC.Generics

As suggested in this Haskell libraries mailing list thread. Among other benefits, this would allow use of generic functions with Proxy t when t is of a kind other than *.

There seem to be more changes required than just putting {-# LANGUAGE PolyKinds #-} in GHC.Generics, however, since I tried doing that myself and found myself unable to properly derive Generic(1) instances in GHC.Generics. Here is a snippet of the resulting error message:

libraries/base/GHC/Generics.hs:826:1: error:
    Couldn't match type ‘M1 i0 c0 (M1 i1 c1 U1) p0’ with ‘M1 D x’
    Expected type: Rep (Proxy t) x
      Actual type: M1 i0 c0 (M1 i1 c1 U1) p0
    Relevant bindings include
      from :: Proxy t -> Rep (Proxy t) x
        (bound at libraries/base/GHC/Generics.hs:826:1)
    In the expression: M1 (M1 U1)
    In an equation for ‘from’: from Proxy = M1 (M1 U1)
    When typechecking the code for ‘from’
      in a derived instance for ‘Generic (Proxy t)’:
      To see the code I am typechecking, use -ddump-deriv

libraries/base/GHC/Generics.hs:826:1: error:
    Couldn't match type ‘M1 t0 t1 (M1 t3 t4 U1) t2’ with ‘M1 D x’
    Expected type: Rep (Proxy t) x
      Actual type: M1 t0 t1 (M1 t3 t4 U1) t2
    Relevant bindings include
      to :: Rep (Proxy t) x -> Proxy t
        (bound at libraries/base/GHC/Generics.hs:826:1)
    In the pattern: M1 (M1 U1)
    In an equation for ‘to’: to (M1 (M1 U1)) = Proxy
    When typechecking the code for ‘to’
      in a derived instance for ‘Generic (Proxy t)’:
      To see the code I am typechecking, use -ddump-deriv
    In the instance declaration for ‘Generic (Proxy t)’

Trac metadata

Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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CC	dreixel
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