:kind! does not work under type constructors

Say I have the following:

{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies #-}

module Scratch where

data Nat = Zero | Succ Nat
type family a + b where
  Zero + a = a
  (Succ n) + m = Succ (n + m)

I load this into ghci. See what happens next:

GHCi, version 7.8.0.20140228: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Scratch          ( Scratch.hs, interpreted )
Ok, modules loaded: Scratch.
*Scratch> :kind! Zero + Succ Zero
Zero + Succ Zero :: Nat
= 'Succ 'Zero
*Scratch> :kind! Succ (Zero + Zero)
Succ (Zero + Zero) :: Nat
= 'Succ ('Zero + 'Zero)

Note the last line. It doesn't reduce under the Succ!

I will fix shortly.

Trac metadata

Trac field	Value
Version	7.8.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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